Effect Of Phosphorus On The Biosynthesis And Characteristics Of Starch In Developing Wheat Grains

Starches are important for the quality of wheat.Phosphorus is one of the three essential nutrients for crops,have certain influence on the synthesis and accumulation of starch.This study was conducted to investigate the mechanism of changes in starch biosynthesis and characteristics under different phosphorus levels.The main wheat varieties in Xinjiang,Xindong20 and Xindong23 were used for this study,three levels of phosphorus(P0: P2O5 0 kg/ hm2;LP: P2O5 105 kg/ hm2;HP: P2O5 210 kg/ hm2)were applied at regreening.During grain development,sampled at different developmental stages,the wheat starch content,particle size distribution of starch granules,chain-length distribution of amylopectin,phosphorus content of wheat grain,activity of enzymes involved in starch synthesis and degradation,expression of genes coding for starch synthesis and degradation enzymes,in situ localization of ?-and ?-amylase gene transcripts under different phosphorus levels(P0: P2O5 0 kg/hm2;LP: P2O5 105 kg/hm2;HP: P2O5 210 kg/hm2)were studied,the morphology and microstructure,enzymolysis and thermal characteristics of starch granules in mature grain in response to different phosphorus levels were researched.The results are as show below:1.During grain development,phosphorus application increased the starch(total starch,amylopectin and amylose)content and ratio of B-chain in amylopectin,as well as increased the activity of ?-and ?-amylase in endosperm,but reduced the ratio of amylose content to amylopectin content and ratio of A-chain in amylopectin,while had little influence on the phosphorus content in grains.Moreover,in response to phosphorus application,the activity of sucrose synthase(SS)and peak activity of starch branching enzymes(SBE)was significantly increased and the peak activity of starch debranching enzymes(DBE)was ahead in endosperm of Xindong 20,the ratio of B-type starch granules in mature grains of Xindong20 was also increased.The total starch,amylopectin and amylose content in the grains were very low during early grain-filling stage,then increased with grain development,and reached the peak under LP treatment at maturity in these two varieties.The activity of ?-amylase was little changed with grain development and much lower than that of ?-amylase,while the ?-amylase activity was increased with grain development.The phosphorus content of wheat grains was little changed at different grain filling periods and there was no significant difference between three phosphorus levels at maturity.2.12 genes involved in starch synthesis(agp1,agp2,gbss1,gbss2,ss1,ss2,ss3,ss4,sbe1,sbe2 a,sbe2b,iso1)and 11 genes involved in starch degradation(amy1,amy2,amy3,amy4,bam1,bam2,bam3,bam4,bam5,bam6,bam7)expressed within endosperm during wheat grain development.These genes were up-regulated under the LP treatment compared with the P0 and HP treatments,and most of the genes coding for starch synthesis enzymes show peak expression at early-or mid-stage of grain filling,while most of amylase genes show peak expression at mid-or late stage of grain filling.3.The paraffin sections of wheat grains in different developmental stages were made and the RNA probes of amy4,bam1 and bam5 were synthesized for the in situ localization within endosperm.The results revealed that the amy4,bam1 and bam5 transcripts were detectable in both pericarp and early endosperm at 7 DPA under three phosphorus treatments.With the development of wheat grains,the transcripts of amy4,bam1 and bam5 were detectable in the entire endosperm until maturity under the P0 condition.In response to the LP and HP treatments,the amy4,bam1 and bam5 transcripts were accumulated in the border of endosperm at 28 and 35 DPA.Further,this regulation was more pronounced under the LP treatment.4.The results of study on starch in mature grains were as show: the observation results of SEM showed that phosphorus application did not change the shape of starch granules,but the” pinholes” along the equatorial grooves and on flat surfaces of the granules were easier to observe than those under P0 conditions,and starch granules under phosphorus application were substantially increased fluorescence after treatment with proteolytic enzymes and staining with methanolic merbromin and CBQCA,illustrated that the structure of channels within starch granules were changed;Secondly,the enzymatic hydrolysis on starch granules revealed that phosphorus application increased the concentration of reducing sugars after the starch granules were hydrolyzed with ?-amylase and amyloglucosidase,these results postulated related to the changes of microstructure and histochemical patterns of starch granules under different phosphorus levels.Moreover,the onset temperature,end temperature,peak temperature and delta H of starch granules isolated from Xindong23 was enhanced.In this study,we postulated that the high transcriptional level of genes involved in starch synthesis and the changes of starch synthesis enzymes activity under the appropriate amount phosphorus fertilization increased the micro-tubes of amyloplast,then promoted the starch synthesis and accumulation,furthermore,the residual of micro-tubes produced more channels within starch granules;on the other hand,the high transcriptional level of amylase gene in the periphery of endosperm increased the activity of ?-and ?-amylase,then promoted the change of channels and pores within starch granules.