Molecular evolution and functional relevance of the chalcone synthase genes of pea

We have isolated seven genomic chalcone synthase (CHS) genes and six classes of CHS cDNA from elicitor-treated pea tissues. Comparison of the nucleotide sequences of the coding regions revealed the existence of eight members of the CHS gene family in pea. These can essentially be divided into three groups (PSCHS1, 2 and 8; PSCHS3, 4 and 5; and PSCHS6 and 7) on the basis of nucleotide and/or amino acid sequence comparisons of the coding regions, introns and promoter regions. We previously reported that the accumulation of CHS mRNAs is induced by elicitor treatment. Accumulation of CHS mRNA was observed mainly in roots and very little was found in floral organs. To specifically detect expression of each CHS gene in various types of pea cells, S1 nuclease protection assays were performed. Interestingly, the classification of the eight members of the CHS gene family based on the sequence identity was found to reflect their expression patterns as determined by the S1 nuclease protection assay. The first group of CHS genes, PSCHS1, 2 and 8, was strongly induced not only by elicitor treatment and UV irradiation but is also constitutively expressed in root and flower tissues. The second group, PSCHS3, 4 and 5, was also strongly induced by elicitor treatment and UV irradiation but is constitutively expressed only in root. Expression of the third group, PSCHS6 and 7 was barely detectable in any of the organs tested and was not influenced by environmental stimuli such as elicitor or UV. Furthermore, sequence analysis of the promoter region of each member of the CHS gene family revealed that putative cis-regulatory elements, such as Box-I, Box-II and G-Box, were conserved only in PSCHSl, 2, 3, 4 and 5. From these results we propose that an ancestral CHS gene might have given rise to defense response-related (UV irradiation- and elicitor-responsive) and -unrelated (unresponsive) genes at an early stage of evolution, followed by divergence within these subclasses based upon the developmental program in pea.