Grain Filling Characteristics And Their Relations With Endogenous Hormones In Several Rice Mutants

Grain filling in cereals plays a vital role in determining grain weight and quality. The grain filling rate and grain plumpness differ greatly among varieties as well as at different positions on a panicle for a same variety. Usually, the superior spikelets in the upper part of the panicle flower earlier and fill faster and better, and the inferior spikelets in the lower part of a panicle flower later and fill slower and poorer. Although many studies have been conducted to investigate the mechanism underlying poor filling of the inferior spikelets, the mechanism is yet to be understood. Plant hormones play very important roles in regulating plant growth and development. However, the information on whether and how plant hormones regulate grain filling of rice is very limited. This study chose different rice materials and observed their grain filling characteristics and changes in contents of endogenous zeatin+zeatin riboside (Z+ZR), gibberellins (GAs), auxin (IAA), abscisic acid (ABA), putrescine (Put), spermidine (Spd) and spermine (Spm) in grains, and analyzed the relationship between grain filling and plant hormones. The main results are as follows:1. Three kinds of rice mutants, large-grain mutant (AZU-M), small-grain mutant (ZF802-M) and single spikelet on a branch mutant (Zhehui7954-M) and their wide types (AZU-WT, ZF802-WT, and Zhehui7954-WT) were used and were field-grown. Yield components of the three kinds of mutants and their wide types were observed. The results showed that, when compared with their respective wide type, AZU-M had a significant heavier grain weight, less panicle number, higher seed-setting rate, and a lower grain yield; ZF802-M had more panicle number, lower grain weight and seed-setting rate, and lower grain yield. Zhehui7954-M showed significant increases in seed-setting rate, grain weight and grain yield.2. The grain filling rate showed a smaller difference and was synchronous between superior and inferior spikelets for the three kinds of mutants. However, the difference was significant in grain filling rates between superior and inferior spikelets, i.e., superior spikelets filled faster and showed a larger grain weight than inferior spikelets, for the three wide types. The division rate and number of endosperm cells exhibited similar trends with grain filling rate and grain weight.3. The contents of endogenous Z+ZR, IAA and ABA were low in the early grain filling stage, and were gradually increased with the grain filling process, and gradually declined after reaching the maximum. They showed small difference between superior and inferior spikelets for the three kinds of mutants, and were positively correlated with the grain filling rate and endosperm cell proliferation rate. During the early grain filling period, the GAs content was high and was decreased with the filling process, and was not significantly correlated with grain filling rates. Contents of endogenous Put, Spd and Spm in grains were low at the early grain filling stage, and were increased with the grain filling process, and then were decreased after reaching a maximum. Contents of Put, Spd and Spm were higher in superior spikelets than in inferior spikelets at the early grain filling stage, and were reversed at the late grain filling stage, and showed small difference between superior and inferior spikelets for the three kinds of mutants. Contents of Put, Spd and Spm were significantly and positively correlated with grain filling rates and endosperm division rates.4. Spraying exogenous IAA increased contents of Z+ZR and IAA in grains. Spraying ABA increased the content of the ABA. The number of endosperm cells and grain weight were increased after spraying exogenous IAA and ABA. Application of ABA also increased the mean grain-filling rate. Spraying exogenous Spd and Spm increased the contents of Spd and Spm in inferior spikelets, the number of endosperm cells, grain filling rates and grain weight. Spraying MGBG [Methylglyoxal-bis (guanylhydrazone), an inhibitor of polyamine (Spd and Spm) synthesis] reduced Spd and Spm contents in both superior and inferior spikelets, increased the Put content, and significantly reduced the endosperm cell number, grain filling rate and grain weight. Spraying exogenous Put reduced the number of endosperm cells, grain filling rate and grain weight.The results indicate that poor grain filling of inferior spikelets in rice is closely associated with low contents of Z+ZR, IAA and ABA in the spikelets. The grain filling would be improved and grain yield be increased by regulating the levels of above hormones in inferior spikelets through chemical control.