Abstract
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.
Highlights
The traits of existing crop cultivars can be improved by crossing cultivars or lines to introduce beneficial traits, such as resistance or tolerance to disease or stress, to susceptible cultivars
Increases in the height of the F1 hybrids nearly halted at 50 days after sowing (DAS), whereas that of the parents continued increasing (Figure 2A)
HWA2 loci exhibit growth termination and leaf yellowing after the seedlings developed three or four study determined that F1 hybrids from the cross of ‘A.D.T.14 and ‘P.T.B.7 rice exhibited limited leaves
Summary
The traits of existing crop cultivars can be improved by crossing cultivars or lines to introduce beneficial traits, such as resistance or tolerance to disease or stress, to susceptible cultivars. Plants 2019, 8, 450 their parents, and the phenomenon has been reported to occur in the offspring of crosses involved a number of species, including Oryza sativa [1,2,3,4], Nicotiana spp. One of the causal genes is related to disease resistance (R), and interactions between the R gene and other causal gene cause autoimmune responses in the hybrid offspring [2,7]. The autoimmune responses include the accumulation of reactive oxygen species such as H2 O2 , cell death, upregulation of disease resistance-related genes, and downregulation of photosynthesis-related genes [2,7,15,16]
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