THE ARABIDOPSIS ACT11 ACTIN GENE IS STRONGLY EXPRESSED IN TISSUES OF THE EMERGING INFLORESCENCE, POLLEN, AND DEVELOPING OVULES

ACT11 represents a unique and ancient actin subclass in the complex Ar abidopsis actin gene family. We have isolated and characterized the Ar abidopsis ACT11 actin gene and examined its expression. Southern blott ing with a 5' gene-specific probe showed that ACT11 was a single-copy gene in the genome. Northern analysis with a 3' gene-specific probe an d reverse transcriptase-mediated PCR (RT-PCR) using gene-specific prim ers detected ACT11 mRNA at low levels in seedling, root, leaf, and sil ique tissue; at moderate levels in the inflorescence stem and flower: and at very high levels in pollen. The 5' region of the ACT11 gene, in cluding the promoter region, the 5'-untranslated leader, the intron wi thin the leader, and the first 19 actin codons, was fused to a beta-gl ucuronidase (GUS) reporter gene. The expression of the ACT11/GUS fusio n was examined histochemically in numerous independent transgenic Arab idopsis plants. Strong ACT11/GUS activity was detected in rapidly elon gating tissues and organs (e.g., etiolated hypocotyls, expanding leave s, stems) and in floral organ primordia. As the floral buds developed into mature flowers, strong GUS activity was gradually restricted to m ature pollen and developing ovules. ACT11 appears to be the only Arabi dopsis actin gene expressed at significant levels in ovule, embryo, an d endosperm. The unique expression patterns in reproductive organs and the sequence divergence of the ACT11 actin gene suggest that the ACT1 1 isovariant plays distinct and required roles during Arabidopsis deve lopment.