Studies On The Initial Physiological Pathway Of Inferior Grain Filling In Large Panicle Rice

It is an important way to improve the rice yield by increasing the grain weight and the number of grains per panicle,cultivating large panicle rice varieties.In modern production,large panicle type varieties have been popularized and planted in a large area.However,because of the long period of grain filling stagnation,poor filling and low seed setting rate,the potential of increasing yield has not been fully realized.Photoassimilates is the main source of rice grain filling material,and its supply and utilization are closely related to inferior grain filling.Auxin is an important growth regulator in plants,which affects the distribution of assimilates and grain filling process of inferior spikelets(IS)through apical dominance mediated by it.In this study,two large panicle japonica rice varieties(CJ03 and W1844),which had obvious differences between grain filling of superior spikelets(SS)and IS,with the same sink capacity,were selected as materials.The spikelets removing treatments were set up to simulate the increasing supply of sugar(to IS).The mechanism of initial grain filling was analyzed from the aspects of photoassimilates unloading,transportation and utilization in the grain.Besides,the exogenous auxin treatment was set up to verify the regulation of auxin and to clarify the physiological pathway of initial inferior grain filling,in order to provide theoretical basis for breeding and high yield cultivation of improved rice varieties in the future.The main results are as follows:1.Adequate assimilation supply is the key to the initial inferior grain fillingIn this study,the source-sink ratio and carbohydrate-spikelet ratio of the two large panicle varieties under spikelets-removing treatments reached the range of 146%of 241%and 139%of 246%compared to origin,respectively.Under adequate sugar supply,the stagnant period of IS was shortened.The initiation of grain filling was advanced by at least 4 days,and the final grain weight and seed setting rate were also close to those of SS.The study of grain filling characteristics also found that spikelets removing treatments increased the initial growth potential of IS and shortened the time to reach the maximum filling rate,indicating that the grain filling process of IS was accelerated and adequate assimilation supply was very important for the initial inferior grain filling.2.Sucrose phloem unloading is a limiting factor for initial inferior grain fillingAfter spikelets removing,the adequate supply of assimilation did not immediately start the inferior grain filling.Compared to the SS,the IS still had a short stagnant period,indicating that there was still other limiting factor in the process of initial inferior grain filling.Dorsal vascular bundles are the only channel for nutrients to enter grains,suggesting that the amount of sugar exported to grains is absolutely controlled by it.In this study,it was found that the vascular bundles of SS had hollow sieve tubes,more plasmodesmata and stronger dredging ability at 4 days after anthesis,while there were a large number of sieve tubes without dredging ability and less plasmodesmata in the IS.After spikelets removing,the development of vascular bundles in IS was improved,and then a large amount of soluble sugar was accumulated rapidly in the developing grains,which indicated that unobstructed sieve tubes was a physical obstacle to sugar reception in grains at initial grain filling period.The signal from a large amount of sugar supply promoted the development of phloem cells and increased the space for sugar input.In addition,assimilation also needs the help of sucrose transporters to enter the grain.The main sucrose transporter gene responsible for unloading is OsSUT1.The relative expression of OsSUT1 in the IS after spikelets removing were always higher than that of control,and even higher than that of the SS on the day of initial grain filling.It indicates that the high expression of OsSUT1 may be coordinated with of vascular bundles development,which promotes more sugar to enter the grain to improve the initial grain filling.At the same time,the related gene expression of sugar transporters in IS increased rapidly after spikelets removing,which pushed the sugar unloading from phloem and then transported into endosperm.Finally starch synthesis was completed quickly to promote grain filling.3.IAA works as an upstream factor to regulate the initial inferior grain fillingIn this study,it was found that the content of IAA in IS began to increase quickly after anthesis,even earlier than the changes of soluble sugars in grains,indicating that the effect of auxin regulation may be earlier than that of assimilation supply.After IAA application,the initial grain filling of W1844 inferior spikelets were briefly advanced,and the effect pathway was similar to that of spikeletes removing treatment.It indicated that auxin may be an upstream factor involved in the regulation of initial grain filling of IS.However,the grain filling progress of CJ03 IS was not shorten by IAA application,indicating that there were differences in sensitivity and response to exogenous IAA spraying among varieties,which may be related to that the IAA concentration of 20mg/L would not be the appropriate concentration to improve the initial grain filling of CJ03 IS.After IAA application,the expression of two auxin efflux genes OsPIN1b and OsPIN3t,which related to the development of vascular bundles in IS under spikelets removing,were up-regulated in the early stage,indicating that auxin may promote the development of vascular bundles in the grain through efflux transport.The promotion would liberate the ability of sucrose unloading,accelerate the synthesis and accumulation of starch and finally promote the initial grain filling ofIS.According to the above results,the primary assumption of a physiological model of initial inferior grain filling regulated by auxin was described in this study:the auxin produced by filial cells was outputted with the help of auxin efflux protein PIN,so that the development of dorsal vascular cells was promoted and the dredging ability of phloem was improved to make more assimilates enter the grain.Numerous assimilates were continuously unloaded from the phloem of the dorsal vascular bundle into maternal cells such as nucellar projection,then crossmembrane transported into the aleurone and endosperm cells by sugar transporters(SUT and SWEET),and finally transformed into starch for accumulation in the endosperm.