A population-scale isoform atlas enables isoform-based trait association and regulatory analysis in lettuce

Abstract

Genomic variation shapes phenotypic diversity, yet how genetic variation contributes to transcript isoform diversity remains poorly understood at the population scale. Here, we integrated transcriptomic data from 240 lettuce accessions with a lettuce pangenome to construct a comprehensive pan isoform atlas comprising 531,657 isoforms from 49,729 transcribed loci. Isoform presence absence variation (iPAV) captured extensive transcript diversification during domestication and horticultural differentiation, providing additional resolution beyond locus-level analyses, particularly within loci shared across the population. To facilitate the systematic analysis of isoform trait associations, we developed ISONET-GWAS, a computational framework that integrates isoform-based association mapping, functional annotation, and network-based candidate prioritization. ISONET-GWAS recovered the known chlorophyll regulator LsGLK, identified 455 metabolite isoform associations across 77 metabolites, and prioritized candidate regulators of chlorophyll metabolism supported by functional validation. Genome-wide splicing quantitative trait locus (sQTL) mapping identified 10,498 sQTL associations, revealing widespread distal genetic associations with alternative splicing and candidate trans-sQTL hotspots. Notably, we identified a trans-sQTL hotspot overlapping LsPPR, which encodes a pentatricopeptide repeat RNA-binding protein. Perturbation of LsPPR resulted in widespread changes in alternative splicing of floweing-related genes, accompanied by delayed bolting and flowering and altered splicing of flowering-related genes. Together, our study establishes a population-scale pan-isoform resource and a broadly applicable analytical strategy for isoform-level trait association, candidate prioritization, and complementary analysis of the genetic regulation of alternative splicing in crops.