SlCESA6 and SlCEL2 antagonistically affect tomato fruit firmness by influencing cellulose deposition

Abstract

The firmness of fleshy fruits is closely associated with primary cell wall (PCW) structure and polysaccharide metabolism. A tomato (Solanum lycopersicum) mutant, yellow-fruited tomato 1 (yft1, derived from M82) was found to exhibit a firmer fruit phenotype than wild-type cultivar M82, as well as reduced expression of the ethylene signaling gene ETHYLENE INSENSITIVE 2 (SlEIN2) and a higher levels of cellulose. Two cellulose deposition-associated genes, CELLULOSE SYNTHASE subunit A6 (SlCESA6) and ENDO-β-1,4-GLUCANASE 2 (SlCEL2), were induced and repressed respectively in yft1. In this study, SlCESA6 and SlCEL2 were expressed at high levels in immature fruit and mature fruits, respectively. The fruit-specific expression patterns suggest their involvement in regulating tomato fruit firmness. Thus, the level of SlCESA6 and SlCEL2 transcription was suppressed using RNA interference (RNAi), and double mutant was generated via crossing, termed slcesa6, slcel2 and slcesa6 slcel2, respectively. Phenotypic analysis revealed that slcesa6 fruits exhibited significantly reduced firmness at 35, 47, and 54 days post-anthesis (dpa), accompanied by a marked decrease in pericarp cellulose content and thinner cell walls. Furthermore, slcesa6 fruits displayed thinner cuticles, resulting in increased postharvest water loss, compromised fruit integrity, and shortened shelf life. Intriguingly, the softening phenotypes of the slcesa6 fruit were complemented in the slcesa6 slcel2 double mutant to the levels of M82, although fruit of the slcel2 mutant showed no difference in fruit firmness compared with wild type during ripening. Collectively, these results demonstrate that SlCESA6 positively regulates tomato fruit firmness through promoting cellulose deposition, whereas SlCEL2 acts antagonistically within the same pathway to influence fruit firmness.