Drought Stress In Maize Research Articles

Quantitative Proteomic Analyses Identify ABA-Related Proteins and Signal Pathways in Maize Leaves under Drought Conditions.

Drought stress is one of major factors resulting in maize yield loss. The roles of abscisic acid (ABA) have been widely studied in crops in response to drought stress. However, more attention is needed to identify key ABA-related proteins and also gain deeper molecular insights about drought stress in maize. Based on this need, the physiology and proteomics of the ABA-deficient maize mutant vp5 and its wild-type Vp5 under drought stress were examined and analyzed. Malondialdehyde content increased and quantum efficiency of photosystem II decreased under drought stress in both genotypes. However, the magnitude of the increase or decrease was significantly higher in vp5 than in Vp5. A total of 7051 proteins with overlapping expression patterns among three replicates in the two genotypes were identified by Multiplex run iTRAQ-based quantitative proteomic and liquid chromatography-tandem mass spectrometry methods, of which the expression of only 150 proteins (130 in Vp5, 27 in vp5) showed changes of at least 1.5-fold under drought stress. Among the 150 proteins, 67 and 60 proteins were up-regulated and down-regulated by drought stress in an ABA-dependent way, respectively. ABA was found to play active roles in regulating signaling pathways related to photosynthesis, oxidative phosphorylation (mainly related to ATP synthesis), and glutathione metabolism (involved in antioxidative reaction) in the maize response to drought stress. Our results provide an extensive dataset of ABA-dependent, drought-regulated proteins in maize plants, which may help to elucidate the underlying mechanisms of ABA-enhanced tolerance to drought stress in maize.

Drought stress in maize causes differential acclimation responses of glutathione and sulfur metabolism in leaves and roots.

BackgroundDrought is the most important environmental stress that limits crop yield in a global warming world. Despite the compelling evidence of an important role of oxidized and reduced sulfur-containing compounds during the response of plants to drought stress (e.g. sulfate for stomata closure or glutathione for scavenging of reactive oxygen species), the assimilatory sulfate reduction pathway is almost not investigated at the molecular or at the whole plant level during drought.ResultsIn the present study, we elucidated the role of assimilatory sulfate reduction in roots and leaves of the staple crop maize after application of drought stress. The time-resolved dynamics of the adaption processes to the stress was analyzed in a physiological relevant situation –when prolonged drought caused significant oxidation stress but root growth should be maintained. The allocation of sulfate was significantly shifted to the roots upon drought and allowed for significant increase of thiols derived from sulfate assimilation in roots. This enabled roots to produce biomass, while leaf growth was stopped. Accumulation of harmful reactive oxygen species caused oxidation of the glutathione pool and decreased glutathione levels in leaves. Surprisingly, flux analysis using [35S]-sulfate demonstrated a significant down-regulation of sulfate assimilation and cysteine synthesis in leaves due to the substantial decrease of serine acetyltransferase activity. The insufficient cysteine supply caused depletion of glutathione pool in spite of significant transcriptional induction of glutathione synthesis limiting GSH1. Furthermore, drought impinges on transcription of membrane-localized sulfate transport systems in leaves and roots, which provides a potential molecular mechanism for the reallocation of sulfur upon prolonged water withdrawal.ConclusionsThe study demonstrated a significant and organ-specific impact of drought upon sulfate assimilation. The sulfur metabolism related alterations at the transcriptional, metabolic and enzyme activity level are consistent with a promotion of root growth to search for water at the expense of leaf growth. The results provide evidence for the importance of antagonistic regulation of sulfur metabolism in leaves and roots to enable successful drought stress response at the whole plant level.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0940-z) contains supplementary material, which is available to authorized users.

Identification of Discriminant Factors after Exposure of Maize and Common Bean Plantlets to Abiotic Stresses

Adverse environmental conditions limit crop yield and better understanding of plant response to stress will assist the development of more tolerant cultivars. Maize and common bean plantlets were evaluated under salinity, high temperature, drought and waterlogged conditions to identify biochemical markers which could be useful for rapid identification of putative stress tolerant plants. The levels of phenolics (free, cell wall-linked, total), aldehydes including malondialdehyde andchlorophylls (a, b, total) were measured on stressed plantlets. Only two indicators were statistically non-significant: chlorophyll b in maize plantlets stressed with sodium chloride and malondialdehyde content in drought stressed maize. The most remarkable effects of abiotic stresses can be summarized as follows: (i) salinity increased levels of free phenolics in maize plantlets and chlorophylls (a, b, total) in common bean; (ii) high temperature (40 °C) elevated levels of chlorophylls (a, b, total) in maize but decreased chlorophylls (a, b, total) and free phenolics in common bean; (iii) drought increased phenolics and decreased chlorophylls (a, b, total) in maize and increased chlorophyll pigments (a, b, total) in common bean; (iv) waterlogging increased free phenolics and decreased chlorophylls (a, b, total) in maize and increased chlorophyll (a, total) incommon bean. Free phenolics and chlorophylls, especially a, were the most responsive indicators to stress and can, therefore, be considered putative biochemical markers for abiotic stress tolerance in maize and common bean. The use of Fisher’s linear discriminant analysis to differentiate non-stressed and stressed plants in breeding programs is also a novel aspect of this report. Fisher’s linear discriminant functions classified correctly 100% of non-stressed or stressed originally grouped plants.

Genome-Wide Analysis of Alternative Splicing during Development and Drought Stress in Maize.

Alternative splicing plays a crucial role in plant development as well as stress responses. Although alternative splicing has been studied during development and in response to stress, the interplay between these two factors remains an open question. To assess the effects of drought stress on developmentally regulated splicing in maize (Zea mays), 94 RNA-seq libraries from ear, tassel, and leaf of the B73 public inbred line were constructed at four developmental stages under both well-watered and drought conditions. This analysis was supplemented with a publicly available series of 53 libraries from developing seed, embryo, and endosperm. More than 48,000 novel isoforms, often with stage- or condition-specific expression, were uncovered, suggesting that developmentally regulated alternative splicing occurs in thousands of genes. Drought induced large developmental splicing changes in leaf and ear but relatively few in tassel. Most developmental stage-specific splicing changes affected by drought were tissue dependent, whereas stage-independent changes frequently overlapped between leaf and ear. A linear relationship was found between gene expression changes in splicing factors and alternative spicing of other genes during development. Collectively, these results demonstrate that alternative splicing is strongly associated with tissue type, developmental stage, and stress condition.

Silicon fertilization improves the maize (Zea maysL.) performance under limited moisture supply

Field crops are subjected to numerous inconsiderate climatic hazards that negatively affect physiological processes, growth and yield. Drought is one of the major abiotic factors that limits the agricultural productivity especially in the arid and semi-arid areas of the globe. Silicon (Si) is a naturally occurring beneficial nutrient which modulates plant growth and development events and has been known to improve the crop tolerance to abiotic stresses. With the objective to investigate the role of silicon nutrition on maize hybrids under limited moisture supply, a two year field study was conducted during 2010–11 at Post Graduate Research Station (PARS), University of Agriculture Faisalabad, Pakistan. We evaluated growth of two maize hybrids P-33H25 and FH-810 under well watered (100% field capacity) and water deficit situation (60% field capacity) as affected by Si application. Silicon was added in soil @ 100 mg/kg using Calcium Silicate as source. Water deficit condition significantly reduced agro-morphological and physiological attributes of maize plants. Silicon application significantly increased the plant height, leaf area index, yield and related attributes along with improvement in photosynthetic rate, leaf water status and osmotic adjustment under limited moisture supply. It was concluded that silicon application to droughtstressed maize enhanced its growth and yield owing to improved photosynthetic rate, higher osmotic adjustment, increased water status and lowered transpiration.

Changes in chloroplast ultrastructure in leaves of drought-stressed maize inbred lines

Chloroplasts are commonly the site of the earliest abiotic injury visible in plant ultrastructure. In this study, six inbred lines of maize (Zea mays L.) were used to analyze changes in the ultrastructure of chloroplasts and related physiological parameters under conditions of drought stress simulated by 20% polyethylene glycol 6000 (−0.6 MPa) for two days. Chloroplasts of three maize lines proved to be more sensitive. They showed changes in the ultrastructure in response to drought, including damage of thylakoid membranes, an increase in the number and size of plastoglobuli, swelling of thylakoid membranes both stromal and granal, disorganization of the thylakoid membrane system, an obvious increase in the intrathylakoid space, and a decrease in the length-to-width ratio and area of chloroplasts. In addition, the contents of malondialdehyde increased markedly in the sensitive lines. Contrary to the sensitive lines, stable structures and shapes of chloroplasts were observed in the drought-resistant lines; it could be considered as an advantage contributing to drought tolerance in the plants. In addition, the drought index of leaf fresh mass (LMDI) in the drought-sensitive lines was ≤ 0.5, which was also associated with a lower content of leaf chlorophyll. In contrast, drought tolerance coincided with lesser growth reduction, and higher LMDI and leaf chlorophyll content.

Metabolic response of maize plants to multi-factorial abiotic stresses.

Clarification of the metabolic mechanisms underlying multi-stress responses in plants will allow further optimisation of crop breeding and cultivation to obtain high yields in an increasingly variable environment. Using NMR metabolomic techniques, we examined the metabolic responses of maize plants grown under different conditions: soil drought, soil salinity, heat and multiple concurrent stresses. A detailed time-course metabolic profile was also performed on maize plants sampled 1, 3 and 7 days after initiation of soil drought and heat stress. The metabolic profile of maize plants subjected to soil drought was more similar to plants exposed to salt stress than to heat-stressed plants. Drought-stressed maize plants subjected to salt or heat stress showed distinct integrated metabolic profiles compared with those exposed to either stressor individually. These differences show the considerable metabolic plasticity of maize in response to different growth conditions. Moreover, glucose, fructose, malate, citrate, proline, alanine, aspartate, asparagine, threonine and one unknown compound fluctuated obviously between maize plants grown in controlled growth cabinet and a natural regime. These changes were associated with the TCA cycle and core nitrogen metabolism, and could be related to their multiple functions during plant growth. The evident stress-induced trajectory of metabolic changes in maize indicated that the primary metabolic responses to soil drought, heat and combined drought and heat stresses occurred in a time-dependent manner. Plasticity at the metabolic level may allow maize plants to acclimatise their metabolic ranges in response to changing environmental conditions.

Investigation of Comparative Regulation on Antioxidant Enzyme System under Copper Treatment and Drought Stress in Maize (<i>Zea mays</i> L.)

The present study was conducted to present the responses of drought-sensitive ‘Shemal’ and drought-tolerant ‘71MAY69’ maize cultivars under drought condition (20% Polyethylene glycol, -0.40 MPa) and three different copper concentrations (0.5 mM, 1 mM, 1.5 mM CuSO4.5H2O) for 5 days to determine the enzymatic responses of copper treatment in maize leaves.Copper treatments alone did not change stomatal conductance, relative water content, malondialdehyde, proline, hydrogen peroxide content and abscisic acid level according to control groups. Combined treatment (drought and copper) alleviated the damage of PEG- induced drought stress in maize leaves. Superoxide dismutase (SOD), catalase (CAT), glutatione reductase (GR) activity increased and glutathione -S transferase (GST) activity decreased, while ascorbate peroxidase (APX) activity didnot change under drought stress in the tolerant cultivar. SOD, CAT and APX were decreased and GST activities were increased while GR did not change in ‘Shemal’. Also SOD, APX and CAT activity increased by copper treatment alone in both cultivars. Otherwise combined treatment increased SOD, APX and CAT activity at all concentrations, but GR and GST activity increased only by (PEG+1.5 mM) treatment when compared with PEG treatment alone in sensitive ones. As a result, exogenous copper alleviated drought stress, while it induced an oxidative damage by increasing antioxidant enzyme activities differently from drought tolerance. Copper tolerance in maize is not a common response of its defense mechanism because of different response to copper and drought in the same cultivar.

Physiological and protein profiling response to drought stress in KS141, a Korean maize inbred line

Understanding the complex response mechanism of a crop to drought is the major step in the developing of tolerant genotypes. In our study, to investigate physiological traits and proteome dynamics, an inbred maize (Zea mays L.) line (KS141) was subjected to 10 days of water-withholding at the V5 or V6 leaf stage. The subsequent analysis of their physiological parameters revealed a decreased relative leaf water content, Fv/Fm, stomatal conductance, net CO2 assimilation rate, leaf transpiration, and water use efficiency, resulting in severe growth retardation of leaf area, stem length and width, aerial part, and root dry matter at 3 and 10 days after withholding water. However, aerial part and root dry matter were little changed during drought stress for 3 days. To understand the proteome dynamics during the 10-day drought stress in maize leaves, comparative proteome analysis was carried out between the well-watered and drought-treated leaves. Proteins were extracted using phenol extraction method from leaves with/without drought stress, and then separated by 2-DE. After 2-DE gel analyses, 14 differentially expressed protein spots were identified by MALDITOF mass spectrometry. Out of 14, eleven and three protein spots were found to be up- or down-regulated, respectively. Interestingly, stress-related proteins such as glutathione S-transferase, abscisic stress-ripening proteins, and pathogenesis-related proteins were increased by drought stress. Our study may provide molecular mechanisms and selective markers for drought tolerant maize genotypes.

Oviposition acceptance and larval development of Chilo partellus stemborers in drought‐stressed wild and cultivated grasses of East Africa

AbstractMaternal host choices during oviposition by herbivorous insects determine the fitness of their offspring and may be influenced by environmental changes that can alter host‐plant quality. This is of particular relevance to ‘push‐pull’ cropping systems where host preferences are exploited to manage insect pest populations. We tested how drought stress in maize and companion plants that are used in these systems affect oviposition preference, larval feeding, and development of the spotted stemborer, Chilo partellus Swinhoe (Lepidoptera: Crambidae). Five host species were tested (all Poaceae): maize (Zea mays L.), Napier grass (Pennisetum purpureum Schumach), signal grass [Brachiaria brizantha (A. Rich) Stapf], Brachiaria cv. ‘Mulato’, and molasses grass [Melinis minutiflora (Beauv.)]. Under drought stress, maize experienced as much oviposition as control unstressed maize in choice and no‐choice experiments. Similarly, larval leaf damage was not significantly different in drought‐stressed vs. unstressed maize. In contrast, oviposition occurred less on drought‐stressed than on unstressed Napier and signal grass. Oviposition acceptance and leaf damage remained low in both drought‐stressed and unstressed molasses grass and Mulato. Larval survival and development remained high in drought‐stressed maize, but not in Napier, signal, and molasses grass and Mulato, where survival and development were low in both drought‐stressed and unstressed plants. Our results indicate that herbivore responses to drought‐stressed plants depend on the plant species and that drought stress can change host preference and acceptance rankings. In particular, trap‐crops such as Napier grass may not divert oviposition from the main maize crop under drought stress conditions.

Protein kinase structure, expression and regulation in maize drought signaling

Plants respond to survive under water deficit conditions via turning on or off a series of genes that further alter plant physiology and morphology, which allows a plant to tolerate, escape or avoid drought stress. These gene products are either functional (such as aquaporins and the enzymes of osmoprotectant biosynthesis) or regulatory (such as protein kinases). In order to investigate the role of maize protein kinases (PKs) in drought signal transduction pathways, genome-wide profiling and analyses of three major families of protein kinases, RLKs, CDPK-SnRKs and MAPKs, were carried out. Our selection pressure analysis found evidence for positive selection in 24 % of the ZmRLK genes, but sites under selection were predominantly located in the extracellular region. Phylogenetic analysis of CDPK-SnRKs revealed considerable conservation in protein sequences across five types—CDPKs, CRKs, PPCKs, PEPRKs and SnRKs. 19 MAPKs, 18 MAPKKs, 84 MAP3Ks and 9 MAP4Ks were identified, and the potential MAPK signaling cascades were proposed. Some PK genes based on transcriptome analysis including microarray, RNA-sequencing and real-time quantitative PCR (qRT-PCR) showed drought-induced expression. Protein structure, subcellular location and gene expression of putative ABA receptors were analyzed to provide novel molecular insights into maize drought signaling. The drought stress signal transduction pathway was first evaluated and characterized in this study. To understand microRNA (miRNA) involvement in maize drought stress response and resistance, signaling pathway-associated miRNAs and potential miRNA-target pairs were predicted. Furthermore, real-time quantification of miRNAs by stem-loop qRT-PCR demonstrated that nine miRNAs might function as positive or negative regulators in drought stress signal transduction, and these miRNAs can be used as biomarker and regulatory target in genetic analysis and improvement of drought resistance in maize. These efforts can broaden the understanding of the physiological and molecular mechanisms of resistance to drought in maize and facilitate the creation of cultivars with increased drought tolerance.

Alleviation of drought stress in maize by exogenous application of gibberellic acid and cytokinin

The study was conducted to investigate the ameliorative roles of GA3 and CK on adverse effects of drought in maize. Drought stressed maize plants were applied with GA3 and CK at 50, 100, and 150 mg L−l as foliar spray at the vegetative and the reproductive stages. Plant height, internode length, stem diameter, leaf chlorophyll index, and dry matter production were significantly affected by drought. In most cases, GA3 and CK significantly improved the depressed plant traits, but in varying degrees depending on the growth stage encountering hormones, and their types and concentrations. Both GA3 and CK were found to be very effective in alleviating drought-imposed adverse effects on maize at the vegetative phase. Such alleviating effects varied depending on the concentration of the hormones. Application of CK at 150 mg L−l was excellent resulting in a 106% yield advantage compared to drought stress and 79.9% increase relative to well-watered controls. Conversely, GA3 at 50 mg L−l performed well showing 78.8% increase in grain yield. However, both GA3 and CK had very little effect on improving the depressed growth and yield attributes in maize at the reproductive phase. The relative yield advantages for the hormones were mainly attributed to improving the cob and seed-bearing capacity of drought-stressed maize plants.

Differential Expression of MicroRNAs in Response to Drought Stress in Maize

Drought is one of the major abiotic stresses that limit maize productivity. Apart from the principal transcriptional regulation, post-transcriptional regulation mediated by microRNAs appears to be the prevalent response of plants to abiotic stress. In this study, the differential expression of microRNAs in the previously evaluated drought-tolerant inbred lines R09 under drought stress was detected by microarray hybridization. The target genes of the differentially-expressed microRNAs were predicted by bioinformatics software WMD3 for plant target gene prediction. The possible regulation of the differentially-expressed microRNAs as well as their target genes in maize response to drought stress was analysed according to Gene Ontology. Sixty-eight microRNAs in 29 microRNA families were detected to be differentially expressed in the seedling of the drought-tolerant inbred line R09, accounting for 5.97% of the total number of the probes. The expression profiles were different between the two time points of the drought stress. The functions of the genes targeted by the differentially-expressed microRNAs involve multiple physiological and biochemical pathways of response to abiotic stress, such as transcription regulation, metabolism, signal transduction, hormone stimulation, and transmembrane transport. Under drought stress, the differential expression of microRNAs regulates the expression of their target genes, resulting in multiple responses of physiological and biochemical pathways relative to drought tolerance of maize. miR156, miR159 and miR319 families may play more important roles. The different members of the same family may play similar regulation effects in most cases.

More beneath the surface? Root versus shoot antifungal plant defenses

Let's get dirty! Plant roots are deeply embedded in clammy soil, and working with them means digging into the mud, hoping not to crush the fragile root branches and flimsy root hairs. In short, working with roots can be a tedious activity. This might partly explain why, over decades, roots could maintain a subsistence fairly hidden from the scrutinizing eyes of plant researchers—until the past few years. Plant breeding has been mainly driven by the search for aboveground traits; as a consequence, roots are now even claimed to be a potential key for a second green revolution (Gewin, 2010). Promising studies aiming to improve crop yields focus on root architecture and the genetics underlying root development. For instance, wheat lines exhibiting deeper and fast-growing roots were isolated and crossed with commercial cultivars, in the hope of generating drought-tolerant lines (Kirkegaard et al., 2007). Similarly, drought-stressed maize lines were shown to have a higher yield when specifically adapting their root structure by building an intercellular air space compared to maize varieties that were not able to adapt their root architecture (Zhu et al., 2010). As if handling abiotic stress would not be already a substantial task, roots are also garrisoned at the frontline against pathogens and pests. Soil-borne pathogens such as Fusarium, Pythium, or Phytophthora are infamous for being vicious parasites that can devastate entire harvests (Okubara and Paulitz, 2005). However, the lack of a convenient accessibility of root systems renders the task of controlling such pathogens more than challenging. Consequently, the knowledge about belowground defense responses is limited, in contrast to well-dissected aboveground mechanisms (Rasmann and Agrawal, 2008). Moreover, studies comparing root and shoot immune responses are scarce. In animal model systems, organ-specific antifungal immune responses are a well-known phenomenon (Lionakis et al., 2011). In plant-microbe models, pathological studies were historically mainly focused on either roots or shoots. Recent studies highlight an organ-specific immunity against fungal pathogens, pointing toward the fact that plants employ a “defense in depth,” a multi-layered organ-specific local and systemic defense system. Elucidating such a defense in depth for given pathosystems will be an important task for future studies in plant pathology.

Modulation Role of Abscisic Acid (ABA) on Growth, Water Relations and Glycinebetaine Metabolism in Two Maize (Zea mays L.) Cultivars under Drought Stress

The role of plant hormone abscisic acid (ABA) in plants under drought stress (DS) is crucial in modulating physiological responses that eventually lead to adaptation to an unfavorable environment; however, the role of this hormone in modulation of glycinebetaine (GB) metabolism in maize particularly at the seedling stage is still poorly understood. Some hydroponic experiments were conducted to investigate the modulation role of ABA on plant growth, water relations and GB metabolism in the leaves of two maize cultivars, Zhengdan 958 (ZD958; drought tolerant), and Jundan 20 (JD20; drought sensitive), subjected to integrated root-zone drought stress (IR-DS) simulated by the addition of polyethylene glycol (PEG, 12% w/v, MW 6000). The IR-DS substantially resulted in increased betaine aldehyde dehydrogenase (BADH) activity and choline content which act as the key enzyme and initial substrate, respectively, in GB biosynthesis. Drought stress also induced accumulation of GB, whereas it caused reduction in leaf relative water content (RWC) and dry matter (DM) in both cultivars. The contents of ABA and GB increased in drought-stressed maize seedlings, but ABA accumulated prior to GB accumulation under the drought treatment. These responses were more predominant in ZD958 than those in JD20. Addition of exogenous ABA and fluridone (Flu) (ABA synthesis inhibitor) applied separately increased and decreased BADH activity, respectively. Abscisic acid application enhanced GB accumulation, leaf RWC and shoot DM production in both cultivars. However, of both maize cultivars, the drought sensitive maize cultivar (JD20) performed relatively better than the other maize cultivar ZD958 under both ABA and Flu application in view of all parameters appraised. It is, therefore, concluded that increase in both BADH activity and choline content possibly resulted in enhancement of GB accumulation under DS. The endogenous ABA was probably involved in the regulation of GB metabolism by regulating BADH activity, and resulting in modulation of water relations and plant growth under drought, especially in the drought sensitive maize cultivar JD20.

Root Morphology and Gene Expression Analysis in Response to Drought Stress in Maize (Zea mays)

Water-deficit stress tolerance is a complex trait, and water deficit results in various physiological and chemical changes in maize (Zea mays L.) and exacerbates pre-harvest aflatoxin contamination. The objective of this study was to characterize the variations in morphology, physiology, and gene expression in two contrasting inbred lines, Lo964 and Lo1016, in order to understand the differences in response to water-deficit stress. The results revealed that Lo964 was less sensitive to water-deficit stress, and had a strong lateral root system and a higher root/shoot ratio in comparison to Lo1016. In response to water-deficit stress by comparing stressed versus well-watered conditions, abscisic acid syntheses were increased in leaves, roots, and kernels of both Lo964 and Lo1016, but by different magnitudes. Indole-3-acetic acid (IAA) was undetectable in the leaves and roots of either genotype regardless of treatments, but increases of 58% and 8% in IAA concentration were observed in 20 DAP kernels, in response to water-deficit stress, respectively. The expression of the MIPS was up-regulated 7-fold in leaf tissues of Lo964 compared to Lo1016 at watered conditions, but decreased significantly to similar levels in both genotypes at water-deficit conditions. ZmPR10 and ZmFer1 expressions tended to up-regulate although ZmPR10 was expressed higher in root tissue while ZmFer1 was expressed higher in leaf tissue. Further study is needed to confirm if Lo964 has reduced aflatoxin contamination associated with the drought tolerance in the field in order to utilize the resistant trait in breeding.

From epoxycarotenoids to ABA: The role of ABA 8′-hydroxylases in drought-stressed maize roots

The ability of plants to withstand drought, a potentially major constraint to yield and production, is influenced by abscisic acid (ABA). ABA is synthesized in the cytosol from plastid carotenoid pathway derived precursors, and later inactivated by the action of ABA hydroxylases. Endogenous accumulation of ABA is controlled by both its synthesis and catabolism. Enzymatic activity of ABA 8′-hydroxylase (ABA8Ox), also referred to as CYP707A, is considered one of the key steps in modulating ABA levels that control numerous physiological processes. To investigate the role of this enzyme, maize ABA8Ox gene family members were identified. ABA8Ox gene expression was then analyzed in different tissues and roots during the drought-stress response in maize. These genes were found to be expressed in all tissues, with a high degree of specificity to each tissue and some degree of overlap. Maize ABA8Ox1a and ABA8Ox1b were shown to be the major transcript components for regulating ABA catabolism in drought-stressed roots. Phylogenetic and gene-structure analyses were performed to extend the implications and infer the cause of ABA catabolism in other cereal crops.

Differential expression of candidate genes for lignin biosynthesis under drought stress in maize leaves

In order to provide information for the development of molecular selection markers for drought tolerance improvement, the methods of prometric analysis, quantitative real-time PCR and field evaluation were employed for the identification of the differential expression of candidate genes under drought stress in maize. At seventeen, twenty-four and forty-eight hours of polyethylene glycol-simulated drought stress at the seventh leaf stage, leaf samples were collected from two drought-tolerant inbred lines for prometric analysis by two-dimensional electrophoresis and peptide mass fingerprinting. Fifty-eight proteins out of more than 500 were found in response to drought stress. Three drought-induced spots 2506, 3507 and 4506 showed sequence similarity with cinnamyl alcohol dehydrogenase, cytochrome protein 96A8 and S-adenosyl-L-methionine synthase, respectively. The expression of two key enzymes to lignin biosynthesis was quantified by quantitative real-time PCR among three drought-tolerant and one drought-sensitive inbred lines under drought stress and well-watered control conditions. After a decrease at the beginning of drought stress, the expression of cinnamyl alcohol dehydrogenase and caffeate O-methyltransferase recovered at twenty-four hours of the drought stress in the three drought-tolerant lines, but not in the drought-sensitive lines. Leaf lignin content, anthesis-silking interval and grain weight per plant were investigated with six inbred lines of varying drought tolerance under drought stress and well-watered control. Drought tolerance coefficients of these three characters were calculated and the correlation coefficients among these drought tolerance coefficients were estimated. Significant difference in leaf lignin content was found among the inbred lines and in response to drought stress. Close correlations were observed between the drought tolerant coefficients for leaf lignin content and grain weight per plant, and between the drought tolerant coefficients for leaf lignin content and anthesis-silking interval. These results indicate that leaf lignin content is a useful index for evaluation of drought tolerance in maize. Molecular selection markers can be developed on the basis of differential expression of the candidate genes and applied to maize improvement for drought tolerance.

Functional repair of embolized vessels in maize roots after temporal drought stress, as demonstrated by magnetic resonance imaging

Xylem sap under high tension is in a metastable state and tends to cavitate, frequently leading to an interruption of the continuous water columns. Mechanisms of cavitation repair are controversially discussed. Magnetic resonance (MR) imaging provides a noninvasive, high spatial and temporal resolution approach to monitor xylem cavitation, refilling, and functionality. Spin density maps of drought-stressed maize taproots were recorded to localize cavitation events and to visualize the refilling processes; c. 2 h after release of the nutrient solution from the homemade MR imaging cuvette that received the root, late metaxylem vessels started to cavitate randomly as identified by a loss of signal intensity. After c. 6 h plants were rewatered, leading to a repair of water columns in five out of eight roots. Sap ascent during refilling, monitored with multislice MR imaging sequences, varied between 0.5 mm min(-1) and 3.3 mm min(-1). Flow imaging of apparently refilled vessels was performed to test for functional repair. Occasionally, a collapse of xylem vessels under tension was observed; this collapse was reversible upon rewatering. Refilling was an all-or-none process only observed under low-light conditions. Absence of flow in some of the apparently refilled vessels indicates that functionality was not restored in these particular vessels, despite a recovery of the spin density signal.

Isolation and Analysis of Drought-Induced Genes in Maize Roots

Maize roots are important component for plant adaptation to soil water deficits because they are supposed to take up water and necessary solutes from the soil. In the present study, the drought-induced genes were isolated in maize roots. A suppression subtractive hybridization protocol was applied to construct a forward subtractive cDNA library from CN165 for drought-stressed maize roots and a number of drought-induced genes were isolated. Totally, 126 uniESTs (containing 82 singlets and 44 contigs) were obtained from 503 available ESTs sequences after macroarray hybridization. UniESTs were analyzed using BLASTN and BLASTX and the results showed that 92% of the uniESTs had homolgous sequences in maize nr database by BLASTN. About 89% of uniESTs appeared the homlogous amino acid sequences in rice protein database but not in maize protein database by BLASTX, implying that those genes are likely new functional genes in maize. Function analysis showed that those genes were involved in a broad spectrum of biological pathways, mainly in signaling and regulatory pathways related to stress tolerance.