Genome‐Wide Identification of Terpene Synthase ( <scp>TPS</scp> ) Gene Family in <i>Atalantia buxifolia</i> and the Role of <scp>AbTPS08</scp> in Asian Citrus Psyllid Defense

Huanglongbing/citrus greening, vectored by the Asian citrus psyllid (ACP), is one of the greatest threats to citrus production worldwide. While certain citrus species and citrus relatives are particularly susceptible to ACP, others are repellent or resistant including the Chinese box orange (Atalantia buxifolia) which is a citrus relative. These resistant species are suspected to contain a diverse suite of terpenoids, plant secondary metabolites that are critical to defense and biosynthesized by terpene synthase (TPS) enzymes. Here, we performed a genome-wide survey of terpene synthase (TPS) genes in A. buxifolia and identified 34 AbTPS genes representing five TPS subfamilies. Analyses of the phylogenetic relationships, intron and exon distributions, and conserved motifs among these and homologous genes suggest that the TPSs are highly conserved across plant species. The distinctive expression patterns of the AbTPS genes among plant tissues highlight their tissue-specific roles in the regulation of terpene biosynthesis. Importantly, AbTPS08 was significantly upregulated by both ACP infestation and methyl jasmonate (MeJA) treatment. Biochemical assays of the recombinant AbTPS08 protein confirmed that AbTPS08 could convert farnesyl pyrophosphate (FPP) to α-farnesene in vitro. Furthermore, Agrobacterium tumefaciens-mediated transient overexpression in citrus callus revealed that AbTPS08 exhibits α-farnesene synthase activity in vivo. AbTPS08 was localized in the cytoplasm, the primary compartment for sesquiterpene synthesis. Overexpression of AbTPS08 in lemon (Citrus limon) was repellent to ACPs in dual-choice feeding assays, increased jasmonic acid (JA) accumulation, and induced the transcription of JA biosynthesis-related genes. This study provides novel insights into sesquiterpene biosynthesis in A. buxifolia and demonstrates that α-farnesene can improve ACP defense directly and indirectly.