Abstract
Salicylic acid (SA) is a phenolic phytohormone widely believed to regulate plant growth and stress response. Despite its significance, the genetic basis of SA-mediated resistance to biotic stressors in tea plants is little understood. Our study investigated the genetic diversity, population structure, and linkage disequilibrium (LD) patterns of 299 tea accessions using 79 560 high-quality single nucleotide polymorphisms (SNPs) obtained from genotyping-by-sequencing (GBS) data. Our genome-wide association study identified CSS0033791.1, an essential gene encoding 9-cis-epoxycarotenoid dioxygenase (CsNCED1), which catalyzes a vital step in abscisic acid (ABA) biosynthesis. Exogenous ABA treatment and transgenic overexpression of the CsNCED1 gene lowered SA content in the respective tea plants by inhibiting the expression of the ICS gene. Further analysis revealed that ABA could reduce the expression levels of the SA receptor gene (NPR1) and NPR1 target genes (PR1 and WRKY18), increasing the plant's susceptibility to biotic stressors. Furthermore, the feeding behavior of Spodoptera litura revealed that the insect bite area on transgenic leaves was substantially more extensive than that in wild type (WT), implying that the CsNCED1 gene had a negative regulatory role in SA-mediated immune response. This study thus provides the foundation for future insect resistance breeding, sustainable tea plant resource usage, and molecular marker-assisted (MAS) tea plant breeding.