Abstract
Within the large Brassica S gene family, SLG (S locus glycoprotein) and SRK (S locus receptor kinase) participate to the control of pollen-stigma self-incompatibility. In the self-compatible species maize, S gene family members are predominantly expressed in vegetative organs but are also expressed to a lesser extent in the stigma (silk). To determine if the expression of any S gene family members correlates with female receptivity, we analyzed their expression in developing maize silks. We show that a large family of maize S transcripts is expressed in developing silks. Surprisingly, we isolated a cDNA complementary to a large portion of the antisense strand of the maize receptor kinase S domain. Rapid amplification of cDNA ends (RACE)-polymerase chain reaction, RNase protection, and Northern hybridization with single-stranded riboprobes confirmed that natural antisense S transcripts exist in leaves and seedling shoots and in all sexual tissues tested except mature pollen. These natural antisense S transcripts co-exist with several less abundant sense S transcripts. The accumulation of sense and antisense S transcripts is differentially regulated during pollen and silk development. Thus, these results support a role for S gene family members in sexual tissue development and/or compatible pollination and reveal a new level of complexity in the regulation and function of the S gene family in maize.
Highlights
Flowering plants have evolved self-incompatibility systems that prevent or substantially reduce the level of self-fertilization
Rapid amplification of cDNA ends (RACE)-polymerase chain reaction, RNase protection, and Northern hybridization with single-stranded riboprobes confirmed that natural antisense S transcripts exist in leaves and seedling shoots and in all sexual tissues tested except mature pollen
Northern blot analysis of silk mRNAs was conducted to analyze whether the accumulation of S domain-related transcripts during development correlated with the acquisition of female receptivity
Summary
Flowering plants have evolved self-incompatibility systems that prevent or substantially reduce the level of self-fertilization. In the genus Brassica, at least two stigmatically expressed genes at the self-incompatibility locus or S locus are required for the recognition and the rejection of self pollen by the stigma: the S locus receptor kinase (SRK) and the S locus. The gene encoding the pollen S determinant that would provide the ligand for the SRK receptor has been identified; it is unrelated to SLG or SRK but belongs instead to a family of genes encoding pollen coat proteins. This gene, termed SCR (S locus cysteine-rich protein) is a single copy, S locus-encoded, anther-expressed gene [15,16,17]. Different members are expressed in reproductive tissues in self-incompatible as well as self-compatible species, but others are expressed predominantly in vegetative structures, so that it is obvious that the S gene family may have diverse roles in plants
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