Group of Functional Biomolecular Discovery. Group of Plant Stress Physiology. Group of Plant Molecular Physiology.
Ecophysiology, stress and adaptation
Group of Plant-Microbe Interactions. Group of Plant-Insect Interactions.
Group of Plant Environmental Microbiology. Plant Light Acclimation Research Group We investigate photosynthesis and chloroplast development, the fundamental to photoautotrohic growth of plants.
Fluctuation of sun light constantly damages photosynthetic apparatuses located in the chloroplasts, and the excess light energy absorbed by them ultimately leads to growth defect. Our research aims in understanding the acclimation to such light stress, by elucidating photoprotection mechanisms. We also investigate chloroplast differentiation from its precursor proplastid, because it requires the proper assembly and quality control of the photosynthetic apparatuses.
Using various experimental methods, we explore these mechanisms at molecular level, which will be useful for modification of photosynthetic activity and genetic engineering. In order to survive, plants evolved a large suite of adaptions to face sudden changes in external environment. These experiments thus show that drought stress perception and signaling occur at an early stage of mDr treatment, and they enable a molecular genetic and physiological dissection of subsequent responses to the stress.
Under mDr treatment, plants showed an early response with a drastic decrease in LRWC and stomatal conductance, but photosynthesis rate remained normal. Moreover, the expression pattern of stomatally related genes showed a peak at an early stage of drought stress. Under stress, the outward channels are induced and inward channels are repressed. In agreement with this, the outward channel gene GORK showed the highest expression at day 1 of drought stress treatment. Another gene, RPK1 , was found to be induced at an early stage of drought stress. This is consistent with its role in mediating an early response in ABA signaling and the regulation of guard cells under stress, with a function in the improvement of abiotic stress tolerance Osakabe et al.
MYB60 has an important role in the regulation of guard cells, with the knockout mutation resulting in stomatal closure Cominelli et al. Under mDr time-course experiments, MYB60 expression is lowest at day 1, corresponding with the drastic decrease in stomatal conductance under drought treatment. Photosynthesis rate determined by instantaneous gas-exchange measurements was not affected by the mDr stress treatment, which was supported by the expression profile of photosynthesis-related genes. Starch accumulation was also normal during mDr treatment data no shown.
In agreement with these observations, previous studies found that photosynthesis usually is not affected by mild drought and mDr Cornic and Massacci, ; Flexas and Medrano, Moreover, analysis of the publicly available expression profiling data under drought and salt stress showed a nonsignificant effect of mild drought on the expression of photosynthetic genes both qualitatively and quantitatively Chaves et al. ROS are produced in different compartments of the plant cell, both under normal and stressful conditions Grene, When plants are challenged by drought or other abiotic stresses, ROS are generated as a result of the inhibition of photosynthesis and the predominance of photorespiration Noctor et al.
ROS are found to have a dual function in plants: they are needed as signaling molecules, but a high concentration is detrimental Kwak et al. High ROS concentration is hence a stress symptom, and plants have to maintain the ROS within a certain level that is required for normal cellular homeostasis. The induction of members of the antioxidant system is highly correlated with the severity of the stress. Under severe abiotic stresses such as high light, low temperature, high temperature, salt stress, severe drought, and a combination of stresses, antioxidant enzymes are differentially and highly induced Kliebenstein et al.
This is consistent with the normal photosynthesis rate under mDr, proven both at the physiological and molecular levels. However, GPX6 and TPX1 are slightly, yet significantly, up-regulated approximately 2-fold at the very early stage of drought stress, and these peroxidases might be involved in reducing very early ROS responses, partly from stress signaling Kwak et al.
In conclusion, the mDr level is below the threshold required for the generation of a highly destructive concentration of ROS. Therefore, there was a net normal level of antioxidant enzymes except for the dismutases, which were repressed. The repression of the dismutases can be explained by the low concentration of superoxide and efficient photosynthesis, due to the incomplete closure of the stomata under stress conditions Cruz de Carvalho, In addition, studies on many crops showed a discrepancy regarding the expression of the antioxidant enzymes in response to drought.
In some cases they were induced, but in other cases they were repressed Cruz de Carvalho, , suggesting that different ROS balance and levels are required at different responses. One of the first acclimation responses to drought is the decrease in leaf growth, which results in the maintenance of cell turgor and reduces the transpiration area Mathews et al. In addition to cell turgor, cell wall biochemical and biophysical characteristics play an important role in cell growth Mathews et al.
In Arabidopsis, leaf size is a result of both cell division and cell expansion Horiguchi et al. Cell expansion is a process of cell wall modification and loosening catalyzed by enzymatic and nonenzymatic protein components of the cell wall Cosgrove, , which is composed of cellulose and hemicelluloses in a matrix of pectins and proteins Cosgrove, Expansins are the key cell wall-loosening proteins, which act by the breakage of hydrogen noncovalent bonds between cellulose and the surrounding matrix, leading to slippage of the cell wall components under acidic pH acid growth and, consequently, the increase in extensibility of the cell wall McQueen-Mason et al.
Physical properties of the cell wall play a crucial role in the response of plants to water deficit Bacon, Transcriptome analysis of pDr showed the repression of many expansin genes Bray, , while mild osmotic stress revealed the induction of expansin genes Skirycz et al. Cell expansion in response to drought was characterized in the maize Zea mays root system as an adaptation to low water potential Wu and Cosgrove, In addition, there are many studies on leaf growth under water deficit in maize leaves and other crop plants such as sunflower Helianthus annuus as well as in Arabidopsis Mathews et al.
Despite the plethora of studies on cell expansion in response to drought, very little is known about the molecular basis of this process in plant responses to internal and external stimuli. In this research, microarray and qRT-PCR analyses revealed the up-regulation of cell expansion genes under mDr treatment.
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In contrast, under pDr prewilting and pDr wilting drought treatments, most of the expansin genes were down-regulated. The expression profiles of four expansin genes in the time-course analysis of mDr showed a pattern of repression at the beginning of drought day 0 , induction at day 1, and repression thereafter. A fifth expansin gene, EXLB1 , has a different expression pattern, with a peak in expression at day 1 and a decrease starting at day 2. This early peak in expansin expression can be interpreted as an acclimation to mDr by cell wall adjustment.
A study of the resurrection plant Craterostigma plantagineum showed an increase in expansin expression and activity at an early stage of dehydration, resulting in a flexible cell wall as an adaptation to dehydration Jones and McQueen-Mason, Consistently in our study, there was no significant RB at an early stage of mDr compared with a later stage day 5 and afterward , supported in part by the slight increase in expansin expression as an adaptation to stress that occurs as an early response.paradisehydrographix.com/1032-best-cell-tracking.php
:: Stress Physiology and Molecular Biology Laboratory ::
Differential spatial expression of expansins was shown in maize leaves, tomato Solanum lycopersicum shoot apex, and tomato embryos Chen et al. Therefore, the spatial and temporal patterns of expansin expression and activity need to be studied in response to drought. The plant cell wall is required not only for mechanical support but for growth and adaptation to hostile environments.
There is still a lot to be learned about cell wall modification under different abiotic stresses at the molecular, cellular, tissue, and whole-plant levels. Studies on the effects of overexpression of expansin genes show enhanced growth in rice and high sensitivity to hormones and salt stress in Arabidopsis Choi et al.
These suggest the important role of expansins in acclimation and adaptive responses of plants to abiotic stresses. Gas-exchange measurements at the late stage of mDr Day10 showed no significant differences in stomatal conductance and photosynthesis of drought-treated plants compared with the well-watered control data not shown. Moreover, no stomatally related genes were differentially expressed at this stage. In contrast, microarray and qRT-PCR analyses of the early stage of mDr Day01 showed that many stomatally related genes were either up-regulated or down-regulated.
Hence, the early stage Day01 showed reduced stomatal conductance, whereas normal stomatal conductance and reduced growth were shown at late stage of mDr Day JA biosynthesis and signaling were among the main enriched GO categories in the down-regulated genes at the late stage of mDr. Jasmonates have been found to have a potential role in response to drought stress in soybean Glycine max , as they showed an early increase within 2 h of dehydration and a decrease in concentration afterward Creelman and Mullet, Moreover, jasmonates were found to cause stomatal closure Raghavendra and Reddy, This role was confirmed by the impaired stomatal response to exogenous jasmonates in jasmonate-insensitive mutants jar1 and coi1 ; Suhita et al.
There is cross talk between jasmonates and ABA as they utilize a similar cascade of events to stimulate stomatal closure Suhita et al. Under our mDr treatment, the coi1 and jin1 mutants were found to be significantly resistant or insensitive to drought stress compared with the wild type, with biomass accumulation under drought not different from the well-watered control.
Plant abiotic stress
This suggests that the reduced growth as a response to drought stress, as a developmental program for acclimation, is not switched on in the absence of JA signal perception and response. This is supported by studies that show that the JA-mediated inhibition of seedling and root growth is suppressed in the coi1 mutant Xie et al. In experiments on stomatal closure, a characteristic drought response, jasmonates induce closure that is suppressed in the coi1 mutant, which retains normal ABA responsiveness Munemasa et al.
Since JA was also shown to repress photosynthesis genes Reinbothe et al. At the early stage of mDr Day01 , plants accumulate high ABA concentration with induction of ABA biosynthesis and signaling genes but with no significant differential expression of JA pathway genes. However, JA signaling and biosynthesis genes are significantly down-regulated. We propose that at the early stage of mDr, endogenous JA in combination with high ABA level is enough to stimulate the preparatory response needed for drought acclimation e.
JA is probably not required at high concentration under drought stress, and an increase in its concentration might negatively affect plant response in growth. Indeed, interaction between cellulose synthesis and a high concentration of JA revealed a negative effect of JA on cell wall modification and plant growth, which enhanced plant resistance to fungal pathogens Ellis et al.
Biochemical, Physiological and Molecular Avenues for Combating Abiotic Stress Tolerance in Plants
Moreover, JA induction in response to wounding and herbivory freezes the cell cycle, inhibits cell expansion, and results in stunted growth Zhang and Turner, ; Onkokesung et al. To minimize the inhibitory effect of JA on plant growth under prolonged drought late mDr , the down-regulation of JA biosynthesis and signaling pathways can act in establishing new homeostasis in the acclimation process. The response to mDr, extended over a period of time, can be distinguished into multiple stages, from early to intermediate to late Fig.
During the early priming or preconditioning stage, stress perception, signaling, and reprogramming of gene expression take place. Many of these immediate responses, such as ABA response genes and ROS scavengers, probably involved in signaling responses, are also observed in pDr.
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The drought response pathways can be traced by the expression pattern of individual genes of known function.
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