Dissection of a pollen-specific promoter from maize by transient transformation assays

We have previously reported the isolation and characterization of a gene (Zm13) from Zea mays which shows a pollen-specific pattern of expression. Stably transformed tobacco plants containing a reporter gene linked to portions of the Zm13 5' flanking region show correct temporal and spatial expression of the gene. Here we present a more detailed analysis of the 5' regions responsible for expression in pollen by utilizing a transient expression system. Constructs containing the beta-glucuronidase (GUS) gene under the control of various sized fragments of the Zm 13 5' flanking region were introduced into Tradescantia and Zea mays pollen via high-velocity microprojectile bombardment, and monitored both visually and with a fluorescence assay. The results suggest that sequences necessary for expression in pollen are present in a region from -100 to -54, while other sequences which amplify that expression reside between -260 and -100. The replacement of the normal terminator with a portion of the Zm13 3' region containing the putative polyadenylation signal and site also increased GUS expression. While the -260 to -100 region contains sequences similar to other protein-binding domains reported for plants, the -100 to -54 region appears to contain no significant homology to other known promoter fragments which direct pollen-specific expression. The microprojectile bombardment of Tradescantia pollen appears to be a good test system for assaying maize and possibly other monocot promoter constructs for pollen expression.