Cultivated tomato has defects in both S‐RNase and HT genes required for stylar function of self‐incompatibility

Summary Cultivated tomato ( Lycopersicon esculentum ), a self‐compatible species, evolved from self‐incompatible (SI) species in the genus Lycopersicon following a breakdown of the self‐incompatibility system. In order to elucidate the molecular basis of this breakdown in L. esculentum , we first analysed the stylar proteins with an in‐gel assay for ribonuclease activity and 2D‐PAGE. No S‐RNase protein or its activity was detected in the style of L. esculentum . We then introduced the S6‐RNase gene from an SI relative, L. peruvianum , into L. esculentum . However, the styles of transgenic plants expressing S 6 ‐RNase at levels comparable to those found in the L. peruvianum style were unable to reject self‐pollen and L. peruvianum pollen in an allele‐specific manner. This indicated that defect in the S‐RNase expression was not the sole reason for the loss of self‐incompatibility in tomato. The asparagine‐rich HT protein, originally identified from the style of Nicotiana alata , is the other stylar factor involved in self‐incompatibility reaction. We cloned and sequenced two distinct genes encoding HT‐A and HT‐B proteins from L. peruvianum ( LpHT‐A and LpHT‐B ) and L. esculentum ( LeHT‐A and LeHT‐B ). A frame shift mutation in the coding sequence of LeHT‐A and a stop codon in the ORF of LeHT‐B were found, and no LeHT‐B transcript was detected in the style of L. esculentum . The results suggest that the breakdown of self‐incompatibility in cultivated tomato is associated with loss‐of‐function mutations in both S‐RNase and HT genes.