HWA1- and HWA2-Mediated Hybrid Weakness in Rice Involves Cell Death, Reactive Oxygen Species Accumulation, and Disease Resistance-Related Gene Upregulation.

Hybrid weakness is a type of reproductive isolation in which F₁ hybrids of normal parents exhibit weaker growth characteristics than their parents. F₁ hybrid of the Oryza sativa Indian cultivars 'P.T.B.7' and 'A.D.T.14' exhibits hybrid weakness that is associated with the HWA1 and HWA2 loci. Accordingly, the aim of the present study was to analyze the hybrid weakness phenotype of the 'P.T.B.7' × 'A.D.T.14' hybrids. The height and tiller number of the F₁ hybrid were lower than those of either parent, and F₁ hybrid also exhibited leaf yellowing that was not observed in either parent. In addition, the present study demonstrates that SPAD values, an index correlated with chlorophyll content, are effective for evaluating the progression of hybrid weakness that is associated with the HWA1 and HWA2 loci because it accurately reflects degree of leaf yellowing. Both cell death and H₂O₂, a reactive oxygen species, were detected in the yellowing leaves of the F₁ hybrid. Furthermore, disease resistance-related genes were upregulated in the yellowing leaves of the F₁ hybrids, whereas photosynthesis-related genes tended to be downregulated. These results suggest that the hybrid weakness associated with the HWA1 and HWA2 loci involves hypersensitive response-like mechanisms.