miR166/ SlHB15 Regulatory Module Controls Ovule Development and Parthenocarpic Fruit Set under Adverse Temperatures in Tomato

pf1 is a tomato mutant showing defective ovules and parthenocarpic fruit set under optimal and extreme heat and cold conditions. PF1 causal gene was cloned by bulk-sequence analysis and shown to correspond to the class-III homeodomain leucine-zipper transcription factor SlHB15. Under cold condition, SlHB15 is subjected to conditional haploinsufficiency and recessive dosage sensitivity controlled by miR166. The comparison of RNAseq transcriptomes of pf1 and wildtype ovules showed that defective ovules mimic wildtype pollinated ovules in parthenocarpic mutant pf1. DAPseq and RNAseq analyses revealed SlHB15 is a bifunctional transcription factor, expressing in the ovule integument. SlHB15 binds the promoters of auxin genes to repress auxin signaling and to ethylene genes to activate their expression. Survey of tomato genetic biodiversity identified pat and pat-1, two historical parthenocarpic mutants, as alleles of SlHB15. Our finding demonstrates the role of SlHB15 as sentinel to prevent fruit set in absence of fertilization and provides a mean to enhance fruiting under extreme temperatures.