Abstract text: The CAZy auxiliary activity 3 (AA3) family enzymes are FAD-containing GMC oxidoreductases that catalyze the oxidation of diverse carbohydrates and alcohols. Although most of the AA3 studies focus on fungi, AA3 encoding genes are also widely distributed across the plant kingdom, yet their biological functions remain largely unknown. Phylogenetic analyses revealed that plant AA3s form a distinct clade from microbial AA3s, suggesting their structural and functional divergence that may give rise to unique enzymatic properties.

In this study, we explored the enzymatic activities and biological roles of largely uncharacterized plant AA3s using Arabidopsis thaliana as a model. Among the twelve members identified in Arabidopsis, two genes (AtAA3A and AtAA3B) are expressed in root and stem, while the remaining members are mostly expressed in reproductive tissues. We identified that AtAA3A and AtAA3B exhibit activity toward monolignols and contribute to cell wall lignification. In addition, we characterized a long-chain alcohol oxidase that is essential in pollen wall formation (AtAA3F).

These findings provide the first insight into the diverse roles of AA3 family enzymes in plant development and metabolism, which highlight their potential role in production and processing of biomass and biopolymer utilization.