Spermidine Enhanced Free Polyamine Levels and Expression of Polyamine Biosynthesis Enzyme Gene in Rice Spikelets under Heat Tolerance before Heading

Abstract

High temperatures (HT) before heading strongly inhibit the development of spikelets in rice. Spermidine (Spd) can improve rice's resistance to HT stress; however, the mechanism underlying this effect has not been elucidated. This study investigated several parameters, including yield, superoxide anion (O₂^.-), protective enzyme activities, and polyamine content, in a heat-sensitive genotype, Shuanggui 1. The yield and yield components decreased dramatically when subjected to HT stress, while this reduction could be partially recovered by exogenous Spd. Spd also slowed the generation rate of O₂^.- and increased protective enzyme, superoxide dismutase (SOD) and catalase (CAT) activities both under normal and high temperatures, which suggested that Spd may participate in the antioxidant system. Furthermore, genes involved in polyamine synthesis were analyzed. The results show that HT before heading significantly increased the expression of arginine decarboxylase OsADC1, Spd synthase OsSPDS1 and OsSPDS3 and had little effect on the expression of the S-adenosylmethionine decarboxylase OsSAMDC2 and ornithine decarboxylase OsODC1. In addition, exogenous Spd considerably reduced the expression of OsSAMDC2, OsSPDS1 and OsSPDS3 under HT but not the expression of OsADC1. The above mentioned results indicate that the exogenous Spd could help young rice spikelets to resist HT stress by reducing the expression of OsSAMDC2, OsSPDS1 and OsSPDS3, resulting in higher levels of endogenous Spd and Spm, which were also positively correlated with yield. In conclusion, the adverse effect of HT stress on young spikelets seems to be alleviated by increasing the amounts of Spd and Spm, which provides guidance for adaptation to heat stress during rice production.

Figure 1

The method of Spd and high temperature treatment, the sample pots.

Figure 2

Changes in the O₂⁻ production rate of the leaves and protective enzyme activities with or without Spd in rice upon heat stress before heading. The genotype SG-1 was pot-grown. Normal temperature (NT) and high temperature (HT) were conducted from 15 days before heading to heading. The chemicals were applied to panicles daily and lasted for 5 days starting on the 2rd day before HT stress treatment. Control plants were applied with deionized water. The data were the average of the three repeats. Different letters indicate statistical significance at P < 0.05 among the different treatments, including 5 and 10 days after heat stress.

Figure 3

Changes in the gene expression involved in polyamine synthesis with or without Spd in rice upon heat stress before heading. The genotype SG-1 was pot-grown. Normal temperature (NT) and high temperature (HT) were conducted during meiosis. The chemicals were applied to panicles daily and lasted for 5 days starting on the 2rd day before HT stress treatment. Control plants were applied with deionized water. Gene expression analysis in panicles was determined 5 days after heat stress and had four repeats. Different letters indicate statistical significance at P < 0.05 between the different treatments, including temperature and Spd.