Insights into the roles of RALF peptides and Cr RLK1L receptors in fruit ripening

The rapid alkalinization factor (RALF) family of small signaling peptides regulates diverse physiological and developmental processes in plants. RALF peptides influence cell wall remodeling and hormonal homeostasis through interactions with CrRLK1L malectin-like receptor kinases and leucine-rich repeat extensin (LRX) proteins anchored to the cell wall. Fruit ripening, characterized by coordinated hormonal regulation and extensive pectin remodeling, provides a relevant yet understudied context for this signaling module. Across fleshy fruits, CrRLK1Ls are expressed during fruit development and ripening but show diverse functions: tomato SlCrRLK1L20 promotes ripening by enhancing ethylene biosynthesis, whereas strawberry FaMRLK47 and peach PpTHE1 repress ripening by dampening ABA and ethylene, respectively. RALF33-like peptides increase susceptibility to fruit fungal pathogens, highlighting a trade-off between softening and defense. Overall, the RALF and CrRLK1L signaling network emerges as a conserved but flexible regulator linking cell wall dynamics, hormonal balance, and pathogen responses during fruit ripening.