The Mechanism And Regulation Underlying The Inhibition On The Assimilates Transport And Metabolism In Phloem Of Rice Caused By Heat Stress

Extreme high temperatures occured frequently these years,severely blocked the assimilate transportation from the source to sink,and disordered the source-sink relation in the most important crops including rice,which influenceed the rice yield and quality.But researches was mainly focused on the source or sink,little was concerned on the assimilates loading,transport,and unloading through phloem,even less about the assimilates transport and regulation under heat stress.Therefore,Nipponbare(NIPP,resistant to high temperature)and its mutant HTS(susceptible to high temperature)were selected in this experiment to study the effect of heat stress on the assimilates transport and metabolism of rice sheath and stem,leaf,and grain.Different concentrations of sucrose were sprayed to rice plants at grain filling stage to explore the mechanism of exogenous sucrose on the assimilates transport under heat stress.The major results are as followes:1.Besides spikelet fertility,heat stress also significantly decreased the kernel weight,but no significant differences were detected in the flag leaf photosynthesis and starch branching enzyme activity in grain between the control and heat stress groups.Thus,the heat stress-induced decline in kernel weight was most likely due to inhibition of assimilates distribution rather than the limitation of source and sink.This assumption was confirmed by the determined distribution of dry mater weight,nonstructural carbohydrates and starch in grain,leaf and sheath-stem induced by heat stress in plants,which might be mainly mediated by the SUT2,SUS2,and SUS3 in grain.Additionally,the callose deposited in the plasmodesmata of leaf and sheath as well as the phytohormones in grain was also found to be involved in sugar transport and metabolism under heat stress.However,the pattern of phytohormones distribution in leaf and sheath-stem was different to those of carbohydrates,indicating that the function of phytohormones in whole-plant carbohydrate partitioning under heat stress was not clear.Accordingly,sugars rather than phytohormones might act as signaling molecule to mediate the source-sink relationship in rice under heat stress.2.Heat stress decreased kernel weight greatly at grain filling stage,the reduction in grain weight was related to the sucrose transport and metabolism as well as the process of grain filling.The relative expression of SUT1,SUT2,INV1 and CIN2 of NIPP increased more than HTS during and after heat stress.As to the grain filling enzyme activities,NIPP was influenced much more than HTS,but increased more than HTS when the heat stress end.Therefore,the difference in heat tolerance between NIPP and HTS may focus in the recovery ability,and NIPP was significantly greater than HTS.3.Transcriptome analysis of stem and sheath,leaf,and grain was used to reveal the regulatory network about the assimilates transport and metabolism under heat stress during grain filling stage,In total,271 GB clean data(1,808,452,322 short reads)was obtained.The results showed that,there are more significant different expression genes in the grain than leaf and stem and sheath under heat stress,at the same time,NIPP contains more up-regulated genes and less down-regulated genes.The GO enrichment and KEGG analysis indicated that the down regulated genes of the grain focus on the response to stimulus and stress,carbon and nitrogen metabolic,secondary metabolic process,and signal transduction,especially the carbon metabolic,plant hormone signal transduction,and biosynthesis of secondary metabolic were influenced heavily.4.Heat stress caused significant damage to rice seedlings(continuous 48 hours heat treatment),especially the heat stress sensitive genotype HTS.The apoplasmic and symplasmic pathways of assimilate transport under heat stress were inhibited,the relative expression of sucrose transporters down-regulated and callose accumulated in plasmodesmata.ABA and the trehalose metabolism may play important roles in this process.Under heat stress,the trehalose content of HTS decreased,and the inhibitory effect of trehalose on SnRK1 was weakened,resulting in greater damage caused by heat stress.The trehalose content in HTS was lower than NIPP,which may ascribe to the significant down-regulation of TPP7 in HTS.5.Exogenous sucrose can alleviated the adverse effects on kernel weight caused by heat stress at grain filling stage,especially the heat stress sensitive genotype HTS.Sucrose regulated dry matter accumulation,and non-structural carbohydrate content in stem and sheath,leaf,and grain by the relative expression of SUT1,SUT2,INV1,SUS2,SUS4 and CIN2 and key enzyme activities on grain filling under heat stress.In addition,sucrose had positive effects on the trehalose signaling pathway and promoted the transport of assimilates to the grain.