Relationship Between Polyamines and the Grain Quality during Grain-Filling Stage Under High Temperature.

Chalky grain caused by high temperature (HT) during the grain-filling stage is a serious problem for rice quality. Our previous studies have shown that applying nitrogen during grain filling under HT may reduce grain chalkiness. Polyamines are nitrogen containing and developmental regulatory molecules; however, the relationship between applying nitrogen and polyamine levels during grain ripening under HT and the role of polyamines with respect to grain development remains unclear. The purpose of this study is to shed light on both aspects. Rice cultivar TK9 with and without nitrogen application has been exposed to HT for 15 days after flowering. During grain filling, polyamine levels, indicators of HT-induced damages and expressions of HT-related and polyamine-related genes have been measured. Following harvesting, grain quality and the content of storage proteins were analyzed. The results showed that HT deleteriously affected yield and quality of the grain. Applying nitrogen preserved storage proteins and reduced chalkiness. Physiological analyses showed that nitrogen moderated the effects of HT by reducing chlorophyll degradation and lipid peroxidation damage within the grain. In developing caryopses, the major polyamine is spermidine, followed by putrescine and spermine, regardless of temperature treatments. Nitrogen maintained the polyamine levels under HT. Gene expression analysis proved that HT repressed starch and protein synthesis-related gene-expression, and promoted the expression of the polyamine oxidation gene (PAO) and hypoxia fermentation-related genes. Nitrogen suppressed the PAO gene expression under HT. Our findings suggest that nitrogen application can mitigate deleterious effects of HT by sustaining polyamine levels and regulating the expression of the PAO gene.