Approach and mechanism in enhancing the remobilization of assimilates and grain-filling in rice and wheat

Abstract

Monocarpic plants such as rice and wheat need to initiate the whole plant senescence before their pre-stored assimilates can be remobilized for the grain filling. However, plant senescence and photosynthesis are a pair of trade-off processes. How to coordinate plant senescence, photosynthesis, and remobilization of assimilates is a long-term unsolved problem in cereals. Solving this problem would have great significance in not only understanding the mechanism of the remobilization of assimilates to grain and grain-filling but also effectively coping with the poor remobilization of assimilates from vegetative tissues to grain and slow grain-filling for some cultivars or under the condition of heavy use of nitrogen in the current rice and wheat production systems. This paper reviewed crop management and physiological mechanism in enhancing the remobilization of assimilates to grain and grain-filling in rice and wheat. There are three important findings. (i) Moderate soil-drying (MSD) during post-anthesis can regulate the relationship of plant senescence, photosynthesis, and remobilization of assimilates, and accelerate grain-filing. With post-anthesis MSD, plant can rehydrate overnight but not severely inhibit leaf photosynthesis at day time. More importantly, it can timely and appropriately initiate the process of plant senescence, trigger assimilate remobilization from vegetative tissues to grain, accelerate grain-filling, especially for the late-flowering inferior caryopses, and lead to increases in grain yield, harvest index, and water use efficiency. The MSD therefore provides a new way to coordinate the relationship among plant senescence, photosynthesis, and remobilization of assimilates during grain-filling and to achieve the dual goal of increasing grain yield and saving water in cereals. (ii) Abscisic acid (ABA) and its interaction with ethylene and gibberellins (GAs) regulate assimilate remobilization and grain-filling. During the active grain-filling period, the filling of grain requires a higher level of endogenous ABA and higher ratios of ABA to ethylene and to GAs. Post-anthesis MDS enhances the remobilization of assimilates to grain and grain-filling through increases in ABA levels and the ratios of ABA to ethylene and/or to GAs, which reveals the mechanism in which plant hormones regulate assimilate remobilization and grain-filling, and provides a physiological regulation to improve grain-filling in cereals. (iii) A modest increase in ABA level in plants can enhance activities of the key enzymes involved in carbon metabolism, and thereby increase assimilate loading and unloading capacity. An elevated ABA level through MSD or applying ABA with a low concentration can increase activities of the key enzymes in starch or fructan degradation and sucrose synthesis in vegetative tissues, leading to the enhancement in assimilate loading capacity, and enhance activities of the key enzymes in sucrose-to-starch conversion in the sink organ, and consequently, increase assimilate unloading capacity and starch synthesis in grain, which reveals the mechanism in which ABA regulates assimilate remobilization and grain-filling. Further research is needed to understand the molecular mechanism, such as from the aspect of signal transduction, in which assimilate remobilization and grain filling are regulated in cereals, to investigate the regulatory mechanism underlying nitrogen remobilization from vegetative tissues to grain, and to elucidate the mechanism in which the MSD reduces spikelet degeneration and improves grain quality of cereals.