Abstract
Ethylene is crucial in climacteric fruit ripening. The ethylene signal pathway regulates several physiological alterations such as softening, carotenoid accumulation and sugar level reduction, and production of volatile compounds. All these physiological processes are controlled by numerous genes and their expression simultaneously changes at the onset of ripening. Ethylene insensitive 2 (EIN2) is a key component for ethylene signal transduction, and its mutation causes ethylene insensitivity. In tomato, silencing SlEIN2 resulted in a non-ripening phenotype and low ethylene production. RNA sequencing of SlEIN2-silenced and wild type tomato, and differential gene expression analyses, indicated that silencing SlEIN2 caused changes in more than 4,000 genes, including those related to photosynthesis, defense, and secondary metabolism. The relative expression level of 28 genes covering ripening-associated transcription factors, ethylene biosynthesis, ethylene signal pathway, chlorophyll binding proteins, lycopene and aroma biosynthesis, and defense pathway, showed that SlEIN2 influences ripening inhibitor (RIN) in a feedback loop, thus controlling the expression of several other genes. SlEIN2 regulates many aspects of fruit ripening, and is a key factor in the ethylene signal transduction pathway. Silencing SlEIN2 ultimately results in lycopene biosynthesis inhibition, which is the reason why tomato does not turn red, and this gene also affects the expression of several defense-associated genes. Although SlEIN2-silenced and green wild type fruits are similar in appearance, their metabolism is significantly different at the molecular level.
Highlights
The gaseous phytohormone ethylene (C2H4) is essential for developmental and physiological regulation in a variety of higher plants [1] and its effect is more obvious during the ripening process of climacteric fruits [2]
As SlEIN2 is an important component of signal transduction, mutations occurring in this gene will effectively block the signal transduction pathway, resulting in plant insensitivity to ethylene
Differences in carbon fixation between pTRV GM and pTRV-SlEIN2 fruits were found and pTRV orange ripe (OR)/pTRV-SlEIN2 Selected DEGs (SDEGs) were enriched on photosynthesis
Summary
The gaseous phytohormone ethylene (C2H4) is essential for developmental and physiological regulation in a variety of higher plants [1] and its effect is more obvious during the ripening process of climacteric fruits [2]. EIN2’s cleaved CEND can inhibit the expression of the F-box genes EB1 and EBF2, disrupting the accumulation of EIN3 by recognizing their 30-untranslated regions and transferring them to the P-body. Fruit ripening is a complicated process, including the accumulation of volatile components, flavonoids formation, pectin degradation, and carotenoid biosynthesis [24,28,29,30] These diverse processes are regulated by numerous transcription factors and signal transduction pathways [24], among which the ethylene signal pathway is typically found. Previous studies have shown that non-ripening tomatoes were obtained by silencing SlEIN2 through VIGS, and recently a new paper from Gao et al (2016) showed that in yellow-fruited tomato 1(yft1) mutant, lower expressed of EIN2 would lead to impaired ethylene biosynthesis and perception, as well as abnormal carotenoid production [22,31]. Increases the knowledge on the molecular mechanisms regulating fruit ripening by signal transduction pathways
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