S RNase and interspecific pollen rejection in the genus Nicotiana: Multiple pollen-rejection pathways contribute to unilateral incompatibility between self-incompatible and self-compatible species

In self-incompatible (SI) plants, the S locus acts to prevent growth of self pollen and thus promotes outcrossing within the species, Interspecific crosses between SI and self-compatible (SC) species often show unilateral incompatibility that follows the SI x SC rule: SI species reject pollen from SC species, but the reciprocal crosses are usually compatible, The general validity of the SI x SC rule suggests a link between SI and interspecific pollen rejection; however, this link has been questioned because of a number of exceptions to the rule, To clarify the role of the S locus in interspecific pollen rejection, we transformed several Nicotiana species and hybrids with genes encoding S-A2 Or S-C10 RNase from Si N. alata, Compatibility phenotypes in the transgenic plants were tested using pollen from three SC species showing unilateral incompatibility with N. alata. S RNase was implicated in rejecting pollen from all three species. Rejection of N. plumbaginifolia pollen was similar to S allele-specific pollen rejection, showing a requirement for both S RNase and other genetic factors from N. alata. In contrast, S RNase-dependent rejection of N. glutinosa and N. tabacum pollen proceeded without these additional factors. N. alata also rejects pollen from the latter two species through an S RNase-independent mechanism, Our results Implicate the S locus in all three systems, but it is clear that multiple mechanisms contribute to interspecific pollen rejection.