Factors affecting inter- and intraspecific pollen rejection in Nicotiana

Inter- and intra-specific compatibility systems in the genus Nicotiana have been used to identify factors that control pollen recognition. nr. alata has a classic gametophytic self-incompatibility (SI) system in which the specificity of pollen rejection is determined by a multiallelic S-locus. S-RNases are products of the S-locus, and are the factors that determine specificity in the style. In vitro mutagenesis experiments have been conducted to determine how allelic specificity is encoded in the S-RNase sequence. Plant transformation experiments have shown that S-RNases also act as factors controlling interspecific pollen rejection. By examining the effect of S-RNases on inter- and intra-specific compatibility in different generic backgrounds, four different pollen rejection mechanisms can be recognized. S-RNases are implicated in three of these mechanisms. The dependence of pollen rejection on genetic background shows that S-RNases interact with other factors. In general, such factors can be classified in three groups based on their mode of interaction with the S-locus and other pollen-pistil interaction pathways. Some of these factors are now cloned. As more factors are cloned and characterized, it is becoming apparent that pollen-pistil interactions that were once thought to be distinct are actually interrelated. (C) 2000 Annals of Botany Company.