Loss Function of ClARF4 Reduces Plant Height and Cell Size in Watermelon

Auxin response factors (ARFs) are key transcription factors regulating plant growth and development, but their functions and molecular mechanisms in stem elongation of cucurbit crops remain unclear. In this study, we identified a nuclear-localized ARF member, ClARF4, in watermelon. ClARF4 knockout lines were generated using CRISPR-Cas9 technology and exhibited significant reductions in plant height, internode number, and internode length. Cytological analysis indicated that the dwarf phenotype resulted from inhibited longitudinal cell elongation in stems. Transcriptome analysis revealed that ClARF4 modulates cell size by regulating the expression of genes involved in auxin signaling and response pathways. Furthermore, using yeast two-hybrid screening, we identified ClPetC, a component of the photosynthetic electron transport chain, as an interacting protein of ClARF4; bimolecular fluorescence complementation (BiFC) assays further confirmed their direct interaction in the plant nucleus. This study not only reveals the key role of ClARF4 in regulating plant height in watermelon, but also provides important insights into the function of chloroplast–nucleus signaling crosstalk in plant architecture establishment by identifying the interaction between ClARF4 and the chloroplast protein ClPetC in the nucleus.