Abstract
Moderate soil drying imposed at the post-anthesis stage significantly increases starch accumulation in inferior grains of rice, but how this process is regulated at the level of gene expression remains unclear. In this study, we applied moderate drying (MD) treatments to the soil at the post-anthesis stage and followed the dynamics of the conversion process of soluble sugars to starch in inferior grains using RNA-seq analysis. An elevated level of ABA induced by MD was consistently associated with down-regulation of ABA8ox2, suggesting that lower expression of this gene may be responsible for the higher ABA content, potentially resulting in better filling in inferior grains. In addition, MD treatments up-regulated genes encoding five key enzymes involved sucrose-to-starch conversion and increased the activities of enzymes responsible for soluble-sugar reduction and starch accumulation in inferior grains. Differentially expressed transcription factors, including NAC, GATA, WRKY, and M-type MADS, were predicted to interact with other proteins in mediating filling of inferior grains as a response to MD. Transient expression analysis showed that NAC activated WAXY expression by binding to its promoter, indicating that NAC played a key role in starch synthesis of inferior grains under MD treatment. Our results provide new insights into the molecular mechanisms that regulate grain filling in inferior grains of rice under moderate soil drying.
Highlights
Rice panicles are composed of many spikelets, each of which is considered as an individual unit in the inflorescence (Mohapatra and Sahu, 1991)
The weight of inferior grains subjected to moderate drying (MD) during grain filling was significantly increased (Fig. 1A).The soluble sugar content of the inferior grains was significantly reduced in the MD treatment (Fig. 1B), whilst the starch content was significantly increased (Fig. 1C)
Since the endogenous abscisic acid (ABA) level is regulated by both biosynthesis and catabolism, we investigated the relevant genes in all the libraries.We found that ABA8ox2 was predominantly detected in our samples (Fig.5B),while ABA8ox1 and ABA8ox3 exhibited much lower transcription levels, suggesting that ABA8ox2 plays a key role in grain filling.The expression levels of ABA8ox2 were verified by RT-qPCR (Fig. 5C), which showed the same expression pattern as the RNA-seq results (Fig.5B)
Summary
Rice panicles are composed of many spikelets, each of which is considered as an individual unit in the inflorescence (Mohapatra and Sahu, 1991). On both the primary and secondary branches are referred to as spikelets.The grain filling of spikelets is based on the order within a single panicle. Spikelets located on apical primary branches usually flower earlier and generate larger and heavier grains, referred to as ‘superior grains.’. Some spikelets located on the proximal secondary branches reach anthesis later and generate smaller or even no grains; these are referred to as ‘inferior grains’ (Mohapatra et al, 1993). The differences observed between superior and inferior grains, such as ethylene production and the grain-filling rate, are more significant in large-panicle rice, especially in so-called ‘super rice’ varieties, which possess numerous spikelets with a large sink capacity and have a high yield potential (Panda et al, 2009; Sekhar et al, 2015). Starch accumulation in grains is of great importance, as it is the major constituent of the seeds (Yoshida et al, 1976)
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