For plants themselves, the important physiological function of fruits is to provide a shelter for seed development (before maturity) and a carrier of transmission (after maturity). Therefore, most fruits are not delicious before they mature and contain various defensive substances that are harmful to animals and microorganisms. This is because before the fruit ripens, the seeds are not yet fully developed, and plants use various defense mechanisms, including the jasmonic acid signaling pathway, to protect the normal development of the seeds. Once the seeds mature, the fruit enters the ripening stage, becoming fully colored, fragrant, and releasing resistance. These "friendly" signals displayed by plants will attract animals and microorganisms to "feed", thereby helping them spread seeds. For plants, this complex and intricate mechanism formed during long-term evolution helps them reproduce and thrive. For humans, fruits are more susceptible to necrotrophic pathogens and decay after ripening, which often leads to serious post harvest losses. Although delaying fruit ripening can enhance the resistance of fruits to dead nutritional pathogens, the nutritional and flavor qualities of fruits can only be fully formed after ripening, making quality and resistance incompatible. Therefore, in-depth research on the molecular mechanisms of quality formation and resistance reduction during fruit ripening, as well as their interaction mechanisms, has guiding significance for breaking the negative correlation between quality and high resistance in breeding.

Li Chuanyou, a researcher in the Institute of Genetics and Developmental Biology of the Chinese Academy of Sciences, has long studied the molecular mechanism of plant defense against pests and diseases and fruit quality formation using tomato as a model, and applied it to breeding. The team's previous work analyzed the molecular network of ethylene regulating tomato fruit ripening and answered the question from the perspective of transcriptional regulation, "Why does fruit ripen with complete color, aroma, and flavor?". They found that the core transcription factor EIL of the ethylene signaling pathway forms a functional complex with the transcription mediator subunit MED25, which directly or indirectly (through downstream secondary transcription factors) regulates the expression of quality formation related genes such as fruit softening, pigment metabolism, and flavor substance synthesis. Meanwhile, EIL-MED25 also forms a positive and negative feedback loop with downstream transcription factors to maintain ethylene homeostasis during the maturation process.

On February 7th, Li Chuanyou's team and others published a research paper titled TomatoCYP94C1inactive bio active JA Ile to attract jasmonite mediated defense during fruit ripening online in Molecular Plant. Related studies have found that EIL, while regulating the expression of genes related to tomato quality formation, targets and activates the expression of the jasmonic acid metabolism gene CYP94C1, thereby reducing jasmonic acid mediated resistance, answering the question of "why mature fruits are more susceptible to the invasion of necrotrophic pathogens.".

Research has found that during the ripening process of fruits, the biosynthesis of ethylene rapidly increases, while the level of active jasmonic acid JA Ile sharply decreases.

In Arabidopsis, members of the Cytochrome P450 94 (CYP94) family pass through ω- Oxidation deactivates JA Ile. Among them, AtCYP94B1 and AtCYP94B3 convert the active form of JA Ile into low activity 12-OH-JA Ile, while AtCYP94C1 can further convert 12-OH-JA Ile into inactive 12-COOH-JA Ile. These three genes in Arabidopsis are all induced to express by mechanical damage. The sequence analysis results showed that the tomato genome encodes a total of 3 CYP94B oxidases (CYP94B1, CYP94B2, and CYP94B3) and 2 CYP94C oxidases (CYP94C1 and CYP94C2). Interestingly, unlike other members, the expression of CYP94C1 is strongly induced by maturation rather than mechanical injury. In vitro enzyme activity tests showed that CYP94C1 can convert JA Ile into inactive 12-COOH-JA Ile. Knocking out CYP94C1 can significantly increase the JA Ile content in mature fruits, thereby enhancing the resistance of fruits to necrotrophic pathogens. The above results indicate that CYP94C1 is specifically induced to express during fruit ripening, leading to the inactivation of JA Ile and thus reducing the resistance response mediated by jasmonic acid.

Further research has found that the core transcription factor EIL of the ethylene signaling pathway directly binds to the promoter region of CYP94C1, thereby activating its expression. It can be seen that before the fruit ripens, plants use the defense hormone jasmonic acid to protect developing seeds. Once the seeds mature, plants utilize the maturation hormone ethylene to achieve the effect of killing two birds with one stone. On the one hand, utilizing ethylene to promote quality formation and attract animals to feed. On the other hand, ethylene is used to metabolize the active form of jasmonic acid, thereby "relieving" jasmonic acid mediated resistance and making the fruit more susceptible to necrotrophic pathogens and decay. Both of these factors contribute to the spread of seeds.

It should be emphasized that knocking out CYP94C1 specifically enhances the resistance of fruits to necrotrophic pathogens without altering the fruit ripening process, thus having no effect on sugar content, acid content, lycopene content, and vitamin C content. Therefore, this study not only reveals the molecular mechanism of reduced resistance during fruit ripening, but also provides genetic targets and technical means for solving the contradiction between quality and resistance in breeding and reducing post harvest losses.

Relevant research work has been supported by the National Natural Science Foundation of China and the Chinese Academy of Sciences' strategic leading science and technology project.



EIL targeted activation of CYP94C1 inactivates JA Ile and reduces jasmonic acid mediated resistance