Abstract
Heat stress occurrence during seed filling leads to the formation of a chalky portion in the limited zone of the starchy endosperm of rice grains. In this study, isolation of aleurone, dorsal, central and lateral tissues of developing endosperm by laser-microdissection (LM) coupled with gene expression analysis of a 44 K microarray was performed to identify key regulatory genes involved in the formation of milky-white (MW) and white-back (WB) grains during heat stress. Gene regulatory network analysis classified the genes changed under heat stress into five modules. The most distinct expression pattern was observed in modules where most of the small heat shock proteins and cellular organization genes were changed under heat stress in dorsal aleurone cells and dorsal starchy endosperm zones. The histological observation supported the significant increase in cell number and size of dorsal aleurone cells in WB grains. With regard to the central starchy endosperm zone, preferential down-regulation of high molecular weight heat shock proteins (HMW HSPs), including a prominent member encoding endoplasmic reticulum (ER) chaperones, by heat stress was observed, while changes in expression of starch biosynthesis genes were minimal. Characterization of transgenic plants suppressing endosperm lumenal binding protein gene (BiP1), an ER chaperone preferentially down-regulated at the MW zone under heat stress, showed evidence of forming the chalky grains without disturbing the expression of starch biosynthesis genes. The present LM-based comprehensive expression analysis provides novel inferences that HMW HSPs play an important role in controlling redox, nitrogen and amino acid metabolism in endosperm leading to the formation of MW and WB chalky grains under heat stress.
Highlights
The appearance of rice grains, such as the degree of translucency, is extremely important to capture higher revenues in the market
We indicate the possibility that reduced expression of BiP1 increased the unfolded and immature proteins, which are important for starch and prolamin synthesis, thereby causing the MW type of chalk in the early storage phase under heat stress
In the present study, integrated experiments on morphology, comprehensive expression analyses and measurements of storage compounds were carried out focusing on the MW and WB types of chalk formed at the early storage phase under heat stress
Summary
The appearance of rice grains, such as the degree of translucency, is extremely important to capture higher revenues in the market. A proportion of chalky grains >15% has a negative impact on eating quality (Kim et al 2000), due to the decrease in palatability of the rice grain (Kim et al 2000, Cheng et al 2005). The proportion of chalky grains is negatively correlated with head rice yield (Lyman et al 2013). Designing the development of climate-resilient rice with acceptable grain quality for heat stress is a pressing issue (Sreenivasulu et al 2015). Starch accumulation in the developing endosperm is asynchronous. Active accumulation of starch starts in the central zone at an early stage, it proceeds to the peripheral parts as seed maturation progresses (Ishimaru et al 2009). The timing of heat exposure during onset of seed formation and grain filling causes differences in the types of chalky grains (Tashiro and Wardlaw 1991). Even among tissues in starchy endosperm, the center and back zones are likely to be chalky, whereas a chalky phenotype is not observed at the lateral zone under high-temperature stress
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