SlHDZ19 Promotes Tomato Thermotolerance via a PLA2-Dependent Lipid-Metabolic Transcriptional Program

Heat stress (HS), increasingly intensified by climate change, severely restricts tomato growth and productivity. Although core heat shock factor-mediated transcriptional networks have been extensively characterized, how lipid metabolic reprogramming is transcriptionally coordinated during thermotolerance remains unclear. Using SlHDZ19 overexpression and mutant lines together with transcriptomic, biochemical, promoter-binding, and gene-silencing analyses, we show that the homeobox-leucine zipper transcription factor SlHDZ19 promotes tomato (Solanum lycopersicum) thermotolerance by activating a PLA2-dependent lipid-metabolic transcriptional program in leaves. SlHDZ19 overexpression generally improved heat-stress performance, while SlHDZ19 mutant lines exhibited heightened sensitivity, including more severe wilting, higher electrolyte leakage, and reduced proline accumulation and CAT activity under heat stress. Transcriptomic analysis revealed that SlHDZ19 is required for the full induction of canonical heat-responsive genes and that the linoleic acid metabolism pathway was repeatedly implicated in SlHDZ19-dependent transcriptional changes. SlHDZ19 binds to and activates the promoters of SlPLA2α, which encodes a phospholipase A2 involved in releasing linoleic acid from membrane lipids, and three lipoxygenase genes (SlLox7, SlLox8, and SlLoxC), accompanied by elevated overall PLA2 and LOX activities in SlHDZ19-overexpressing plants. Moreover, genetic silencing of SlPLA2α in both wild-type and SlHDZ19-overexpressing backgrounds supported its functional requirement downstream of SlHDZ19 in thermotolerance. Collectively, our findings support a thermotolerance module in which SlHDZ19 transcriptionally regulates PLA2- and LOX-associated steps of linoleic acid metabolism, potentially linking lipid-associated signaling and membrane remodeling with heat stress adaptation in tomato.