Role of ethylene in non-volatile compounds during strawberry maturation and changes in sugar metabolism during melon maturation – new studies about non-climacteric fruit ripening.

Abstract

Based on respiration, ethylene production and ethylene response, fleshy fruit are classified as climacteric or non-climacteric. Generically, climacteric fruit are those whose maturation occurs concomitantly to a peak in respiration and increase in ethylene production; are extremely responsive to exogenous ethylene; and are able to complete ripening detached from the mother plant if physiological maturation occurs prior to harvest. On the other hand, non-climacteric fruit are classified as having no respiratory peak nor rise in ethylene production during maturation; do not respond to exogenous ethylene in most cases; and are unable to ripen detached from the mother plant, since physiological ripeness must coincide with the time of harvest. Studies conducted mainly on tomatoes, a model climacteric fruit, generated important information on ethylene production, perception, and signal transduction, as well as their influence on change of color, flavor, texture and aroma, which allowed the development of more efficient production, harvesting and post-harvest technologies. However, in the maturation of non-climacteric fruits, although much research has been conducted, there are still many questions to be elucidated. Among the non-climacteric fruits, strawberry (Fragaria ananassa L. Dutch) is the most studied system for understanding the role of ethylene in ripening regulation with several genes related to ripening already characterized. Recently, the melon fruit (Cucumis melo L.), is emerging as a new model to study fruit ripening due to the presence of climatic and non-climatic cultivars, allowing the comparison of results among model fruit of the same species. Therefore, in the present study ‘Albion’ strawberry fruit on the plant at four developmental stages (Green, White, Pink and Red) were immersed in three different treatments (Ethephon – an ethylene-generating compound, 1-MCP - an ethylene perception inhibitor, or Water - containing solvents and diluents), plus one absolute control that received no treatment. At ripeness all fruit were harvested and evaluated to show the effect of ethylene on important physical-chemical attributes and non-volatile quality compounds. Ethephon treatment was observed to affect fruit dimension, firmness, anthocyanins and amino acid content. Treatment with 1-MCP at any developmental stage had no effect on any of the variables measured. A second study was conducted with a non-climacteric melon fruit (Cucumis melo cv. ‘Yellow’) to characterize the expression of important sugar metabolism genes during four fruit development stages (small green, large green, color change and full ripe) to relate sugar metabolism of non-climacteric melon with possible linkage to sugar metabolism and shelf life of these fruits. The target genes were choosen using previous information from a RNAseq sequencing of ‘Yellow’ melon. Genes encoding enzymes that metabolise sugar for energy were observed to be more expressed early in development at the large green development stage, while genes encoding enzymes that synthesize sugar and/or direct this sugar to storage were observed to be expressed later in development, at the color change and full ripe stages. Both studies provided important information about non-climacteric fruit ripening that can be used to improve management, harvest and post-harvest technologies in the future.