Three 4-coumarate : coenzyme A ligases in Arabidopsis thaliana represent two evolutionarily divergent classes in angiosperms

The enzyme 4-coumarate:CoA ligase (4CL) plays a key role in channelling car bon flow into diverse branch pathways of phenylpropanoid metabolism which s erve important functions in plant growth and adaptation to environmental pe rturbations. Here we report on the cloning of the 4CL gene family from Arab idopsis thaliana and demonstrate that its three members, At4CL1, At4CL2 and At4CL3, encode isozymes with distinct substrate preference and specificiti es. Expression studies revealed a differential behaviour of the three genes in various plant organs and upon external stimuli such as wounding and UV irradiation or upon challenge with the fungus, Peronospora parasitica. Phyl ogenetic comparisons indicate that, in angiosperms, 4CL can be classified i nto two major clusters, class I and class II, with the At4CL1 and At4CL2 is oforms belonging to class I and At4CL3 to class II. Based on their enzymati c properties, expression characteristics and evolutionary relationships, At 4CL3 is likely to participate in the biosynthetic pathway leading to flavon oids whereas AtCL1 and At4CL2 are probably involved in lignin formation and in the production of additional phenolic compounds other than flavonoids.