Abstract：Caffeic acid O-methyltransferase (COMT) catalyzes the preferential formation of syringyl (S) monolignol subunits, and acts as a key enzyme in lignin synthesis. COMT is implicated in multiple physiological processes in plants, e.g. the functioning of plant vasculature, and defense responses to biotic and abiotic stresses. Up to now, no literature has been available in the cloning and characterization of COMT genes in Hevea brasiliensis (para rubber tree). Previously, we showed that the levels of a COMT protein increased markedly with tapping in the latex of reopened rubber trees. The expressions of this COMT protein correlated well with the patterns of tapping-enhanced latex yields. Here, by searching the assembled latex EST library (20126 high-quality Sanger ESTs, with average length of 575 bp), a contig annotated as COMT was obtained. Based on the sequence of this contig, the full-length cDNA (1 312 bp) that corresponded to a Hevea COMT gene was then cloned by PCR, and named as HbCOMT1(GenBank accession No. GI: 443908530). The genomic clone of HbCOMT1 (1 926 bp) that comprised the complete protein-coding region was also cloned, consisting of four exons and three introns. HbCOMT1 predicted a protein of 368 amino acids with a molecular weight of 40.58 kD and an isoelectric point of 5.46. HbCOMT1 shared the hallmarks of typical plant O-methyltransferases. Phylogenetic analysis put HbCOMT1 and the COMTs of Ricinus communis and Vitis vinifera into one cluster, and the COMTs of the other 11 plant species into the other cluster. Using Real-time quantitative PCR, the expression patterns of HbCOMT1 were examined in different Hevea tissues and in response to a series of treatments. HbCOMT1 mRNA accumulated most abundantly in latex, then in leaf and bark, low in flower and bud, and undetectably in seed. The expression of HbCOMT1 in latex was up-regulated by tapping in re-opened rubber trees, having about 3-fold increase at the fifth tapping. The transcripts of HbCOMT1 were stimulated markedly by wounding, showing an increase of more than 15-fold after 24 h of wounding treatments. Compared with the expression levels in the latex of healthy rubber trees, HbCOMT1 expressions were up-regulated in the trees affected with moderate tapping panel dryness (TPD), but down-regulated in the trees with severe TPD. However, HbCOMT1 expressions were affected little by Ethrel (2-chloroethylphosponic acid, an ethylene releaser) treatment. In brief, the paper reported the cloning and molecular characterization of a COMT gene from H. brasiliensis, which predicted a role in stress response and the regulation of latex flow in the laticifers. The results will be beneficial to further functional characterization of this COMT gene.
