The Origin and Evolution of Mimosine Degrading Enzymes within the Mimosoideae (Leguminosae)

Mimosine is a novel non-protein amino acid found in some members of the Mimosoideae subfamily of legumes. This unique secondary metabolite has been discussed as important in pest/disease resistance and as a carbon/nitrogen reservoir. However, the presence of mimosine induces various physiological disorders in vertebrate herbivores including infertility, birth defects and hair loss. This action limits the potential utility of these plants being used as a fodder. With the dry weight of seeds and leaves in some species reaching 6-10% mimosine, the evolution of novel mimosine degrading enzyme (MIMOSINASE) systems is of general interest to both basic and applied science. MIMOSINASE enzymes have at least two independent origins, one in mimosoid legumes (e,g., Leucaena and Mimosa) and the other in a few bacteria associated with mimosoid legumes. However, little is known about the genomic origin(s) or diversification of MIMOSINASE encoding genes in legumes. In the present study, we used the available L. leucocephala cDNA sequence as a reference in BLAT searches against 27 de novo assembled mimosoid transcriptomes. The recovered transcripts included sequences most similar to the reference (likely functional MIMOSINASES) and members of the CYSTATHIONINE BETA-LYASE gene family. Consequently, we conducted a series of phylogenetic analyses including all our sequences and other similar sequences from GenBank. Well supported phylogenetic results were consistent with the putative MIMOSINASE sequences being derived from CYSTATHIONINE BETA-LYASE. The future characterization of MIMOSINASE activity derived from these diverse sequences may be of significance to plant breeding programs focused on the manipulation of mimosine content.