FRUITFULL2 controls tomato fertility through style length and pollen quality

Abstract

Tomato reproductive success and yield are particularly vulnerable to the negative effect of heat stress leading to stigma exsertion (protrusion) and lower pollen viability, both interfering with fertilization. Thus, understanding the regulation of these two traits in tomato is crucial for the yield and quality of the crop. Here, we found that knocking out the tomato MADS-domain transcription factor FRUITFULL2 (FUL2) function leads to a higher incidence of parthenocarpy in tomato. This phenotype was primarily due to impeded self-pollination as a consequence of the higher frequency of stigma exsertion and lower fertilization rates due to reduced pollen quality. Stigma exsertion in ful2 mutants, in contrast to heat stress-induced exsertion, was caused by style elongation, particularly in the younger flowers of a truss. Interestingly, Quantitative Trait Loci for style elongation, stigma exsertion, and pollen viability map close to the position of FUL2 on chromosome 3, making it a candidate gene underlying these QTLs. At the molecular level, ful2 mutant styles have higher expression of Style2.1 and SE3.1, which are known as positive regulators of style length. In addition, after reducing the impact of style exsertion and low pollen quality by manual pollination with wild-type pollen, ful2 mutants exhibited reduced fruit size independent of seed number. This study reveals the contributions of flower number, style length, and pollen quality, as affected by FUL2, to tomato fertility and fruit size.