SlIDL6–SlHSL1/2/3 ligand-receptor pairs regulate tomato pedicel abscission

We isolated three HAESA-like homologs, SlHSL1/2/3, which are involved in tomato flower abscission. SlHSL1/2/3 are the receptors of SlIDL6 in pedicel abscission and flower abscission under low light. Ethylene action inhibitor 1-methylcyclopropene (1-MCP) can significantly depress the expression of SlHSL1/2/3 and the abscission regulated by SlHSL1/2/3 were partially dependent on ethylene. Flower and fruit abscission reduce crop yield, so decreasing abscission is a significant agricultural issue. HAESA (HAE) and HAESA-like2 (HSL2) kinases and its ligand, INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) peptide, have been confirmed to be the core elements regulating floral organ abscission in Arabidopsis thaliana . Our earlier research revealed that SlIDL6, a homolog of IDA in tomato, functions similarly to AtIDA regulating the abscission of tomato flower organs. Here, we further isolated three HAESA-like homologs, SlHSL1/2/3, which are involved in tomato flower abscission. SlHSL1/2/3 are highly expressed in the abscission zone (AZ). The knockout mutant lines of Slhsl1, Slhsl2 , and Slhsl3 showed lower flower pedicel abscission than wildtype (WT). The double mutant of Slhsl1Slhsl2, Slhsl1Slhsl3 , and Slhsl2Slhsl3 further depressed abscission than each of the single mutant lines, while triple mutants Slhsl1Slhsl2Slhsl3 exhibited the lowest abscission, indicating that SlHSL1/2/3 mediated abscission is non-redundancy, at least partially. Treating tomato pedicel explants with SlIDL6 peptide significantly accelerated pedicel abscission in WT, but had little effect on the abscission rate of SlHSL1 /2/3 knockout lines, indicating that SlHSL1/2/3 are the receptors of SlIDL6 in pedicel abscission. Ethylene action inhibitor 1-methylcyclopropene (1-MCP) can significantly depress the expression of SlHSL1/2/3 . Ethylene can significantly accelerate the abscission of WT, while the less abscission was found in SlHSL1/2/3 knockout lines. Taken together, our findings indicate that SlHSL1/2/3 can act as receptors for SlIDL6 to positively regulate tomato pedicel abscission and the abscission regulated by SlHSL1/2/3 were partially dependent on ethylene.