Silencing of <i>MeCOMT8a/b</i> Enhances Adventitious Root Development and Drought Tolerance in Cassava by Modulating <i>O</i> -Methylated Flavonoid Metabolism

Caffeic acid O-methyltransferase (COMT) is a pivotal enzyme in phenylpropanoid biosynthesis. We identified 39 COMT genes in cassava. Among these, the proximal duplicate gene pair MeCOMT8a and MeCOMT8b (MeCOMT8a/b) was a key candidate associated with lignin and flavonoids. Co-silencing of MeCOMT8a and MeCOMT8b promoted adventitious root (AR) growth but reduced lignin and flavonoid content. Multiomics analyses revealed downregulation of phenylpropanoid- and flavonoid-related genes, and decreased O-methylated flavonoids, such as laricitrin. In vitro enzymatic assays confirmed that both MeCOMT8a and MeCOMT8b efficiently catalyzed the conversion of myricetin to laricitrin. Silencing of MeCOMT8a/b also led to elevated auxin levels and upregulation of auxin-responsive genes, providing a mechanistic explanation for the enhanced AR phenotype. Consequently, MeCOMT8a/b-silenced plants exhibited enhanced drought tolerance, attributable to a more robust root system and reduced oxidative damage. Thus, silencing of MeCOMT8a/b enhances adventitious root development and drought tolerance in cassava by modulating O-methylated flavonoid metabolism.