The C2H2-type zinc finger protein SlHair5 acts as a regulatory node coordinating trichome development and leaf morphogenesis in tomato

SlHair5 is a pivotal regulatory node downstream of the H-H4 module that coordinates epidermal differentiation with leaf morphogenesis and plant architecture through a dosage-sensitive transcriptional network and a reciprocal feedback loop. C2H2-type zinc finger proteins (C2H2 ZFPs) are essential regulators of plant development, yet the mechanisms governing their functional diversification and hierarchical organization in tomato (Solanum lycopersicum) remain largely elusive. In this study, we identified and characterized SlHair5 (H5), a previously unassigned C2H2 ZFP that operates downstream of the established H-H4 regulatory module. H5 localizes to the nucleus, where its expression is indirectly activated by the H-H4 module. Functional characterization using CRISPR-Cas9-mediated knockout (h5-sko) and overexpression (H5-OX) lines demonstrated that H5 is a pivotal developmental regulatory node. At the cellular level, H5 promotes epidermal differentiation by modulating trichome density and stalk cell elongation in a type-specific manner. Intriguingly, H5 exhibits dosage-sensitive and non-linear regulatory effects; while trichome traits showed a relatively linear response to H5 levels, organ-level traits-such as leaf shape and plant height-displayed a threshold effect, with pronounced growth retardation and narrowed leaflets observed in H5-OX lines. Transcriptomic profiling identified 569 differentially expressed genes, revealing that H5 orchestrates a broad network including regulators of trichome development (JAZ4 and WRKY57), leaf morphogenesis (Tf, GH3.15, BOPa, and CKX2), and stem elongation (GT-26 and ER). Furthermore, H5 exerts an indirect feedback control over H3 and H4, thereby fine-tuning the ZFP regulatory hierarchy to maintain developmental homeostasis. Collectively, our findings establish H5 as a higher-order regulatory hub that coordinates cellular patterning with organismal architecture, providing critical insights into the transcriptional programs governing vegetative development in tomato.