Summary: High temperature significantly alters the amylose content of rice, resulting in mature grains with poor eating quality. However, only few genes and/or quantitative trait loci involved in this process have been isolated and the molecular mechanisms of this effect remain unclear. Here, we describe a floral organ identity gene, OsMADS7, involved in stabilizing rice amylose content at high temperature. OsMADS7 is greatly induced by high temperature at the early filling stage. Constitutive suppression of OsMADS7 stabilizes amylose content under high temperature stress but results in low spikelet fertility. However, rice plants with both stable amylose content at high temperature and normal spikelet fertility can be obtained by specifically suppressing OsMADS7 in endosperm. GBSSI is the major enzyme responsible for amylose biosynthesis. A low filling rate and high expression of GBSSI were detected in OsMADS7 RNAi plants at high temperature, which may be correlated with stabilized amylose content in these transgenic seeds under high temperature. Thus, specific suppression of OsMADS7 in endosperm could improve the stability of rice amylose content at high temperature, and such transgenic materials may be a valuable genetic resource for breeding rice with elite thermal resilience.

Grain quality is one of the key targets to be improved for rice breeders and covers cooking, eating, nutritional, appearance, milling, and sensory properties. Cooking and eating quality are mostly of concern to consumers and mainly determined by starch structure and composition. Although many starch synthesis enzymes have been identified and starch synthesis system has been established for a long time, novel functions of some starch synthesis genes have continually been found, and many important regulatory factors for seed development and grain quality control have recently been identified. Here, we summarize the progress in this field as comprehensively as possible and hopefully reveal some underlying molecular mechanisms controlling eating quality in rice. The regulatory network of amylose content (AC) determination is emphasized, as AC is the most important index for rice eating quality (REQ). Moreover, the regulatory mechanism of REQ, especially AC influenced by high temperature which is concerned as a most harmful environmental factor during grain filling is highlighted in this review.

QRT-PCR is one of the most popular approaches to analyze specific gene expression level, and stably expressed reference genes are essential to obtain reliable results. However, many reference genes are only stable under certain circumstances and different reference genes might be required in different experiments. High temperature is a common stress that affects rice endosperm development and it has become a hot topic recently. Although study about reference genes at different developmental stages in rice has been reported, these genes may not be suitable to study high temperature mediated responses especially in endosperm. In our quest for proper reference genes to quantify gene expression in rice endosperm under high temperature, we studied 6 candidate genes selected from the transcriptome data and 11 housekeeping genes. All genes were analyzed with qRT-PCR and the expression stability was assessed with software geNorm and NormFinder. Fb15 and eIF-4a were identified as the two most stable genes in endosperm at different developmental stages, while high temperature treatment has a least effect on expression of Fb15 and UBQ5 in rice endosperm. Our results provide some good candidate reference genes for qRT-PCR normalization in rice endosperm under different temperatures.

Amylose content (AC) is a critical factor for the quality of rice. It is determined by the biosynthesis gene Waxy (Wx) and a variety of quantitative trait loci (QTLs). Although many QTLs have been reported to affect rice AC, few of them have been investigated under varying growth conditions, especially various temperatures, which are known to greatly influence the AC. We analyzed the AC at different temperatures and planting seasons in a set of chromosome segment substitution lines (CSSLs) which were derived from a cross between the indica variety 9311 and the japonica variety Nipponbare carrying the same Wxb allele. A joint analysis detected a single locus, qSAC3, with a high logarithm of odds (LOD) score in four different conditions. The qSAC3 from indica 9311 (qSAC3ind) substantially increased the AC in japonica Nipponbare under all tested growth conditions. Furthermore, introducing the qSAC3ind into the soft rice variety Nangeng9108 with Wxmq, a mutant allele of Wxb, also moderately increased its AC and improved its appearance quality significantly by reducing the chalkiness of the polished rice. Our results indicate that the qSAC3ind could increase the AC of japonica rice in different environments as well as in the background of different Wx alleles and that qSAC3 is a valuable locus for fine-tuning the rice AC and ameliorating the dull endosperm in rice varieties with the Wxmq allele.