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

Abstract

In self-Incompatible (Sl) plants, the S locus acts to prevent growth of self-pollen and thus promotes outcrossing within the species. Interspecific crosses between Sl and self-compatible (SC) species often show unilateral incompatibility that follows the Sl x SC rule: Sl species reject pollen from SC species, but the reciprocal crosses are usually compatible. The general validity of the Sl x SC rule suggests a link between Sl and interspecific pollen rejection; however, this link has been questioned because of s 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 Sl 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 alle-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 letter 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.