Characterization and expression of beta-1,3-glucanase genes in peach

Beta-1,3-glucanase is one of the pathogenesis-related (PR) proteins involved in plant defense responses. A peach beta-1,3-glucanase gene, designated PpGns1, has been isolated and characterized. The deduced amino acid sequence of the product of PpGns indicates that it is a basic isoform (pI 9.8), and contains a putative signal peptide of 38 amino acids but has no C-terminal extension. Amino acid sequence comparisons revealed that PpGns1 is 69% and 67% identical to citrus and soybean beta-1,3-glucanases, respectively. Southern analysis of total genomic DNA also indicates that at least three genes for beta-1,3-glucanases exist in peach, forming a small gene family. Characterization of four additional clones by PCR has identified a second beta-1,3-glucanase gene, PpGns2. PpGns2 has been partially sequenced, and when compared to PpGns1, it shows high sequence homology, 96% and 99% nucleotide identity in the first and (partial) second exons, respectively. The deduced partial sequence of the PpGns2 product displays only two differences from PpGns1 in the signal peptide and one in the (partial) mature protein (141 amino acids). The 5'-flanking promoter regions of these two genes share 90% identity in nucleotide sequences interrupted by five major gaps (4-109 nt long). The promoter region contains various sequences similar to cis-regulatory elements present in different stress-induced plant genes. In leaves and stems of peach shoot cultures grown in vitro, PpGns1 is induced within 12 h after exposure to a culture filtrate of Xanthomonas campestris pv. pruni or ethephon. However, it is not induced following treatment with mercuric chloride.