A functional FRIa variant drives turnip adaptation to the Qinghai-Xizang Plateau

Flowering time is crucial for plant adaptation to regional climate conditions and for reproduction in agricultural production. Turnip (Brassica rapa ssp. rapa) is a traditional adaptive vegetable cultivated on the Qinghai-Xizang Plateau. However, little is known about the potential molecular mechanisms underlying the adaptive evolution of flowering time in turnip in different regions, especially on the Qinghai-Xizang Plateau. In this study, the key adaptive trait of flowering time, which is positively correlated with altitude, was analyzed in 104 turnip accessions grown worldwide. Whole-genome resequencing and Sanger sequencing discovered that the FRIa gene, a key regulator of flowering time in turnip, had sequence variations, with five distinct alleles and eight genotypes being defined. By combining the association analyses of FRIa genotypes and flowering time as well as genetic validation, FRIa1 was identified as a highly functional allele that contributes to delayed flowering of turnip for adaptation to the Qinghai-Xizang Plateau. Four specific SNP variations were determined to be critical for the strong function of FRIa in turnip. Our results provide new insights into the adaptation of flowering time in turnip along altitudinal gradients and shed light on the adaptive evolution of FRIa in turnip on the Qinghai-Xizang Plateau.