Genome-Wide Analysis of the Metallocarboxypeptidase Inhibitor Family Reveals That AbMCPI8 Affects Root Development and Tropane Alkaloid Production in Atropa belladonna

<i>Atropa belladonna</i> is a medicinal plant and an important source for the commercial production of tropane alkaloids (TAs), such as scopolamine and hyoscyamine, which are used clinically for their anticholinergic properties. In this study, we identified 16 metallocarboxypeptidase inhibitor (MCPI) genes from <i>A. belladonna</i> (<i>AbMCPIs</i>), which are grouped into three subgroups based on phylogenetic relationships and are distributed across 10 chromosomes. Promoter analysis showed that most <i>cis-regulatory</i> elements were related to defense and stress responses, such as drought, low-temperature, ABA (abscisic acid), GA (gibberellin), auxin, light and MeJA responsiveness. A gene encoding a putative metallocarboxypeptidase inhibitor (<i>AbMCPI8</i>) is cloned from <i>A. belladonna</i> and characterized. <i>AbMCPI8</i> shows similar tissue expression pattern to TA biosynthesis genes such as <i>AbPMT</i>, <i>AbAT4</i>, <i>AbTRI</i>, etc., with exclusive expression in the roots. When <i>AbMCPI8</i> is silenced by virus-induced gene silencing (VIGS), the root growth is markedly inhibited and the production of hyoscyamine and scopolamine is significantly reduced. Our findings indicate a positive role of AbMCPI8 in root development, which could positively affect TA production in <i>A. belladonna</i>.