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The Acyl-CoA ligase-like (ACLL) gene family in Arabidopsis and poplar Souza, Clarice de Azevedo


Many genes of unknown function have been annotated in plant genome projects, and many of these may encode undiscovered enzymes. For example, completion of the Arabidopsis thaliana genome sequence revealed large families of phenylpropanoid-like enzymes of unknown functions. Using an in silico similarity search based on the amino-acid sequences of known Arabidopsis genes encoding 4-coumarate:CoA ligase (4CL), I identified nine putative genes as members of the Arabidopsis acyl-CoA ligase-like (ACLL) gene superfamily which encode a plant-specific clade of enzymes closely related to true 4CLs. I also identified all ACLLs in the fully sequenced poplar and rice genomes. Phylogenetic analysis of amino-acid sequences revealed five ACLL clades, each containing at least one ACLL member from each species, suggesting conserved biochemical functions for ACLL enzymes. In four of five clades, most of the ACLL representatives have the PTS1 peroxisomal target sequence, indicating a likely function in that organelle. I established tissue expression profiles and the wound and herbivory responsiveness of Arabidopsis and poplar ACLL genes, and this revealed similar expression patterns for potentially orthologous genes. Finally, I mined publicly available microarray databases for co-expressed Arabidopsis genes, and this data provides clues for potential ACLL biochemical functions. The only non-peroxisomal clade is the one most closely related to true 4CLs and contains a single copy gene in Arabidopsis (ACLL5) and poplar (ACLL13). These genes are flower and anther-preferred in expression, and because of the apparent conservation in sequence and in expression, were chosen for functional analysis. ACLL5 is transiently expressed in tapetum cells just prior to release of microspores from tetrads, suggesting a role in pollen wall and/or sporopollenenin formation. In support of this, an acll5 transposon insertion mutant is male sterile and fails to produce pollen grains. These data suggest that ACLL5 and similar enzymes from other species, produce CoA ester intermediates used in an unknown pathway required for pollen wall formation. In silico co-expression analysis in Arabidopsis has revealed potential other members of this pathway, also conserved across angiosperms. This work highlights the utility of the Arabidopsis model system in the discovery of genes in other plant species with genome sequence information.

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