| Drought is one of the major environment stresses during plant growth and development,and the disaster due to drought becomes more and more severe and frequent. Wheat (Triticumaesitivum L.) is one of the important food crop world widely. Water deficit is one of thecommon stresses during wheat growth, and the lost of wheat yield by water deficit is verylarge. Therefore, it is very important and significant to study the mechanism of droughttolerance in wheat and to breed the new drought tolerant wheat cultivars.Plant senescence is an important period of plant development, which is a programmedprocess that is subjected to gene regulation. The structure, function, metabolism and geneexpression of the plant cell go through a series of coordinated changes during plantsenescence, two of the distinct changes are the degradation of chlorophyll and the decline ofphotosynthesis. The grain-filling of wheat is frequently deteriorated by drought and heat stresscombination at the late stage. Senescence results in deterioration of the quality of vegetables,poor grain quality and reduced crop yield. Delay leaf senescence is a useful way to increasephotosynthetic time, which is beneficial to increase wheat yield.The wheat stay-green mutant, tasg1, was previously generated by applying the ethylmethane sulphonate (EMS) mutagen to HS2, a common wheat cultivar as the WT. In thisstudy, all the experiments were conducted in2sets of experriments: one, in the field and theother, in the laboratory. We used WT and tasg1, as materials to study the changes of plantphysiology and its stay green mechanism under water stress condition. The results willprovide a theoretical basis to wheat breeding in drought resistance and senescence delay. Themain results are as follows:(1) tasg1represent a functional stay green mutant1) Under normal field conditions, no significant difference between tasg1and WT wasobserved in plant development and phenotype before the flag leaves appeared. The stay-greenphenotype of tasg1was expressed at the beginning of anthesis and was especially apparentwith late natural senescence. Drought stress accelerated the plant senescence in both wheatvarieties, but it was delayed in tasg1compared to the WT. Compared with WT, tasg1showsan obvious stay green phenotype at later stage of nature senescence.2) No obvious difference was found in chlorophyll (Chl) a content between tasg1and WT at the initial phase of senescence. However, from the22th to the30th day after anthesis(DAA), Chl a content in tasg1was always higher than in WT, especially under drought stress(DS). Similar differences were observed in Chl b content. Net photosynthetic rate (Pn) washigher in tasg1, compared to WT, at30DAA, which was consistent with the differences inChl content. Somewhat similar differences were observed in transpiratory rate (E) andstomatal conductance (Gs), but intercellular CO2concentration (Ci) was significantly lower intasg1than WT. This may be related to the higher photosynthetic activity in tasg1.3) The yield of tasg1was9.5and7.0%higher than WT under controls (CK) and droughtstress (DS) conditions, respectively, but these differences were not significant. The greaternumber of kernels per wheat spike was the major factor contributing to the higher yield oftasg1, although the number of wheat spikes in each plot in tasg1also contributed to theincreased yield. The size and mass of each kernel was lower in tasg1than WT. From theobservations above, tasg1represents a functional stay green mutant.(2) The characteristics of the chloroplasts and thylakoid in tasg1and WT1) In normal water conditions, chloroplasts were arranged regularly along the cell wall,but their shape was slightly different in the two genotypes. Chloroplasts were approximatelyroundish in WT, but prolonged in tasg1. After drought stress, some damage to the chloroplastenvelope was found in WT, accompanied with the shift of the organelles from the cell wall tothe center of the cell. Compared to WT chloroplasts, tasg1chloroplasts showed less damageinduced by drought stress. Under normal water conditions, the thylakoid lamellae wereclosely arranged and assembled to form the grana in the WT. Lamellae were more closelyarranged in tasg1. Drought stress resulted in swollen and loosely scattered thylakoid lamellaein WT, but these changes were not obvious in tasg1. We also observed fusion of several granastacks in tasg1under drought stress.2) As the date of drought stress continued, the activity of Hill reaction, Ca2+-ATPase andMg2+-ATPase were significantly higher in tasg1than WT. The activity of thylakoid membraneproteins, including PSII and ATPase, were better maintained in tasg1than in the WT underdrought stress.3) In flag leaves under field conditions, the level of the28kDa polypeptide in tasg1washigher than that in WT under both normal and drought stress conditions on both days in thedrought of10and25d. The polypeptides were similarly detected in WT and tasg1wheatseedlings at the second-leaf stage in the laboratory, and the three observed polypeptides ofabout28,38and50kDa in tasg1were consistently higher than those in WT. This differencesuggested that some polypeptides were degraded during leaf development and senescence. Compared with WT, tasg1could maintain the thylakoid membrane protein complexes withbetter stability against damage by drought stress.4) Expression levels of genes involved in light-harvesting complex I (LHCI), namelyTaLhca1, TaLhca2and TaLhca3, were down-regulated gradually during drought stress in bothWT and tasg1. The expression levels of TaLhcb4and TaLhcb6were higher in tasg1comparedto WT, especially at the last tested time point. From these results, tasg1could increasestability of chloroplast membranes and chlorophyll-protein complexes.(3) The antioxidant activity and ascorbate-glutathione cycle in tasg1and WT duringgain-filling stage1) Superoxide radical (O2ˉ) production rate, hydrogen peroxide (H2O2) accumulation,malondialdehyde (MDA) content, relative electrical conductivity and carbonylation proteincontent in flag leaves increased significantly after anthesis in both tasg1and WT.Nevertheless, tasg1maintained lower MDA content, O2ˉproduction rate and H2O2content inflag leaves than WT. From the results above, we suggest that high antioxidative systemcompetence may be involved in the stay green characteristic of tasg1.2) The activities of several antioxidant enzymes, including superoxide dismutase (SOD),peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) were mostly suppressedby drought stress, but their activities were always higher in tasg1than that in WT. Comparedwith WT, tasg1had higher reduced ascorbate/oxidized ascorbate ratio, reducedglutathione/oxidized glutathione ratio and antioxidant enzyme activities during senescenceunder both normal water and drought stress conditions. These results suggest that thecompetent antioxidative capacity may play an important role in the enhanced droughttolerance in tasg1.From the results above, we suggest that tasg1maintain higher chlorophyll content andphotosynthetic rate than WT at the gain-filling stage, indicating an obviously delaysenescence in tasg1. Compare to WT, the stay-green wheat mutant tasg1could stablymaintain chloroplast and thylakoid ultrastructure better and maintain thylakoid membranepolypeptides at high levels, while its expression of some LHCII related genes remained steadyunder drought stress. The activities of several antioxidant enzymes, including SOD, POD,CAT and APX were mostly suppressed by drought stress, but their activities were alwayshigher in tasg1than in WT. Thus, enhanced stability of thylakoid membrane proteins andantioxidant competence contribute to drought resistance in the tasg1. These data were helpfulto better understand of the stay-green mechanism and to improve the drought resistance ofwheat cultivars. |
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