One of the biggest challenges facing plant scientists is to maximise food production as a result of climate change, particularly during times of abiotic stress such as drought or the presence of high salt levels in ground water. An important starting point is therefore to understand how some plants tolerate drought stress more than others. Of course, the extreme examples are xerophytes (succulents), but major crop plants do not fall into this category! To-date, progress has been made in understanding molecular responses to drought and salt stress in plants such as Arabidopsis thaliana with the identification of many genes which are up- or down-regulated in response to such stress. Further, confirmation of the role of some of these genes has been achieved through plant transformation by inserting these genes into plants, possibly with different promoters, and thereby engineering greater drought or salt resistance. Such a GM solution is not the only technique being considered though and normal breeding techniques are also being used to select more drought tolerant cultivars of crop plants.
An interesting question though is "how are these drought-responsive genes affecting plant metabolism, and, importantly, what are the specific metabolic responses to drought in, for example, potato?"
Plant Drought Responses - Dicots
In an attempt to link together some of the published information on molecular events associated with the response of plants to drought a diagram is being drawn in a similar manner to the 'Metabolic Pathways of the Diseased Potato'. This diagram (Plant Drought Responses - Dicots) will be up-dated regularly as new information is published. Publications used to construct the diagram are listed below. A similar approach, looking at the response of monocots to drought, is also described and can be accessed using the above link to Monocots or via the Downloads Section.
One common feature of drought responses in plants is the role of ABA-responsive genes. Hey et al. (2010) reviewed links between ABA, stress and sugar signalling, and the role of sucrose non-fermenting-1-related protein kinases (SnRKs), SnRK1-activating protein kinases (SnAKs), calcium-dependent protein kinases (CDPKs) and the transcription factors ABA response element binding proteins (AREBPs).
Early work on plant drought responses focused on microarray experiments, particularly with Arabidopsis, which provided useful guidance on which genes could be important. Subsequent work, particularly with transcription factors, has used transformation to confirm the importance of such genes. However, we are still some way from fully understanding signalling cascades from these transcription factors particularly in species other than Arabidopsis (see Arabidopsis signalling PDF).
One problem with trying to define drought response pathways is that many metabolites are associated with multiple metabolic pathways. This can be demonstrated by looking at glutamine which is associated with a number of diverse metabolic pathways as well as its role as an amino acid involved in protein biosynthesis. A PDF can be downloaded (from the Downloads Section) showing several pathways involving glutamine / glutamate as a metabolite. Thus there are difficulties when constructing stress-responsive pathways in assigning some metabolites to specific pathways when other corroborating evidence is lacking. This is particularly pertinent when considering drought responsive pathways as glutamine can accumulate in drought stressed leaves but it is not yet clear which pathway is leading to this glutamine accumulation.
Published information on plant drought responses
Evers et al. (2010) used a combined transcriptomic and targeted metabolite approach to identify drought-responsive genes and compounds using two potato clones with different drought-tolerance phenotypes.
Coello et al. (2011) reviewed the key role that SnRKs play at the interface between metabolic and stress signalling in plants. They suggest that plants have 38 SnRKs which can be subdivided into three sub-families: SnRK1, SnRK2 and SnRK3.
Shin et al. (2010) showed that transcript levels of a putative R1-type MYB-like transcription factor StMYB1R-1 were enhanced in response to several environmental stresses including drought but were unaffected by biotic stresses. Overexpression of StMYB1R-1 in transgenic plants enhanced the expression of drought related genes such as AtHB-7, RD28, ALDH22a1, and ERD1-like.
Gong et al. (2010) identified a number of drought-responsive genes in tomato. Examples of key enzymes in pathways affected by drought include fructose-bisphosphate aldolase (gluconeogenesis), adenylate kinase (purine and pyrimidine nucleotide biosynthesis), aldehyde oxidase (tryptophan degradation), beta-amylase (starch degradation), cystathionine beta-lyase (methionine biosynthesis) and catalase (removal of superoxide radicals).
Ruan and Teixeira da Silva (2010) reviewed the applications of plant metabolomics in characterizing metabolic responses to salt and drought stress and to identify metabolic quantitative trait loci (QTLs).
Legay et al. (2009) used cDNA microarrays to study the response of potato leaves to salt stress which revealed cross talk between abiotic and biotic stress responses during salt exposure.
Rensink et al (2005) used potato cDNA microarrays to identify abiotic stress responses (cold, heat, salt) in potato. They showed that stress-regulated clones could be separated into either stress-specific or shared-response clones suggesting the existence of general response pathways as well as more stress-specific pathways.
Proline-, glycine- and tryrosine-rich proteins (PGYRPs) are thought to be associated with abiotic stress responses. For example, Feng et al. (2010) showed that the expression of GmPGYRP1, 3, 5 and 7 (from soybean) was regulated by drought, salt and cold. Cold stress strongly induced the expression of GmPGYRP1, 5, and 7 but repressed the transcription of GmPGYRP3. GmPGYRP3 was also down-regulated by drought. These authors showed that there were 12 PGYRP genes in soybean compared to 14 genes in Arabidopsis.
Muniz Garcia et al. (2010) showed that StPPI1, a proton pump interactor from Solanum tuberosum, is induced by salt stress and cold, and drought and mechanical wounding slightly increased StPPI1 transcript levels.
Wan et al. (2010) reported that consitutive expression of AhUBC2 (ubiquitin-conjugating enzyme from peanut) in Arabidopsis confers improved water stress tolerance through activating an ABA-independent signalling pathway with increased expression of P5CS1, RD29A and KIN1.
Choi et al. (2011) reported that constitutive expression of CaXTH3, a hot pepper xyloglucan endotransglucosylase / hydrolase, enhanced tolerance to salt and drought stresses in tomato plants.
Zhang et al. (2010) showed that expression analyses in wheat of TaSnRK2.8 was involved in the response to PEG, NaCl and cold. Overexpression of TaSnRK2.8 in Arabidopsis resulted in enhanced tolerance to drought, salt and cold.
Arabidopsis plants over-expressing AtTZF1 (a tandem zinc finger protein) had an enhanced tolerance to cold and drought (Lin et al., 2011).
Jin et al. (2011) showed that transcripts of the nucleus-localized AtCPL5 (a serine-2-specific RNA polymerase II C-terminal domain phosphatase) were induced by ABA and drought. AtCPL5 overexpression changed the expression of numerous genes and led to lower transpiration rates on dehydration and enhanced drought tolerance.
Bartels & Sunkar (2005) have provided a comprehensive review of drought tolerance in plants.
de Silva et al. (2011) showed that a calcium-dependent lipid-binding protein (AtCLB) binds specifically to the promoter of Arabidopsis thalianol synthase gene (AtTHAS1). The AtCLB protein was capable of binding to ceramide. The loss of the Atclb gene function confers and enhanced drought and salt tolerance in T-DNA insertion knockout mutants.
Wurzinger et al. (2011) reviewed published information on cross-talk between calcium-dependent protein kinases and MAP kinase signalling. Usefully, they provide a table listing CDPKs and MAPks involved in biotic and abiotic stress responses (including drought stress).
Zhang et al. (2011) reported that mutation of SKB1 (Shk1 kinase binding protein 1) in Arabidopsis resulted in salt hypersensitivity. The authors proposed that SKB1 mediates plant development and the salt response by altering the methylation status of H4R3sme2 and LSM4 and linking transcription to pre-mRNA splicing.
Lee et al. (2010) reported that, in pepper, virus-induced gene silencing of CaRAV1 and CaRAV1/CaOXR1 confers enhanced susceptibility to high salinity and osmotic stresses. Overexpression of CaRAV1 and CaOXR1/CaRAV1 in Arabidopsis resulted in increased tolerance to high salinity and osmotic stress. Overexpression of CaRAV1, CaOXR1 and CaOXR1/CaRAV1 in Arabidopsis also increased resistance to the biotrophic oomycete Hyaloperonospora arabidopsis.
Legay et al. (2011) used cDNA microarrays to look at the response of a drought tolerant and a drought sensitive potato cultivar. They showed that transcription factors related to abiotic stresses and genes relating to raffinose family oligosaccharide synthesis were up-regulated to a greater exent in the resistant clone. Biochemical analysis showed an increase in galactinol and raffinose after drought stress particularly in the more drought tolerant clone.
Nishizawa et al. (2008) suggested that galactinol and raffinose may scavenge hydroxyl radicals to protect plant cells from oxidative damage caused by methylviologen, salinity or chilling.
Park et al. (2005) described results from overexpressing a vacuolar H+-pyrophosphatase (AVP1) in Arabidopsis and showed an increased tolerance to soil water deficit. Expression of AtAVP1 in tomato resulted in plants with more robust root systems allowing them to take up more water during water deficit stress.
Donaldson et al. (2004) showed that salt and osmotic stress cause rapid increases in cyclic GMP in Arabidopsis. Further, salt stress activates two cGMP pathways - an osmotic calcium-independent pathway, and an ionic calcium dependent pathway.
There is an excellent review by Vij and Tyagi (2007) on emerging trends in the functional genomics of the abiotic stress response in crop plants.
Wang et al. (2011) showed that an arginine decarboxylase gene PtADC from Poncirus trifoliata confers abiotic stress tolerance and promotes primary root growth in Arabidopsis.
Sakuma et al. (2006) described a functional analysis of the Arabidopsis transcription factor DREB2A. Arabidopsis plants overexpressing DREB2A showed an increased drought tolerance.
Kang et al. (2011) identified AtSAP5 (a member of the Stress Associated Protein family) in response to salinity, osmotic, drought and cold stress. Over-expression of AtSAP5 in transgenic plants correlated with up-regulation of drought stress responsive gene expression. Recombinant AtSAP5 possessed E3 ubiquitin ligase activity in vitro.
Hussain et al. (2010) reviewed the potential for using transcription factors to engineer enhanced drought tolerance in plants.
Li et al. (2011) showed that the disproportionating enzyme 2 (DPE2) in Arabidopsis (which converts maltose to glucose) is inactivated by cold. Maltose is described as a solute that protects cells from freezing injury. (There is potential for a similar explanation to occur in drought-stressed plants).
Cecchini et al. (2011) describes the role of proline dehydrogenase in Arabidopsis showing it is involved in defence against pathogens. As proline is also associated with drought responses in plants, information in this paper is also relevant to our understanding of some aspects of plant drought responses.
Li et al (2011) reported that transgenic tobacco overexpressing the wheat expansin gene TaEXPB23 lost water more slowly than wild-type plants under water stress.
Abogadallah et al. (2011) showed that overexpression of HARDY an AP2/ERF gene from Arabidopsis improved drought and salt tolerance when expressed in Trifolium alexandrinum.
Frazier (2011) showed that drought and salinity stresses altered microRNA expression in a dose-dependent manner in tobacco.
Chen (2011) used microarray and RT-PCR to show that ZmCIPK genes transcriptionally responded to abiotic stress and that 24, 31, 20 and 19 ZmCIPK genes were up-regulated by salt, drought, heat and cold stresses respectively.
Liu et al. (2011) showed that AtPUB19, a U-box E3 ubiquitin ligase, was up-regulated by drought, salt, cold and ABA. Down-regulation of AtPUB19 led to enhanced ABA-induced stomatal closing and enhanced drought tolerance. Thus, AtPUB19 negatively regulates ABA and drought responses in Arabidopsis.
Garcia et al. (2011) stated that fructans are known to be protective againt drought in plants. They analysed fructan composition and enzymes involved in fructan synthesis in Veronia herbacea, viz sucrose:sucrose 1-frutosyltransferase (1-SST) and fructan:fructan 1-frutosyltransferase (1-FFT), and depolymerization, fructan 1-exohydrolase (1-FEH).
Akashi et al (2001) described the potential role of citrulline as a hydroxyl radical scavenger in drought stressed watermelon plants.
Yokota et al. (2002) showed that wild watermelon accumulated large amounts of citrulline, glutamate and arginine in leaves in response to drought.
Huang et al. (2011) have reviewed information on signal transduction during, cold, salt, and drought stresses in plants.
Chu et al. (2010) investigated the relationship between glucosamine-induced ROS production and abiotic stress responses in Arabidopsis. Scavenging glucosamine by expression of E.coli glucosamine-6-phosphate deaminase enhanced tolerance to drought, oxidative and cold stress.
Krishnaswamy et al. (2011) showed that overexpression of RAP2.6L and DREB19 in Arabidopsis enhanced performance under salt and drought stress without affecting phenotype.
Huang et al. (2008) used microarrays to identify almost 2000 drought-responsive genes in Arabidopsis and showed that most of these genes had returned to normal expression levels by 3hrs after watering. ABA-dependent pathways were predominantly affected by drought and there was extensive cross-talk between responses to drought and otheer environmental factors including light and biotic stresses.
Seo & Park (2011) investigated the role of cuticular wax biosynthesis in relation to drought resistance. They showed that MYB96 affected drought responses and showed that many of the genes involved in wax biosynthesis are up-regulated (KCS, KCR and ECR).
Lei et al. (2011) showed that EIN2 is required for salt tolerance in Arabidopsis as well as the downstream components EIN3 and EIL1. ECIP1 interacts with EIN2.
Oh et al. (2011) reported splice variants for MYB60 in Arabidopsis which were involved in stomatal movement. An initial response to drought root growth behaviour is regulated by MYB60 expression promoting root growth but later stages of drought stress inhibits MYB60 expression resulting in stomatal closure and inhibited root growth.
Han et al. (2011) reported that the expression of SlNAC3 was inhibted by salt, drought and ABA in Solanum lycopersicum.
Bae et al. (2011) examined the expression of of a poplar plasma membrane intrinsic protein (an aquaporin) (PatPIP1). Gene expression was induced by drought, salinity, cold, wounding, gibberellic acid, jasmonic acid and salicylic acid. They proposed that PatPIP1 plays an essential role in defense of plants to water stress.
Rushton et al. (2011) provided a very useful review of the (key) role of WRKY transcription factors in ABA signalling which thereby provides information regarding the role of these transcription factors in response of Arabidopsis to drought.
Liang et al. (2011) showed that the SVM-RFE (Support Vector Machine-Recursive Feature Elimination) method is a useful tool for analysing plant microarray data for studying genotype-phenotype interactions. They analysed the top 10 genes predicted to be involved in water tolerance and showed that 7 were connected to known biological processes in drought tolerance.
Kondrák et al. (2011) showed that potato transformed with the yeast trehalose-6-phosphate synthase 1 gene exhibited increased drought tolerance. They used microarray and RNA gel blots to study regulation of genes in wild type potato and TPS1-transgenic plants.
Yue et al. (2011 and 2012) showed that showed that overexpression of AtLOS5/ABA3 in transgenic tobacco and cotton (respectively) resulted in enhanced drought tolerance.
Shan et al. (2011) investigated the role of chrysanthemum R2R3-MYB transcription factor CmMYB2 and showed that it enhanced drought and salinity tolerance (amongst some other effects) when expressed in Arabidopsis.
Barrera-Figueroa et al. (2011) suggested that miRNAsmay play important roles in drought tolerance in cowpea. Among 44 drought-associated miRNAs, 30 were up-regulated in drought conditions and 14 were down-regulated.
Sun et al. (2011) showed that the Arabidopsis transcription factor bZIP1 is a positive regulator of plant tolerance to salt, osmotic and drought stresses. Overexpressing bZIP1 in wild type Arabidopsis increased tolerance to salt and drought.
AtAIRP1 is a C3H2C3-type RING E3 ubiquitin ligase which is a positive regulator of the ABA-dependent drought response in Arabidopsis. Cho et al. (2011) showed that AtAIRP2 played combinatory roles with AtAIRP1 in Arabidopsis ABA-mediated drought responses.
There are two routes by which plants can synthesise proline. Ku et al. (2011) showed that under drought conditions the major route for synthesis of proline in Nicotiana benthamiana was via Δ(1)-pyrroline-5-carboxylate synthetase (P5CS; EC:126.96.36.199) rather than the alternative route via ornithine-δ-aminotransferase (OAT; EC 188.8.131.52).
Loyola et al. (2011) suggested that, in tomato, isoprenoid compounds synthesised in the plastids are involved in the response to water deficit. They showed that there was higher expression of GGPPS and HPT1 in the drought tolerant Solanum chilense compared to S. lycopersicum. They also suggested that in addition to lower stomatal conductance, α-tocopherol biosynthesis is part of the adaptation mechanism of S. chilense to adverse conditions.
Jin et al. (2011) described how hydrogen improves drought resistance in Arabidopsis. They studied the express of l-cysteine desulfhydrase and d-cysteine desulfhydrase which degrade cysteine to generate hydrogen sulphide.
Qin et al. (2011) showed that the SPINDLY gene SPY (which encodes for an O-linked N-acetylglucosamine transferase) is drought stress inducible and plays a negative role in the Arabidopsis abiotic stress response.
Muñiz García et al. (2011) showed that StABF1, a bZIP transcription factor which is induced in response to ABA, drought, salt stress and cold, is phosphorylated in response to ABA and salt stress in a calcium-dependent manner. They also indentified StCDPK2 that phosphorylates StABF1 in vitro.
Pan et al. (2011) identified SlERF5 from tomato and showed that expression was induced by high salinity, drought, flooding, wounding, and cold. Over-expression of SlERF5 in transgenic tomato increased tolerance to drought and salt stress compared to wild type tomato.
Qin et al. (2011) extended earlier results in tobacco and rice to show the role of isopentenytransferase (involved in cytokinin biosynthesis) in drought tolerance of peanut (Arachis hypogaea L.).
Sun et al. (2011) investigated the rol eof 14-3-3 proteins in the abiotic stress response of cotton (Gossypium hirsutum L.). Salinity and drought stress had a significant effect on the expression of 14.3.3 genes. The 14.3.3 gene CGF14-4 was particularly sensitive to drought and salinity while other 14-3-3s were affected by either drought or salinity.
Guo et al. (2010) showed that the glutaredoxin gene SlGRX1 in tomato was up-regulated by drought and salt stress, and over-expression increased tolerance to drought.
Ko et al. (2006) showed that up-regulation of AtXERICO gene in Arabidopsis increased cellular ABA and adult transgenic plants showed increased drought tolerance. Yeast two-hybrid screening suggested XERICO interacts with E2 ubiquitin-conjugating enzyme AtUBC8 and ASK1-interacting F-box protein AtTLP9.
Pasapula et al. (2011) showed that a vacuolar H+ pyrophosphatase (AVP1) from cotton improved drought and salt tolerance when expressed in Arabidopsis, tomato, rice or cotton.
Ahn et al. (2011) showed that transcripts of GmIMT (myo-inositol methyltransferase) increased in Glycine max in response to drought. Transgenic Arabidopsis over-expressing GmIMT showed improved drought tolerance. IMT converts myo-inositol to O-methyl inositol (d-ononitol).
Tang et al. (2011) identified 6 new NAC genes in wheat. Transgenic tobacco expressing TaNAC2a showed showed higher fresh and dry weight than non-transgenic plants under drought conditions.
Mao et al. (2010) showed that expression of the wheat gene TaSnRK2.4 in Arabidopsis increased tolerance to drought, salt and freezing.
Varshney et al. (2009) provided information on drought and salinity-responsive ESTs in chickpea (Cicer arietinum L.).
Chen et al. (2011) demonstrated a negative role for the glutathione S-transferase gene AtGSTU17 in Arabidopsis in response to drought. Arabidopsis plants were more tolerant to drought when AtGSTU17 was mutated.
Cheng et al. (2012) showed that RGLG2 (a RING domain ubiquitin E3 ligase) interacts with AtERF53 and negatively regulates the drought response in Arabidopsis.
Xie et al. (2009) showed that transgenic Arabidopsis overexpressing soybean GmGT-2A or GmGT-2B showed increased tolrance to drought, salt and freezing.
Tran et al. (2009) showed that 9 GmNAC genes were drought stress inducible in soybean and they showed differential induction levels in shoots and root.
Lee et al. (2011) showed that the YUC7 gene (a flavin monooxygenase involved in tryptophan-dependent auxin biosynthesis) was induced in Arabidopsis (primarily roots) by drought.
Li et al. (2011) studied soybean (Glycine max) miRNAs associated with drought, salinity and alkalinity and showed that 133 conserved miRNAs representing 95 miRNA families were expressed under these three treatments. In addition, 71, 50 and 45 miRNAs were either uniquely or differentially expressed under drought, salinity of alkalinity respectively.
Gao et al. (2011) showed that the zinc finger protein CgZFP1 from chrysanthemum was up-regulated by drought and salt. Expression of CgZFP1 in Arabidopsis conferred resistance to drought and salinity.
Wilson et al. (2009) described the nucleotidase/phosphatase SAL1 as anegative regulator of drought in Arabidopsis
Estavillo et al. (2011) showed that the phosphonucleotide (3'-phosphoadenosine 5'-phosphate [PAP]) accumulates in Arabidopsis in response to drought and high light stress and that the enzyme SAL1 regulates its levels by dephosphorylating PAP to AMP.
Luo et al. (2011) described GsZFP1, a Cys2/His2-type zinc finger protein, as a positive regulator of tolerance to drought and cold.
Choi & Hwang (2011) showed that extraceullular peroxidase 2 (CaPO2) is induced in pepper in response to drought, ABA, cold, salt and infection by Colletotrichum coccodes. Overexpression of CaPO2 in Arabidopsis enhanced tolerance to salt, drought, oxidative stress and resistance to Alternaria brassicicola.
Muñoz-Mayor et al. (2012) showed that overexpression in tomato of the tomato dehydrin gene tas14 increased drought and salinity tolerance.
Li et al. (2012) showed that ROP11 GTPase is a negative regulator of multiple ABA responses including drought responses.
Yao et al. (2012) showed that overexpression of the aspartic protease ASPG1 in Arabidopsis conferred drought avoidance.
Deeba et al. (2012) used MALDI-TOF-TOF to identify proteins up- and down-regulated by drought in cotton (Gossypium herbaceum).
Lee et al. (2012) showed that the Arabidopsis drought responsive NAC transcription factor NTL4 promotes ROS production during drought-induced senescence.
Qin et al. (2012) showed that overexpression of TaMYB33 in Arabidopsis increased resistance to drought and NaCl stress.
Lee et al. (2012) showed that overexpression of AtTZF2 or AtTZF3 in Arabidopsis conferred ABA hypersensitivity, reduced transpiration, and increased drought resistance.
Catala et al. (2007) showed that in Arabidopsis SIZ1 mediated the expression of approx 300 drought responsive genes by a pathway independent of DREB2A and ABA. Arabidopsis possesses approx 1700 genes that are regulated by drought.
Kim et al. (2012) described AtRDUF1 and AtRDUF2 (E3 Ub ligases) as ABA- and drought-induced and knockouts of these genes reduced tolerance to drought stress relative to wild types.
Xu & Chua (2012) showed that mRNA decapping through MPK6-DCP1-DCP5 pathway is a rapid response to dehydration in Arabidopsis.
Kiribuchi et al. (2005) showed that the jasmonic acid inducible transcription factor RERJ1 was upregulated in rice leaves by drought and wounding.
Wan et al. (2012) showed that overexpression of AtCBP60g in Arabidopsis enhanced the defence response, hypersensitivity to ABA, and drought tolerance. They also showed that the isochorismate synthase gene ICS1 expression levels increased in Arabidopsis plants overexpressing CBP60g in response to drought and ABA treatment.
Hasanuzzaman & Fujita (2011) showed that in rapeseed (Brassica napus) the activity of dehydroascorbate reductase, glutathione S-transferase, glutathione peroxidase, and glyoxalase I activity significantly increased under drought stress, while catalase and glyoxalase II activity decreased. A sharp increase in hydrogen peroxide and lipid peroxidation (MDA content) was also induced by drought stress.
Su et al. (2011) showed that expression levels of SbP5CS1 and SbP5CS2 transcripts were upregulated in sorghum seedlings by drought, salt, anf Me JA.
Dong et al. (2011) showed that overexpression of MdVHP1 (a vacuolar H(+)-translocating inorganic pyrophosphatase EC 184.108.40.206) enhanced tolerance to salt, PEG-mimic drought, cold and heat in transgenic apple calluses, and overexpression in transgenic tomato improved tolerance to salt and drought.
Huang et al. (2011) showed that transgenic overexpression of PtrMAPK (from Poncirus trifoliata) in tobacco confers dehydration and drought tolerance.
Yadav et al. (2005) showed that in various plant species methylglyoxal increases in response to drought, cold and salinity
Xie et al. (2012) showed that overexpression of the dehydrin gene MtCAS31 (from Medicago trunctatula) reduced stomatal density and increased drought tolerance in transgeneic Arabidopsis. MtCAS31 interacted with AtICE1.
Lisso et al. (2012) showed that loss of the Arabidopsis NFX1-LIKE2 (AtNFXL2) gene results in elevated ABA levels, elevated hydrogen peroxide levels, reduced stomatal aperture, and enhanced drought stress tolerance.
Kang & Udvardi (2012) described work on response of a drought tolerant and a drought sensitive variety of alfalfa. "Genes for the ROS-generating enzyme, NADPH oxidase were generally induced under drought, while those for glycolate oxidase were repressed. Among the ROS-scavenging protein genes, Ferritin, Cu/Zn superoxide dismutase (SOD), and the majority of the glutathione peroxidase family members were induced under drought in both roots and shoots of both alfalfa varieties. In contrast, Fe-SOD, CC-type glutaredoxins, and thoiredoxins were downregulated".
Wang et al. (2012) showed that transgenic Arabidopsis expressing the histidine kinase gene ZmHK9 were less affected by drought than wild type plants.
Yao et al. (2012) studied the properties of a H(+)-pyrophosphatase gene (KfVP1) from Kalidium foliatum. Transcription of KfVP1 in K. foliatum was induced by NaCl, ABA and PEG and overexpression of KfVP1 in Arabidopsis increased tolerance to salt and drought.
Seo et al. (2012) showed that overexpression of CaRma1H1 (an endoplasmic reticulum (ER)-localized hot pepper RING E3 Ub ligase) in tomato increased resistance to drought and salt stress. In addition, the ER chaperone genes LePDIL1, LeBIP1, and LeCNX1 were markedly up-regulated in 35S:CaRma1H1 tomatoes compared with wild-type plants.
Lu et al. (2012) analysed fructose 1,6-bisphosphate genes and their expression in Arabidopsis and showed they were abiotic stress responsive (including drought).
Wang et al. (2012) analysed the response of chickpea (Cicer arietinum L) to drought using microarrays. They showed that 4,815 differentially expressed unigenes were either ≥2-fold up- or ≤0.5-fold down-regulated in at least one of the five time points during drought stress. 110 pathways in two tissues were found to respond to drought stress. Compared to control, 88 and 52 unigenes were expressed only in drought-stressed root and leaf, respectively.
Cabello & Chan (2012) showed that the expression of the TFs HaHB1 and AtHB13, as well as that of their putative targets AtPR2, AtPR4 and AtGLU, is up-regulated by drought and salinity stresses. Further, transgenic plants (separately) overexpressing these genes exhibited tolerance to sever drought or salinity stresses.
Han et al. (2012) used a beta-glucosidase (AtBG1) known hydrolyze glucose-conjugated ABA to transform creeping bentgrass and increase levels of ABA. The resulting plants showed an increased tolerance to drought and had a dwarf phenotype.
Lee & Park (2012) showed that in Arabidopsis NTL4 regulation of ROS generation underlies drought induced senescence.
Xu et al. (2012) showed that overexpression of a β-glucosidase homolog, AtBG2 in Arabidopsis enhanced resistance to dehydration and salt stress.
Kwak et al. (2007) showed that the expression of HMGB2 and HMGB3 was markedly down-regulated by drought or salt stress in Arabidopsis.
Wang et al. (2012) showed that BcWRKY46 from Pak-choi (B. campestris ssp. chinensis) was expressed in response to low temperature, ABA, salt and dehydration. Constitutive expression of BcWRKY46 in tobacco reduced susceptibility to freezing, ABA, salt and dehydration.
Kim et al. (2012) showed that overexpression of AtTCTP (translationally controlled tumor protein) in Arabidopsis enhanced drought tolerance.
Kesari et al. (2012) described the role of a splicing variant (exon 3-skip P5CS1) of Δ(1)-pyrroline-5-carboxylate synthetase1 in relation to abiotic stress (including drought).
Utsumi et al. (2012) used microarrays to identify approx 1300 drought stress up-regulated genes in cassava and suggested that cassava has similar mechanisms for drought stress response and tolerance as other plant species.
Liu et al. (2012) showed that AtPP2CG1 was up-regulated in Arabidopsis by salt, drought and ABA. AtPP2CG1 up-regulated the genes RD29A, RD29B, DREB2A and KIN1.
Hozain et al. (2012) showed that expression of AtSAP5 in cotton protected photosystem II from drought-induced damage after 4days of drought.
Trivedi et al. (2012) showed that the expression of the histone H1 variant at the transcript and protein levels was induced specifically in the drought tolerant Gossypium herbaceum genotype Vagad.
Chen et al. (2012) isolated CdICE1 from Chrysanthemum dichrum. The constitutive expression of CdICE1 in C. grandiflorum improved the tolerance of C. grandiflorum to low temperature/freezing, drought and salinity.
Kondrák et al. (2012) compared responses of the drought-sensitive potato cultivar White Lady and the drought-tolerant TPS1 transgenic variant. The contents of fructose, galactose and glucose were increased and decreased in the wild-type and TPS1 transgenic leaves, respectively, while the amounts of proline, inositol and raffinose were highly increased in both types in response to drought. They identified four transcription factors uniquely up-regulated in TPS1 transgenic leaves.
Barozai and Wahid (2012) described the in silico identification and characterization of abiotic stress responding genes in potato.
Kumar et al. (2012) showed that the Xerophyta viscosa aldose reductase (ALDRXV4) confers enhanced drought and salinity tolerance to transgenic tobacco plants by scavenging methylglyoxal.
Chen et al. (2012) showed that mRNA level for NRT1.5, a xylem nitrate loading transporter, was down-regulated by salt, drought and cadmium. Functional disruption of NRT1.5 enhanced tolerance to salt, drought and cadmium stresses.
Marshall et al. (2012) reviewed approaches to dealing with drought stress and discuss the role of receptor-like kinases in drought responses.
Cho and Hong (2006) showed that overexpression of NtHSP70-1 increased drought tolerance in plants.
Kang et al. (2012) showed tht in Arabidopsis drought induced the expression of ARR5, ARR7, ARR15 and ARR22 and the cytokinin receptors AHK2 and AHK3 are redundantly involved in dehydration-inducible expression of ARR7, but not that of ARR5, ARR15, or ARR22.
Park et al. (2012) described the identification of differentially transcribed genes in cotton in response to water deficit.
Yu et al. (2012) showed that the transcription factor AtMYB2 can interact with the promoter of choline monooxygenase. (Both AtMYB2 and choline monooxygenase have previoulsy been associated with drought responses.)
Jung et al. (2012) showed that BrRZFP1, a C3HC4-type RING zinc finger protein from Brassica rapa, was induced by salt, cold and ABA. Constitutive expression of BrRZFP1 in Nicotiana tabacum increased tolerance to cold, salt and dehydration.
At the moment we cannot fully describe all the molecular events associated with drought tolerance in potatoes but we can gain some additional information by looking in more detail at those genes regulated by drought in Arabidopsis (see below).
DRASTIC-derived data : Arabidopsis thaliana
Several years ago a gene expression database was available on the DRASTIC (Database Resource for the Analysis of Signal Transduction in Cells) web site - (though this web site is no longer on-line) - containing in excess of 20,000 entries. Using the Pathway Tool to look for genes which were up- or down-regulated for certain treatments it was possible to speculate on which genes may be involved in metabolic or signalling 'pathways'.
For example, the figure below shows the output from selecting a number of Arabidopsis genes that were up- or down-regulated by drought - blue colour indicates the genes were down regulated and red indicates the genes were up-regulated. Not only are some genes down-regulated but in this example the genes At1g09340 (RNA binding protein) and At5g53140 (putative protein phosphatase 2C) are clearly early signalling genes suggesting subsequent down-regulation of whole pathways. The Pathway Tool showed that the genes in column 4 are also affected by ABA, cold and sodium chloride (columns 1, 2 and 16 respectively) in a similar manner and that their regulation was quite different in response to, say, cucumber mosaic virus (column 3). We would therefore expect some overlap in some signalling pathways that are involved in abiotic stress responses though there will almost certainly be some responses which are possibly unique to each stress, as well as some genes being involved in organ-specific responses. These results also indicate how disease resistance mechanisms might be affected by abiotic stress, though conclusive proof would require further experimentation. Interestingly, Liu et al. (2007) reported that transgenic overexpression of UDP-glucose-4-epimerase in A. thaliana conferred tolerance to salt, drought and freezing stress again providing evidence of an overlap in signalling and response pathways between some abiotic stresses.
Another dataset obtained using the DRASTIC pathway tool did in fact show some genes to be diferently regulated by cold and drought (compare columns 3 & 4 below).
A third dataset of genes regulated by drought in shown in the Figure below. This list of genes is also similarly regulated by ABA and salt but none of them are regulated by cold.
One advantage of the DRASTIC gene expression database was that it was possible to up-date the identification of genes and relate the new ('correct') names to previously published data. For example, the first gene in the last dataset (above) was At1g01140 which was originally classified as SNF1-related kinase but is now classified as CBL-interacting protein kinase 9 (CIPK9). CIPK9 has been described as being associated with K+ signalling (Amtmann & Armengaud, 2007). Interestingly, the diagram by Amtmann & Armengaud also shows the involvement of Calcium signalling and in Fig 3 above we have a gene At5g11420 which is now described as a Calcium sensing receptor although its identity was unknown in the original publication.
A list of the genes described in Fig 1, 2 & 3 can be downloaded as a PDF document from here. The function of the genes was up-dated on 15 November 2010. Of these drought-responsive genes from Arabidopsis 18 were on chromosome 1, 15 on chromosome 2, 11 on chromosome 3, 7 on chromosome 4, and 17 on chromosome 5.
Additional published data from the DRASTIC database
Kohler et al. (2006) also included some information from the DRASTIC gene expression database when analysing information on the lignin biosynthetic pathway in Arabidopsis (see their Fig 4). The genes At1g67990, At1g09500, At1g72680 and At2g33590 were cited as responding to ABA, sodium chloride, drought and wounding.