Extensin-Like Protein OsPEX1 Affects Seed Germination by Modulating Water Permeability of Seed Coat in Rice

Cutin and suberin in the rice seed coat serve as a lipid barrier that controls water entry into the seed. However, the molecular mechanisms underlying the permeability properties of seed coats remain largely unknown. In this study, we revealed that the rice EXTENSIN-like protein OsPEX1 influences seed germination by modulating water permeability in the seed coat. The germination rate of the gain-of-function pex1/pex1 mutant was lower than that of the wild-type seeds, although their seed vitality was comparable to that of the wild type. Morphological observations revealed that embryo growth in the pex1/pex1 mutant was inhibited. Further investigation indicated that water absorption in the seed coat around the micropyle was impaired during the early stage of germination. Integrated transcriptional and metabolic analyses suggested that the increased deposition of cutin and suberin may be associated with the overexpression of OsClo5, a transcriptional repressor, in the mutant. It is proposed that abnormal deposition of cutin and suberin in the seed coat may be the major influencing factors contributing to the reduced impermeability of pex1/pex1 seeds. These results indicate that OsPEX1 plays a critical role in modulating water absorption in the seed coat during seed germination.