RNASEI FROM ESCHERICHIA-COLI CANNOT SUBSTITUTE FOR S-RNASE IN REJECTION OF NICOTIANA-PLUMBAGINIFOLIA POLLEN

Unilateral incompatibility often occurs between self-incompatible (SI) species and their self-compatible (SC) relatives. For example, SI Nic otiana alata rejects pollen from SC N. plumbaginifolia, but the recipr ocal pollination is compatible. This interspecific pollen rejection sy stem closely resembles intraspecific S-allele-specific pollen rejectio n. However, the two systems differ in degree of specificity. In SI, re jection is S-allele-specific, meaning that only a single S-RNase cause s rejection of pollen with a specific S genotype. Rejection of N. plum baginifolia pollen is less specific, occurring in response to almost a ny S-RNase. Here, we have tested whether a non-S-RNase can cause rejec tion of N. plumbaginifolia pollen. The Escherichia coli ma gene encodi ng RNaseI was engineered for expression in transgenic (N. plumbaginifo lia x SC N. alata) hybrids. Expression levels and pollination behavior of hybrids expressing E. coli RNaseI were compared to controls expres sing S-A2-RNase from N. alata. Immunoblot analysis and RNase activity assays showed that RNaseI and S-A2-RNase were expressed at comparable levels. However, expression of S-A2-RNase caused rejection of N. plumb aginifolia pollen, whereas expression of RNaseI did not. Thus, in this system, RNase activity alone is not sufficient for rejection of N. pl umbaginifolia pollen. The results suggest that S-RNases may be special ly adapted to function in pollen rejection.