Primary Cell Wall Remodeling for Controlling Fleshy Fruit Softening

Abstract

Texture is a vital component of fruit quality. Softening, a widespread phenomenon across diverse fruits, involves the depolymerization and degradation of primary cell walls, which directly determines fruit storability, transportability and shelf life. In the 1990s, biotechnological approaches were applied to generate storage- and transport-tolerant tomato fruits with suppressed ethylene biosynthesis. Although these transgenic lines exhibited prolonged shelf life, they lost the characteristic flavor traits that develop concomitantly with fruit ripening. Over the past three decades, balancing improved storability and preservation of intrinsic fruit quality traits has remained a critical scientific issue attracting widespread attention from both academic researchers and industrial practitioners in postharvest research.

SlLOB1 was identified via transcriptome mining, and functional characterization using RNA interference (RNAi) and CRISPR/Cas9 genome editing demonstrated that fruits from SlLOB1-RNAi and CRISPR knockout lines displayed a twofold increase in firmness accompanied by markedly extended shelf life, without compromising key fruit quality parameters. From the regulatory network downstream of SlLOB1, we further identified two co-expressed target genes, SlEXP1 and SlCEL2. Whole-fruit compression and pericarp puncture assays were performed to analyze fruit texture. Compared with wild-type controls, fully red ripe fruits with simultaneous knockout of SlExp1 and SlCel2 possessed substantially elevated whole-fruit compressive firmness, and this robust texture phenotype persisted throughout postharvest storage. By contrast, individual knockout of either SlExp1 or SlCel2 failed to induce significant changes in fruit firmness. We additionally found that downstream target SlCSLA2 of SlLOB1 participates in mannan biosynthesis. Knockout of SlCSLA2 enhanced fruit firmness. Monosaccharide and polysaccharide compositional analyses revealed a reduced molar ratio of mannose to mannans and an elevated molar ratio of xylose to xyloglucans in csl2a mutant fruits. These results indicate that a compensatory interplay between mannans and xyloglucans modulates fruit firmness. We also identified SlLRX5, a protein that functions as a molecular brake on fruit softening. lrx5 fruit exhibited severe flesh disintegration and liquefaction. Furthermore, interspecies homologous evolutionary analysis across fleshy-fruit species demonstrated that SlLRX5-mediated regulation of softening is evolutionarily conserved in fleshy fruits.

Publications

https://www.pnas.org/doi/10.1073/pnas.2102486118 (link is external)
https://academic.oup.com/plcell/article/36/3/709/7449576 (link is external)

Homepage

https://person.zju.edu.cn/en/0018501 (link is external)