Genome-wide identification of the EXO70 genes to elucidate their potential roles for intraspecific cross-incompatibility in sweet potato (Ipomoea batatas L.)

Crossbreeding is the primary approach for sweet potato improvement, however, frequent cross-incompatibility during intraspecific hybridization remains a major bottleneck in breeding programs. EXO70 proteins, which regulate vesicle secretion during pollen germination, have been reported to play important roles in self-incompatibility in <i>Brassicaceae</i>. To investigate the potential involvement of <i>EXO70</i> genes in intraspecific cross-incompatibility in sweet potato, members of the <i>EXO70</i> gene family were systematically identified from the sweet potato genome and analyzed for their molecular characteristics as well as expression patterns across different tissues. A total of 35 <i>EXO70</i> genes <i>(IbEXO70)</i> were identified in sweet potato (<i>Ipomoea batatas</i>). Phylogenetic analysis classified these genes into three branches and nine subgroups, showing similar gene number and subgroup distributions to those in the diploid wild relatives of <i>Ipomoea trifida</i> and <i>Ipomoea triloba</i>, although differences were observed in chromosomal distribution and conserved protein motif composition. Gene structure analysis revealed that members of the <i>IbEXO70A</i> subgroup contained a higher number of exons and introns. Tissue specific expression profiling indicated that nine <i>IbEXO70</i> genes were significantly upregulated in compatibly pollinated stigmas compared with incompatible or unpollinated stigmas. Among these, <i>IbEXO70-26</i>, belonging to the <i>EXO70H</i> subgroup, was identified as a strong candidate regulator of cross-incompatibility due to its highest and stigmas and pollen specific expression, particularly under compatible pollination conditions. These results were further supported by transcriptomic comparisons between compatible and incompatible samples, and subcellular localization analysis showed that <i>IbEXO70-26</i> protein was localized to the nucleus. This study provides a comprehensive characterization of the <i>EXO70</i> gene family in sweet potato and lays a solid foundation for future functional studies of compatibility factors involved in pollen-stigma interactions.