StGCPRP, a potato gene strongly expressed in stomatal guard cells, definesa novel type of repetitive proline-rich proteins

Guard cells represent a highly differentiated cell type within the epidermi s of plant leaves and stems. They respond to many endogenous and environmen tal signals and thereby modify the size of the stomatal pore they surround. We identified a novel gene that is highly expressed in guard cells of pota to (Solanum tuberosum). It encodes a repetitive proline (Pro)-rich protein of 54 kD (491 amino acids) and was named StGCPRP (S. toberosum guard cell P ro-rich protein). StGCPRP has a bipartite structure. The C-terminal part of StGCPRP contains a high percentage (46%) of Pro residues organized in dist inct repetitive sequence motifs, whereas its extended N terminus is essenti ally free of Pros. StGCPRP represents the first member of a novel class of hybrid Pro-rich proteins that we designated NHyPRPs. In young but not in ma ture leaves, StGCPRP transcripts were also present at high levels in mesoph yll cells (in addition to guard cells), indicating developmental regulation of StGCPRP gene expression. In addition, StGCPRP expression is regulated b y environmental factors, as shown by a decrease in StGCPRP transcript level s under drought stress. Two proteins similar to StGCPRP were found to be en coded by the Arabidopsis genome, indicating that NHyPRPs are more widely di stributed in higher plants.