Poster: Membrane Transport

Abs # 1235: ACA9, a calmodulin-activated calcium pump involved in pollen growth and fertilization

Presenter:	Harper, Jeffrey F, harper@scripps.edu
Authors	Harper, Jeffrey F (A)   Schiøtt, Morten  (B)   Romanowsky, Shawn  (B)   Palmgren, Michael  (A)
Affiliations:	(A): The Scripps Research Institute (B): The Royal Vetrinary and Agricultural University

Calcium signals play an important role in plant growth and development, including key aspects of pollen tip growth and fertilization. The dynamics of a calcium signal are largely controlled by influx (through channels) and efflux (through pumps and antiporters). Efflux pathways include antiporters and pumps. We have isolated homozygous T-DNA disruptions in all 14 of the Arabidopsis calcium pumps. Mutations in ACA9 (autoinhibited calcium ATPase, isoform 9) were the only examples to show an obvious phenotype with respect to reproductive functions. Homozygous aca9 mutants (three independent alleles) all showed reduced seed-set (85% decrease). This appears to result from a pollen defect, as supported by the observation that wild-type pollen can produce full seed-set when used to fertilize a homozygous aca9. A pollen phenotype is consistent with evidence for pollen-specific expression of ACA9, as revealed by promoter::GUS-fusions. Mutant pollen have at least two defects. First, they show reduced growth potentials in vitro and in vivo. This results in a decreased frequency of pollen tubes reaching ovules in the distal end of the pistil. Second, mutant pollen displayed a block in fertilization in more than 50% of the events in which pollen tubes reached the ovule micropyle. These blocked fertilization events provide an explanation for the partial seed set reduction (25% decrease) observed as a semi-dominant phenotype in heterozygotes. This is the first genetic evidence for the role of a specific calcium transporter in pollen growth and fertilization.