Regulation Of Abscisic Acid And Ethylene To Grain Filling In Rice And Wheat And Its Physiological Mechanism

Both abscisic acid (ABA) and ethylene are important hormones and play important roles in growth and development in responses to stress. It is little known, however, whether and how ABA and ethylene are involved in regulation to grain-filling in rice (Oryza sativa) and wheat (Triticum aestivum). This study investigated the roles of ABA and ethylene in grain filling in rice and wheat and their physiological mechanisms by using typical rice and wheat cultivars. The main results are as follows:1. Regulation of ABA and ethylene to endosperm development and grain filling. The division rate and the number of endosperm cells, grain filling rate, and grain weight of superior grains in rice or wheat were greater than those of inferior grains. At the early grain filling stage, the evolution rate of ethylene and 1-aminocylopropane-1-carboxylic acid (ACC) concentrations were greater in inferior grains than in superior grains. While ABA concentrations and the ratio of ABA to ACC (ABA/ACC) were greater in the superior than in the inferior grains. The cell division and grain-filling rates were significantly and positively correlated with both ABA contents and the ratio of ABA to ACC, whereas significantly and negatively correlated with ethylene evolution rate. Application of cobalt ion (inhibitor of ethylene synthesis) or ABA at early grain filling stage (at 9-13d post-anthesis) significantly increased endosperm cell division rate and cell number, grain-filling rate and grain weight of inferior spikelets. Application of ethephon (ethylene-releasing agent) or fluridone (inhibitor of carotenoid synthesis) had the opposite effects. The results suggest that the antagonistic interactions between ABA and ethylene mediate the endosperm cell division and grain filling in rice and wheat. A higher ratio of ABA to ethylene (ACC) in rice and wheat grains is required to maintain a faster the endosperm cell division rate and grain-filling rate.2. Responses of ABA and ethylene in rice and wheat grains to soil moisture and their relations with grain filling. Both moderate soil-drying (MD) and severe soil-drying (SD) were conducted during the grain filling period, and the treatment of well watered (WW) was taken as control. The responses of ABA and ethylene to soil drying and their relations with grain filling were studied. The results showed that, compared with WW, MD significantly increased, whereas SD significantly reduced, grain-filling rate and grain weight. ABA content in the grains was very low at the early grain filling stage, and reached a maximum when the grain-filling rate was the highest. In contrast to ABA, levels of ethylene and ACC in the grains were very high at early grain filling stage and sharply decreased during the linear period of grain growth. Both MD and SD remarkably enhanced ABA accumulation in the grains, which was enhanced more in SD grains than in MD ones. MD reduced, whereas SD remarkably increased, ethylene evolution rate and ACC content. MD increased, whereas SD reduced, the ratio of ABA to ACC. The grain-filling rates were significantly and positively correlated with both ABA contents and the ratio of ABA to ACC, whereas significantly and negatively correlated with ethylene evolution rate. Application of amino-ethoxyvinylglycine (AVG) or cobaltion (inhibitor of ethylene synthesis) or ABA at 9-13 d post-anthesis significantly reduced ACC concentrations and ethylene evolution rate of grains, and significantly enhanced the activities of sucrose synthase (SuSase), ADP glucose pyrophosphorylase (AGPase) and soluble starch synthase (SSSase), and increased grain-filling rate and grain weight. The results were reversed when ethephon (ethylene-releasing agent) or fluridone (inhibitor of ABA synthesis) was applied. The results suggested that a moderate soil-drying during the grain-filling period of rice and wheat could increase the ratio of ABA to ethylene (ACC), and therefore accelerate grain filling and increase grain weight. The mediation of ABA and ethylene to grain filling is probably through regulating key enzymes involved in sucrose-to-starch pathway, and a high ratio of ABA to ACC enhances grain filling.3. ABA and ethylene in relation with membrane lipid peroxidation and nucleic acid content/concentration in grains at different soil moisture conditions. The MD increased activities of superoxide dismutase (SOD) and catalase (CAT). SD significantly increased those at the early treatment stage, but decreased at the late treatment stage. Among all the treatments, inferior grains had much lower activities of SOD, peroxidase (POD) and CAT than superior grains at the early grain filling stage, and the effect of soil moisture on inferior grains was much grater than on superior grains. There were no significant differences in superoxide anion (O2-) production rate and malondialdehyde (MDA) content of grains between MD and WW plants, but they were much higher in SD plants. MD significantly increased, whereas SD significantly reduced, ribonucleic acid (RNA) contents in grains. There were no significant differences in deoxyribonucleic acid (DNA) contents in grains among all the treatments. The grain-filling rates and ABA contents in grains were significantly and positively correlated with activities of SOD and CAT and RNA contents in grains, whereas significantly and negatively correlated with MDA content of grains. The ethylene evolution rate and ACC content in grains were significantly and negatively correlated with activities of SOD and CAT and RNA contents, and significantly and positively correlated with O2- production rate in grains. Application of cobaltion (inhibitor of ethylene synthesis) or ABA, at 9-13 d post-anthesis significantly enhanced the activities of SOD and CAT, increased RNA and DNA contents of grains, whereas significantly reduced O2- production rate in grains and at MDA content of grains, and the results were reversed when ethephon (ethylene-releasing agent) or fluridone (inhibitor of ABA synthesis) was applied. The results suggested that a moderate soil drying during the grain-filling period of rice and wheat could enhance the ratio of ABA to ethylene (ACC) and the activities of SOD and CAT of grains, and reduce the ethylene evolution rate and membrane lipid peroxidation in grains, and therefore accelerate the filling of inferior grains.4. Roles of ABA and ethylene in grain quality of rice. The MD significantly increased breakdown values, and reduced chalky kernels, chalkiness and setback values. The results were reversed for the SD treatment. Ethylene evolution rates were significantly and negatively correlated with head milled rice and breakdown values, and significantly and positively correlated with chalky kernels, chalkiness and setback values. The ratio of ABA to ACC was significantly and positively correlated with head milled rice and breakdown values, and significantly and negatively correlated with chalkiness. Application of cobaltion (inhibitor of ethylene synthesis) or ABA, at 9-13 d post-anthesis significantly increased head milled rice, amylose contents and breakdown values, whereas significantly reduced chalky kernels, chalkiness, protein contents, and setback values, the results were reversed when ethephon (ethylene-releasing agent) or fluridone (inhibitor of ABA synthesis) was applied. The results indicate that Both ABA and ethylene play roles in the formation of grain quality, and that a higher ratio of ABA to ethylene (ACC) would benefit grain quality of rice.