Study Of Drought Stress On Spikelet Fertility And Panicle Exsertion In Rice

Rice is the staple food for most Asian and Chinese people, but rice production consumes large amounts of water. Presently, drought has been the single greatest factor limiting rice production. It has been reported that drought can contribute to the 50% loss of rice yield each year. The extent depends largely on the development stages at which the plant encounter stresses. Among these stages, reproductive development stages, especially anthesis, is most sensitive to various stresses, such as drought. It can cause 50-70% loss of yield. Drought in this period causes a variety of abnormalities in plant morphology which interfere with pollination or fertilization, and induce abscission of flowers or abortion of newly formed grains. Unraveling the mechanism of plant tolerance to environmental stresses and the physiological and molecular basis of plant responses to drought, especially at the most sensitive stage might provide new strategies to improve the stress tolerance of rice.To understand responses to drought in rice, this paper studied the relationship between spikelet fertility and peduncle elongation on physiological, biochemical and molecular levels at the different reproductive stage under drought. In this study, we conducted a series of experiments to investigate the response to drought by 1) screening the donors to drought in the field at the flowering stage; 2) high-through analysis of phenotypes to drought at the different reproductive stages; 3) transcriptome of peduncle division zone using microarray; 4) gene expression in peduncle zone division using real time RT-PCR.Yield and yield components was analyzed by different genotypes and different reproductive stages, separately. The results indicated that yield in drought condition was only 21% of control in 2006 dry season and the data in 2007 dry season field and pot experiment showed that FL is the most sensitive stage to drought at the different reproductive stages, followed by GF and PM. Among the yield components, the reduction of spikelet fertility was positively with that of yield(r=0.85). Spikelet sterility included two parts:1) directly effects by drought; 2) panicle exsertion.Peduncle elongation played a major in panicle exsertion. The data showed that peduncle elongation started before 5 days of heading. The highest PER was at heading. There was a significant correlation between PER and yield components. There was a highly correlation between PER and plant water status in the field. To understand the impacts of peduncle to spikelet fertility under drought, we tested three criteria using a subset of cultivars in the field and two contrasting in phytotron. It indicated that there was a highly correlation between PER and spikelet fertility (r=0.65, P<0.01) in the field; the contribution of peduncle to spikelet sterility was 50% in IR64 (susceptible) and 26% in Apo (tolerant); there was a high correlation between peduncle elongation rate and FTSW (r=0.84, P＜0.001).The dynamic changes of ABA among source-sink relationship showed that there was a major difference between tolerance and susceptible variety. The leaf ABA was increased firstly and then decreased along the development, but the tolerance was opposite. The opposite change of ABA/GA in peduncle may be the existence of antagonism. Also, drought caused the soluble sucrose increased in flag leaf and decreased in peduncle. The leaf starch was decreased firstly and then increased along the development, but the tolerance was opposite. Combined to the change of leaf ABA, it indicated that this may be the difference between genotypes.There was a highly correlation between flag leaf ABA and soluble and starch content (0.81, P＜0.01 and 0.56, P＜0.05).Anther dehiscence was another factor that drought caused directly the reduction of spikelet fertility. In this study, we investigated the anther dehiscence under drought. The results showed that drought decreased the pollen number on stigma and pollen fertility at the PM and FL significantly. Also pollen tube elongation was blocked by drought. Previous study said there was difference in anther wall structure between contrasting genotypes, but here not got the same results. Among sterile spikelets, there was at the different developmental stages under drought. We observed the distribution of the sterile spikelets to drought at the different reproductive stages. It indicated at GF, most of sterile spikelets were at the abortion and half filled stages. Moreover, there was significant difference at the two stages between two genotypes or treatments. At FL and PM, the most were at the anther exerted but not dehiscence and anther dehiscence but not fertilization stages. Others were similar to control.Among yield components,1000-grain weight (TGW) is thought to be quite constant due to a rigid hull whose size is genetically determined. An automated methodology is presented to analyze grain weight, length and width distributions for grain samples. Frequency distribution analyses on the basis of histograms generally gave bimodal patterns for grain weight (filled and unfilled grains) and monomodal patterns for grain dimensions. These histograms permit the distinction of unfilled, partially filled and fully filled grains. Peak shape and location on the histogram provide further information potentially useful for the diagnosis of physiological stresses affecting grain hull development, spikelet fertility and filling, and may be of value in breeding and grain quality research. The methodology was applied to rice grain samples taken from pot experiment and field experiment at the different reproductive stages. Different reproductive stages were associated with an increased fraction of unfilled spikelets and reduced grain dimensions and weight.Microarrays were used to examine gene expression at 2 days before heading in rice peduncle division zone in response to drought, ABA, and 1 day after re-watering treatments. The results showed that the sensitivity were different in different treatments. Under drought,204 transcripts were up-regulated and 183 transcripts were down-regulated; under ABA treatment,326 transcripts were up-regulated and 396 transcripts were down-regulated. These genes mainly involved in DREB, LEA, cell growth, carbohydrate metabolism, energy metabolism and amino acid metabolism. Among the transcripts of co-regulated under two treatments, BREB and cell growth were affected. After re-watering, most of transcripts were back, but there were also some significant different genes. These transcripts may be the genes response to drought and ABA treatments and be associated with peduncle elongation.The glucan endotransglucosylase could loose the cell wall to elongate cell and also drought and ABA affected their transcript. We used real time RT-PCR to investigate the gene expression of XTHs gene family in peduncle elongation zone. The results showed that the expression profiles varied with within XTH genes. The changes of mRNA levels of OsXTH2 and OsXTH5 were up-regulated and 11 OsXTH genes were down-regulated by drought. In particular, OsXTH10, OsXTH16, OsXTH19, OsXTH22 and OsXTH24 were most sensitive to drought. They were reduced by 5-10times even under the FTSW=0.5. Three OsXTH genes (OsXTH3, OsXTH7, OsXTH26,) were not expressed in peduncle or the mRNA levels of 3 OsXTH genes (OsXTH1, OsXTH6, OsXTH18) were not affected by drought. The changes of mRNA levels of 11 OsXTH genes (OsXTH2, OsXTH5, OsXTH8, OsXTH9, OsXTH11, OsXTH17, OsXTH19, OsXTH20, OsXTH27, OsXTH28 and OsXTH29) were the same positively or negatively to the changes of PER and stomatal conductance. Also, the expression of OsXTH genes showed different patterns of response to drought stress. Of group 1, Transcript levels for OsXTH2, OsXTH5 genes were markedly up-regulated at a series of drought stresses. By contrast,11 OsXTH genes were down-regulated of group 2. OsXTH13, OsXTH14, OsXTH23 of group 3 was after the first increase by mild stress and then dropped with severe stress; in contrast, Of group 4 including OsXTH4, OsXTH10, OsXTH12, OsXTH16, OsXTH21, OsXTH22, OsXTH25, their expression patterns was after the first drop by mild stress and then increased with severe stress; It indicated that interactions among genes may affect PER under drought. As ABA might play key roles in plant responses to stresses and some cell growth genes were affected by ABA, XTH might represent one of candidate genes for dissecting molecular mechanism in response to drought.