Big red: dissecting the role of ethylene in tomato fruit development and ripening

The formation of a big, red tomato (Solanum lycopersicum) starts with the development of the reproductive structures within the flower. After pollination, a green fruit begins to grow, eventually reaches a certain size, and ripens, which involves color changes, softening, and the production of sugars and metabolites. The action of plant hormones has been described in the initial phases of fruit development, with the critical participation of auxin and cytokinins; subsequently, the ripening process is mainly controlled by the ethylene signaling pathway (Brumos, 2021). Various studies have also uncovered the participation of developmental regulators (including MADS-box or NAC transcription factors) and their relationship with the ethylene signaling pathway.

In this issue of The Plant Cell, Wei Huang and colleagues (Huang et al., 2022) dissect the contributions of several members of the ethylene signaling pathway during the early stages of tomato fruit development and provided new insights into tomato ripening. Previous knowledge about the role of ethylene during fruit ripening comes from the study of Never ripe (Nr; Lanahan et al., 1994), a gain-of-function mutant of the ethylene receptor gene SlETR3; however, the ripening defects in Nr are not conserved across cultivars. In this work, Huang and colleagues generated the ethylene-insensitive mutant slein2, a loss-of-function allele of EIN2, which positively regulates ethylene responses (Alonso et al., 1999). This line displayed insensitivity to exogenous ethylene treatment, altered petal abscission, and delayed fruit ripening. The authors also generated mutants for other components of the ethylene signaling pathway: four EIN3-like genes (sleil1-4) encoding regulators of the ethylene response and three EIN3-BINDING F-box protein genes (slebf1-3). These single, double, or higher-order mutants displayed different phenotypes associated with fruit ripening, allowing the discovery of conserved and divergent functions.

In addition to delayed ripening, slein2-1 fruits were small (see Figure); this defect is not observed in other ripening-deficient mutants such as rin (RIPENING INHIBITOR) or nor (NON-RIPENING). The reduced fruit size is not associated with facultative parthenocarpy or nutrient competition between fruits. slein2-1 showed decreased seed number, and the pericarps of slein2-1 fruits had fewer cell layers. The authors found that impaired cell division in the pericarp is caused by the decreased accumulation of auxin derived from seeds. Indole-3-acetic acid content was reduced, and the application of exogenous auxin partially restored fruit growth. Additionally, auxin biosynthesis (TAR and YUCCA) and transport genes (SlPIN3, SlPIN9, SlLAX3, SlLAX5) displayed altered expression in slein2-1 fruits. These results indicate that SlEIN2-mediated ethylene signaling promotes tomato fruit growth and seed development.

Figure.

The ethylene signaling pathway controls tomato fruit growth and ripening. Representative fruits at the ripening stage from various fruit ripening-deficient mutants, overexpression lines of developmental regulators, and combinations of these lines. Adapted from Huang et al. (2022), Figure 5.

This work also addressed the relationship between developmental factors NOR, RIN, and FUL1 and ethylene signaling components. The authors exploited available genetic material to compare slein2-1 plants to the ripening-deficient mutants Nr and rin (Vrebalov et al., 2002), which revealed remarkable similarities between slein2-1 and the rin mutant: both displayed similar ethylene levels, carotenoid profiles, and molecular markers of ripening. They further found that SEIL1 binds to the promoter regions of RIN, NOR, and FUL1 and transcriptionally activates these genes. Additionally, the overexpression of NOR, RIN, or FUL1 partially restored fruit ripening in the slein2-1 mutant. These results indicate that NOR, RIN, and FUL1 act downstream of the ethylene signaling pathway.

The excellent availability of genetic resources (double or higher-order mutants) revealed the existence of a continuum in the ripening program, measured as the time it takes to reach a ripe stage. The authors found a correlation between the ethylene signal intensity (the number and type of available signaling actors) and the ripening process.

In summary, this work integrates developmental regulators and components of the ethylene signaling pathway into a molecular framework that controls tomato fruit ripening.