Functional and evolutionary analyses of pollen coat lipids and proteins in Arabidopsis.

摘要

In both animals and plants, mate selection and fertilization are highly regulated processes governed by many layers of cellular communication between male and female partners. An early step in plant reproduction is the selective hydration of pollen grains by the stigma, the receptive organ at the distal end of the female reproductive structure. Pollen hydration is controlled by factors in the protein-lipid rich pollen coat on the outer surface of the pollen grain; mutants lacking the pollen coat are not recognized by the stigma and remain desiccated. I cloned CER6, a very-long-chain lipid synthesis gene required for the pollen coat function. I identified the lesions in two independent mutant alleles of this gene as well as an intragenic suppressor, cer6-2R. Pollen from cer6-2R plants has intermediate levels of pollen coat lipids, and unlike either of the original mutant alleles, has a morphologically normal pollen coat and is hydrated by the stigma. The predominant proteins in the pollen coat are a family of g&barbelow;lycine-r&barbelow;ich p&barbelow;roteins (GRP) found together in a genomic cluster. The largest and most abundant of these plays a role in hydration of the pollen grain. These genes are similar in structure; the first exon of each encodes a lipid-binding oleosin domain, and the second encodes a basic, repetitive, glycine- or alanine-rich domain. Here, I demonstrate that the glycine rich domains of all but the shortest of these genes are highly repetitive, contain numerous in frame insertions and deletions and are highly variable both within Arabidopsis thaliana ecotypes and between 5 closely related species. The exons encoding the GRP domains are significantly more divergent between these species than the genes adjacent to this cluster. Even though insertions, deletions and mutations are prevalent in these genes, pseudogenes were not identified indicating a strong selective pressure to maintain this cluster of genes. The rapid changes in these genes are consistent with a function in reproduction as many genes mediating reproductive success are highly variable within and/or between species. Moreover, the rapid changes observed in these genes maybe facilitated by their repetitive nature.