Abstract
Many studies have focused on the plant hormone ethylene because of its key role in controlling, among others, climacteric fruit ripening and fruit senescence. These processes can be controlled by applying 1-MCP, which tightly binds to the ethylene receptors thereby blocking the ethylene signaling pathway. 1-MCP is known to inhibit the action of ethylene and to delay the climacteric ripening of tomato fruit. Less is known about its long term effect when the inhibitory effect 1-MCP inhibition is eventually released. Our objective was to study this transient 1-MCP inhibition during tomato fruit ripening in terms of fruit quality, ethylene production, respiration rate and the expression and protein abundance of receptors, CTRs and EIN2. For the identification and quantification of proteins, we used an LC–MS based targeted method of Parallel Reaction Monitoring (PRM), while gene expression was done using real time qPCR. Different color stages of tomatoes were harvested and treated with 1-MCP and subsequently stored to follow up postharvest fruit ripening. The difference with previous 1-MCP studies is that we sampled 1-MCP treated tomatoes at different physiological stages during ripening (and not time), matching the color stages of the untreated control fruit. This allows to properly compare the underlying regulation of the ethylene signaling pathway during a 1-MCP-mediated suppression of ripening. We hypothesized that the levels of the ethylene signaling components would be different for 1-MCP treated fruit due to a reduced ethylene-mediated autocatalytic feed-back. Our results showed that fruit treated with 1-MCP at mature green stage showed a lower respiration rate during subsequent ripening as compared to the untreated fruit, suggesting that climacteric ripening was effectively inhibited by 1-MCP. However, these 1-MCP treated fruit showed a higher ethylene production as compared to untreated fruit. The 1-MCP treated fruit also showed lower to equal levels of gene expression and protein abundance of the ethylene receptors, CTRs and EIN2. As receptors and CTRs are negative regulators of ethylene signaling, decreasing the production of new signaling proteins could subsequently activate downstream ethylene signaling and with that expression of downstream genes. This could lead to higher ethylene production levels, which in turn can compensate 1-MCP mediated inhibition of fruit ripening.
Highlights
Worldwide, tomato is the second most important vegetable crop in terms of production (Food and Agriculture Organization of United Nations, 2016)
In Mata et al (2017) we provided the identification of 8588 tomato pericarp proteins, including four ethylene receptors (SlETR1, SlETR3, SlETR4, and ETR7), three CTR-like proteins (CTRs) (SlCTR1SlCTR3) and SlEIN2
Fragment ions, and mass errors of their MS2 spectra with those of native peptides derived from different ripening stages of tomato, we identified promising candidate peptides for all seven ethylene receptors, three CTRs (1–3) and ETHYLENE INSENSITIVE 2 (EIN2) (Supplementary Table 1)
Summary
Tomato is the second most important vegetable crop in terms of production (Food and Agriculture Organization of United Nations, 2016) It is widely used as a model organism to study fleshy fruit development and climacteric fruit ripening (Giovannoni, 2004; Osorio et al, 2011). The receptors are homologous to bacterial two-component histidine kinases, formed of a sensory histidine kinase and a response regulator domain (Chang et al, 1993). The ethylene receptors interact with the downstream CTR-like protein kinases (Zhong et al, 2008) Four of these tomato CTR-like proteins are homologous to the Raf-like kinase CONSTITUTIVE TRIPLE RESPONSE1 of Arabidopsis, which is a negative regulator of the ethylene response (Kieber et al, 1993; Adams-Phillips et al, 2004; Zhong et al, 2008). The nuclear transcription factors EIN3 and EILs promote the expression of ethylene response factor (ERF) family genes which are downstream regulators of the ethylene responses (Fujimoto et al, 2000; Tieman et al, 2001; Tournier et al, 2003; Liu et al, 2016)
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