STRUCTURAL AND EVOLUTIONARY RELATIONSHIPS AMONG CHITINASES OF FLOWERING PLANTS

The analysis of nuclear-encoded chitinase sequences from various angio sperms has allowed the categorization of the chitinases into discrete classes. Nucleotide sequences of their catalytic domains were compared in this study to investigate the evolutionary relationships between c hitinase classes. The functionally distinct class III chitinases appea r to be more closely related to fungal enzymes involved in morphogenes is than to other plant chitinases. The ordering of other plant chitina ses into additional classes mainly relied on the presence of auxiliary domains--namely, a chitin-binding domain and a carboxy-terminal exten sion--flanking the main catalytic domain. The results of our phylogene tic analyses showed that classes I and IV form discrete and well-suppo rted monophyletic groups derived from a common ancestral sequence that predates the divergence of dicots and monocots. In contrast, other se quences included in classes I and II, lacking one or both types of au xiliary domains, were nested within class I sequences, indicating that they have a polyphyletic origin. According to phylogenetic analyses a nd the calculation of evolutionary rates, these chitinases probably ar ose from different class I lineages by relatively recent deletion even ts. The occurrence of such evolutionary trends in cultivated plants an d their potential involvement in host-pathogen interactions are discus sed.