PPDK’s Role And Its Regulation During Grain Filling Stage Under High Temperature Treatment

The rice grain filling stage involves the complex metabolic pathways and regulation processes including reserve transportation, synthesizing and assembling, distribution and accumulation. Such processes are sensitive to external conditions. High temperature stress is one of the main factors that directly decrease rice production and quality. In order to identify the metabolic changes during grain filling under high temperature, a grain proteomics research has been carried out under different high temperature treatments. That study observed the differential expression of cytosolic pyruvate orthophosphate dikinase(cy PPDK). In this study, two cultivars(9311 and TXZ-25) with different sensitivity to high temperature at grain filling stage were tested under artificial temperature treatments. PPDK’s function that participated in grain metabolic process was discussed from enzyme activity, transcription, translation and post translational regulation levels. The main results are as follows:1. High temperature treatments shortened the grain filling duration of the two cultivars, and significantly reduced the grain’s fresh weight and dry weight. High temperature treatments accelerated the initial grain filling rate of 9311, and had little influence on the maximum and mean grain filling rate of 9311. The initial grain filling rate of TXZ-25 was not affect by high temperature, while the maximum and mean grain filling rate increased a lot. The grain filling duration of XZ-25 had a larger decrease than that of 9311.2. High temperature reduced the two cultivars’ milling quality and physical appearance. After 20 d high temperature treatment, the brown rice rates(BRR), head rice recovery(HRR) and grain width were significantly reduced. The chalkiness of endosperm was significantly increased under high temperature, and the chalk grain type was white core. The chalkiness of TXZ-25 was more sensitive to high temperature.3. The changes of PPDK activity were related to grain filling degree, and had varietal differences. As seed development processing, PPDK activity continued decline, and the high temperature accelerated this decline. Throughout the grain filling duration, 9311 PPDK activity was always higher than that of TXZ-25 regardless of temperature treatments.4. PPDK protein had distinctive post-translational modification for activity regulation. High temperature promoted the PPDK phosphorylation, and lowered the PPDK activity. The total PPDK and phosphorylation level of Thr527 were detected using rice cytosolic PPDK and PPDK-Thr527 phosphorylation specific antibodies. Within 20 DAF of both high temperature and controlled treatments, PPDK protein levels of two cultivars generally showed down-regulated expression, accompanied by up-regulated phosphorylation levels. High temperature treatment intensified those trends. Compared with 9311, PPDK protein level of TXZ-25 was more sensitive to high temperature, and its phosphorylation level was higher. The fast phosphorylation mechanism of PPDK provided a reasonable explanation for the contradiction between PPDK activity and PPDK protein levels.In recent years, more attention is paid on the function of PPDK in non-photosynthetic tissues. This study preliminary confirmed that cy PPDK involved in high temperature stress response of rice grain development, which will provide references for further research about its mechanism and rice physiological breeding.