Effects of RNases on rejection of pollen from Nicotiana tabacum and N-plumbaginifolia

S-RNases are implicated in both inter- and intra-specific pollen rejection in Nicotiana. At least two mechanisms contribute to S-RNase dependent rejec tion of pollen from self compatilble species such as Nicotiana plumbaginifo lia and N. tabacum. Cloned S-RNases from self incompatible N. alata express ed in styles of self compatible N. tabacum cause rejection of N. tabacum po llen through a factor-independent mechanism. However, rejection of N. plumb aginifolia pollen occurs only when S-R-Nases are expressed in conjunction w ith non-S-RNase factors from N. alata (factor-dependent pollen rejection). Here, we compared the pollen rejection activity of four RNases in these two systems. Recombinant RNase expression levels in the factor-dependent N. pl umbaginifolia system were insufficient to cause pollen rejection. However, three S-RNases were active in the factor-independent N. tabacum pollen reje ction system. This system shows the broadest specificity of any system so f ar examined. However, RNaseI from E. coli could not cause N. tabacum pollen rejection. Thus, RNase activity alone is not sufficient for pollen rejecti on. Our results suggest that S-RNases are specially adapted to function in pollen rejection.