Evaluation of 515 expressed sequence tags obtained from guard cells of Brassica campestris

As an attempt to examine the transcripts expressed in a single cell type and to unveil the physiology of guard cells at the molecular level, we generated 515 expressed sequence tags (ESTs) from a directional cDNA library constructed from guard-cell protoplasts of Brassica campestris L. ssp. pekinensis. A comparative analysis of the guard-cell ESTs against the National Center for Biotechnological Information non-redundant protein database revealed that 133 ESTs (26%) have significant similarity to protein coding sequences in the database. Among them were 35 clones related to genes that have not yet been identified in higher plants. Analysis of RNA gel blots of 14 database-matched clones revealed that five clones harbor the sequences for mRNAs expressed most abundantly in guard cells, one of them detecting an mRNA with highly preferential expression in guard cells. Functional categorization of the putatively identified guard-cell ESTs showed, when compared with maize leaf ESTs, that guard cells expressed a higher proportion of signal transduction components and a lower proportion of structural or photosynthetic genes, as is consistent with the roles of guard cells.