Genetic Research Progress: Heat Tolerance in Rice.

Heat stress (HS) caused by high-temperature weather seriously threatens international food security. Indeed, as an important food crop in the world, the yield and quality of rice are frequently affected by HS. Therefore, clarifying the molecular mechanism of heat tolerance and cultivating heat-tolerant rice varieties is urgent. Here, we summarized the identified quantitative trait loci (Quantitative Trait Loci, QTL) and cloned rice heat tolerance genes in recent years. We described the plasma membrane (PM) response mechanisms, protein homeostasis, reactive oxygen species (ROS) accumulation, and photosynthesis under HS in rice. We also explained some regulatory mechanisms related to heat tolerance genes. Taken together, we put forward ways to improve heat tolerance in rice, thereby providing new ideas and insights for future research.

Natural variation of HTH5 from wild rice, Oryza rufipogon Griff., is involved in conferring high-temperature tolerance at the heading stage.

Global warming is a major abiotic stress factor, which limit rice production. Exploiting the genetic basis of the natural variation in heat resistance at different reproductive stages among diverse exotic Oryza germplasms can help breeding heat-resistant rice cultivars. Here, we identified a stable quantitative trait locus (QTL) for heat tolerance at the heading stage on chromosome 5 (qHTH5) in O. rufipogon Griff. The corresponding gene, HTH5, pertains to the pyridoxal phosphate-binding protein PLPBP (formerly called PROSC) family, which is predicted to encode pyridoxal phosphate homeostasis protein (PLPHP) localized to the mitochondrion. Overexpression of HTH5 increased the seed-setting rate of rice plants under heat stress at the heading stage, whereas suppression of HTH5 resulted in greater susceptibility to heat stress. Further investigation indicated that HTH5 reduces reactive oxygen species accumulation at high temperatures by increasing the heat-induced pyridoxal 5'-phosphate (PLP) content. Moreover, we found that two SNPs located in the HTH5 promoter region are involved with its expression level and associated with heat tolerance diversity. These findings suggest that the novel gene HTH5 might have great potential value for heightening rice tolerance to heat stress to the on-going threat of global warming.

Fine mapping of the qHTB1-1QTL, which confers heat tolerance at the booting stage, using an Oryza rufipogon Griff. introgression line.

KEY MESSAGE: The qHTB1-1 QTL, controlling heat tolerance at the booting stage in rice, was fine mapped to a 47.1 kb region containing eight candidate genes. Two positional candidate genes showed significant changes in expression levels under heat stress. High-temperature stress at the booting stage has the potential to significantly limit rice production. An interspecific advanced backcrossed population between the Oryza sativa L. cultivar R53 and the wild Oryza rufipogon Griff accession HHT4 was used as the source material to develop a set of chromosome segment introgression lines to elucidate the genetic mechanism of the qHTB1-1 QTL in heat tolerance. A single-chromosome-segment introgression line, IL01-15, was used to develop secondary populations for the mapping of qHTB1-1 on chromosome 1 for heat tolerance at the booting stage. Using the BC5F2, BC5F3, and BC5F4 populations, we first confirmed qHTB1-1 and validated the phenotypic effect. The qHTB1-1 QTL explained 13.1%, 16.9%, and 17.8% of the phenotypic variance observed in the BC5F2, BC5F3, and BC5F4 generations, respectively. Using homozygous recombinants screened from larger BC6F2 and BC6F3 populations, qHTB1-1 was fine mapped within a 47.1 kb region between markers RM11633 and RM11642. Eight putative predicted genes were annotated in the region, and six genes were predicted to encode expressed proteins. The expression patterns of these six genes demonstrated that LOC_Os01g53160 and LOC_Os01g53220 were highly induced by heat stress in IL01-15 compared to R53. Sequence comparison of the gene-coding regions of LOC_Os01g53160 and LOC_Os01g53220 between R53 and IL01-15 revealed one synonymous and two nonsynonymous SNPs in exons, respectively. Our results provide a basis for identifying the genes underlying qHTB1-1 and indicate that markers linked to the qHTB1-1 locus can be used to improve the heat tolerance of rice at the booting stage by marker-assisted selection.