Hypervariable Domains of Self-Incompatibility RNases Mediate Allele-Specific Pollen Recognition.

Abstract

'Self-incompatibility (SI) in angiosperms is a genetic mechanism that promotes outcrossing through rejection of selfpollen. In the Solanaceae, SI is determined by a multiallelic S locus whose only known product is an S RNase. S RNases show a characteristic pattern of five conserved and two hypervariable regions. These are thought to be involved in the catalytic function and in allelic specificity, respectively. When the Solanum chacoense Si&, genotype is transformed with an Sll RNase, the styles of plants expressing significant levels of the transgene reject Sll pollen. A previously characterized S RNase, S13, differs from the Sl1 RNase by only 10 amino acids, four of which are located in the hypervariable regions. When S1Si4 plants were transformed with a chimeric Sii gene in which these four residues were substituted with those present in the SI, RNase, the transgenic plants acquired the SI3 phenotype. This result demonstrates that the S RNase hypervariable regions control allelic specificity.

S-RNase-Based Self-Incompatibility in Petunia: A Complex Non-Self Recognition System Between Pollen and Pistil

“…The results showed that the non-glycosylated S 3 -RNase retained the full ability to reject self-pollen, suggesting that the recognition function of S-RNase resides in its amino-acid sequence . Domain-swapping experiments were performed to examine the role of the two hypervariable domains in allelic specifi city (Kao and McCubbin 1996 ;Matton et al 1997 ;Zurek et al 1997 ;Matton et al 1999 ). Swapping HVa and HVb between S 11 -RNase and S 13 -RNase of Solanum chacoense (which differ by four amino acids in the hypervariable regions) was suffi cient to switch the allelic specifi city (Matton et al 1997(Matton et al , 1999.…”

“…Domain-swapping experiments were performed to examine the role of the two hypervariable domains in allelic specifi city (Kao and McCubbin 1996 ;Matton et al 1997 ;Zurek et al 1997 ;Matton et al 1999 ). Swapping HVa and HVb between S 11 -RNase and S 13 -RNase of Solanum chacoense (which differ by four amino acids in the hypervariable regions) was suffi cient to switch the allelic specifi city (Matton et al 1997(Matton et al , 1999. However, in the case of P. infl ata and N. alata S-RNases examined, the two hypervariable regions are necessary but not suffi cient for allele specifi city.…”

Mechanisms of self-incompatibility in flowering plants

“…Matton et al [49] investigated the role of the HV regions in allelic specificity using another Solanaceous species, Solanum chacoense. This study differed from those described above in that the S-RNases used, the S 11 -and S 13 -RNases, are almost identical, with only ten different amino acids.…”

“…Three of these amino acids are localized to the HVa region and one to the HVb region [50]. Using site-directed mutagenesis, Matton et al [49] substituted all four amino acids in the HV regions of an S 11 -RNase with those of an S 13 -RNase. Transgenic S 12 S 14 S. chacoense plants expressing the chimeric S 11 -RNase successfully rejected S 13 pollen but not S 11 pollen, therefore suggesting that the HVa and HVb regions together are sufficient to control S allele specificity [49].…”

“…Using site-directed mutagenesis, Matton et al [49] substituted all four amino acids in the HV regions of an S 11 -RNase with those of an S 13 -RNase. Transgenic S 12 S 14 S. chacoense plants expressing the chimeric S 11 -RNase successfully rejected S 13 pollen but not S 11 pollen, therefore suggesting that the HVa and HVb regions together are sufficient to control S allele specificity [49]. Furthermore, a subsequent experiment demonstrated that the modification of only three of the four amino acid residues in an S 11 -RNase to match those of an S 13 -RNase resulted in transgenic S 12 S 14 S. chacoense plants that expressed a dual specific S-RNase capable of rejecting both S 11 and S 13 pollen [51].…”