CHARACTERIZATION OF A RICE POLLEN-SPECIFIC GENE AND ITS EXPRESSION

Pollen development requires a large number of genes expressed in both sporophytic and gametophytic tissues. We have isolated a pollen-specific gene, PSI, from rice. PSI is a unique gene in the rice genome and encodes a 164 amino acid long protein. RNA blot analysis shows that PSI mRNAs accumulate specifically in rice anthers. When introduced into rice tissues by microprojectile bombardment, the PSI promoter drives expression of a marker gene, B-glucuronidase, specifically in rice pollen. The PSI gene and the deduced amino acid sequence of the PSI protein share significant levels of homology with another monocot pollen-specific gene-the maize Zm-13;l gene and its deduced protein, respectively. PSI also shows some homology with the dicot tomato anther-specific gene LAT-52. Interestingly, the structure of the PSI gene is more similar to that of the LAT-52 gene than to Zm-13. The coding regions of both PSI and LAT-52 are interrupted by a single intron, and the positions of the introns are conserved in these genes. Moreover, there is considerable sequence homology in the introns of the PSI and LAT-52 genes in regions immediately upstream of the 3' splice sites. The upstream regulatory sequences of the PSI gene show several regions of homology with other pollen- or anther-specific genes from a number of plant species. The conservation of coding sequences of PSI from rice, Zm-13 from maize, and LAT-52 from tomato suggests a functional conservation of their gene products. Similarities in the regulatory regions of PSI and other anther- or pollen-specific genes among monocotyledonous and dicotyledonous species indicate that at least some regulatory features controlling gene expression in male reproductive tissues are conserved. This is supported by the preservation of pollen-specific expression from the rice PSI promoter when it is introduced into tobacco plants by Agrobacterium Ti plasmid-mediated transformation.