Genotype-dependent and heat-induced grain chalkiness in rice correlates
with the expression patterns of starch biosynthesis genes
To understand the molecular basis of environment-induced and
genotype-dependent chalkiness, six rice genotypes showing variable chalk
levels were subjected to gene expression analysis during reproductive
stages. In the high chalk genotypes, the peak expressions of
ADP-Glucose Pyrophosphorylase (AGPase) Large Subunit 4 (AGPL4)
occurred in the stages before grain filling commenced, creating a
temporal gap with the upregulation of Granule Bound Starch
Synthase I (GBSSI) and Starch Synthase IIA (SSIIA). Whereas, in
the low chalk genotypes, AGPL4 expression generally occurred in later
stages, close to the upregulation of GBSSI and SSIIA.
However, heat treatment altered the expression pattern and created a gap
between the expression peaks of AGPL4, and GBSS1 and
SSIIA. This change was accompanied by transformed granular
morphology, increased protein content, and chalkiness in the grains.
AGPL4 expression pattern may partially explain chalkiness as it
contributes to the pool of ADP-Glucose for producing amylose and
amylopectin, the major components of the starch. Down-regulation of
AGPase during grain filling stages could result in a limited pool of
ADP-Glucose leading to inefficient grain filling and air pockets that
contribute to chalkiness. The study suggests a mechanism of grain
chalkiness based on the coordination of the three starch biosynthesis
genes in rice.