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 (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 SA2 or SC10 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.

NaStEP: A Proteinase Inhibitor Essential to Self-Incompatibility and a Positive Regulator of HT-B Stability inNicotiana alataPollen Tubes

“…However, pollen from other SC species (N. tabacum, N. glauca, and N. benthamiana) clearly does not require NaStEP. This is consistent with other studies showing that multiple mechanisms contribute to interspecific pollen rejection (Murfett et al, 1996). Taken together, our results are congruent with a role for NaStEP as a Figure 6.…”

“…Interspecific pollen rejection occurs through multiple and complex mechanisms, which in Nicotiana spp. happen in both an S-RNase-dependent and an S-RNaseindependent manner (Pandey, 1981;Murfett et al, 1996). Interspecific pollen rejection mediated by SRNases has been shown in some instances to depend strictly on modifier genes common to SI, such as HT-B and 120K (McClure et al, 1999;Hancock et al, 2005).…”

“…SI Nicotiana alata (S 105 S 105 , S C10 S C10 ), SC Nicotiana glauca, and SC Nicotiana tabacum 'Praecox' have been described previously (Murfett et al, 1994(Murfett et al, , 1996Beecher and McClure, 2001). SC Nicotiana plumbaginifolia (inventory no.…”

“…Although S-RNase and SLF/SFB define pollen rejection S-specificity, modifier genes unlinked to the S-locus are required for self-incompatibility (SI; Martin, 1968;Ai et al, 1991;Murfett et al, 1996;Tsukamoto et al, 1999).…”

Unilateral incompatibility gene ui1.1 encodes an S-locus F-box protein expressed in pollen of Solanum species

“…This unidirectional pattern of pollen rejection is referred to as the "SI × SC rule" (12). Although the mechanisms of pollen recognition and rejection by UI are complex (13), several SI factors, including S-RNase, CUL1, and HT, also function in UI (8,14,15).…”