| The gene HVA22 (Hordeum vulgare Abscisic acid induced gene #22) was initially identified as an ABA inducible transcript in barley aluerone layers, and later found to be induced in vegetative tissues exposed to ABA, drought, or cold stress. Although its pattern of expression suggests a role in stress response, its actual function is not clear. Comparison of HVA22 to sequence databases reveals homologs present in diverse eukaryotic organisms, including Arabidopsis thaliana and Saccharomyces cerevisiae, with all of these homologs having been initially identified as part of sequencing projects. The yeast HVA22 homolog, YOP1, was found to be a non-essential gene, but also regulated by stress. Two-hybrid analysis indicates it may form a dimer in vivo , possibly by way of a leucine zipper. Deletion of the YOP1 coding sequence resulted in only a mild growth phenotype at 37°C. To identify genetic interactions with YOP1 and obtain a more informative phenotype, a synthetic enhancement screen was performed in a yop1Δ background. This resulted in the identification and cloning of a single gene, designated SEY1 (Synthetic Enhancement of Yop1Δ). SEY1 is homologous to the Arabidopsis gene RHD3 (Root Hair Defective). RHD3 mutants show defective root hairs that accumulate transport vesicles at their tips. A sey1Δ mutant shows no detectable phenotype in an otherwise wild type background. However, yop1Δ/sey1 double mutants accumulate transport vesicles and display inefficient secretion of invertase, indicating a defect in the secretory pathway. We propose that HVA22/YOP1 acts to regulate secretion in response to environmental conditions and the changing needs of the organism. |
