Ethylene and peach fruit ripening: a functional genomics approach

Fruit quality traits are the result of genetic, agronomic and environmental factors that, alone or in combination, modulate metabolic processes during both pre- and post-harvest phases and affect fruit development and ripening processes. Productivity, size and organoleptic quality should be the main quality criteria adopted by fruit growers: in this context, harvesting time is crucial. Too early harvested fruit may be stored for a long time but their flavour quality is low, whereas late harvested fruit are of better quality but do not withstand long storage periods and theirs shelf-life is reduced. This is particularly true for peaches and nectarines characterized by a quick ripening evolution and a reduced aptitude to prolonged storage: this induces growers to anticipate harvesting and represents the main constraints for supplying high-quality standard level peaches to the consumers. Elucidating mechanisms and basic processes characterizing ripening and responsible for the evolution of quality parameters is the prerequisite to develop strategies aimed to produce high-quality fruit and to maintain these standards throughout the postharvest chain. In climacteric fruit, including peaches, ethylene is a key factor in coordinating and regulating the evolution of several processes characterizing the ripening syndrome. Thus, studying aspects related to ethylene action has been a challenge during the last few decades. Improvements of the basic knowledge of ethylene physiology also came from the identification of specific inhibitors of its biosynthesis and/or action, and from the use of mutants. The development of highthroughput molecular tools (i.e. microarray) and functional genomics approaches represent a great opportunity for a better understanding of the ripening process and the basic mechanisms governing quality-related metabolisms in fruit. Considering peach, the first step toward a functional approach is represented by the development of an Expressed Sequences Tag (ESTs) repertoire, that, together with other ESTs isolated by other units and available in public databases, allowed to select 4806 oligos, corresponding to an equal number of genes expressed in peach fruit, and construct the first peach microarray (?PEACH 1.0). ?PEACH 1.0 has been used to study the effects of exogenous ethylene on different peach genotypes, a "melting flesh" cv and two ripening mutants, Slow Ripening (SR) and Stony Hard (SH). Microarray analysis revealed marked differences in transcript profiling possibly related to the nature of mutation and differences in ethylene physiology. SH fruit has also been used for expression analyses of two elements involved in the ethylene signalling pathway. Besides mutants, specific inhibitors of ethylene biosynthesis and/or action represent invaluable tools for elucidating the ethylene role in the ripening process. One of the most powerful inhibitor of ethylene function is 1-methylcyclopropene (1-MCP) that is practically used on different fruit species, but not on peaches, to prolong shelf-life. Using ?PEACH 1.0, a large-scale analysis of transcriptome has been performed on nectarine fruit treated with 1-MCP in order to elucidate the molecular mechanisms responsible for the limited effect of the inhibitor on this climacteric fruit species. At the end of the treatment (24h) and 48h hours later, a number of genes involved in quality-related ripening processes (such as softening, sugar metabolism and colour development) appeared to be deeply modified in their expression. Changes in the expression profile of Transcription Factors related to ethylene and auxin action confirmed the importance of "cross-talk" between the two hormones in the modulation of the ripening process in peach. In the context of a functional genomics, three different genes (two from peach and one from tomato), identified following transcriptomics approaches, have been used for transgenic experiments in tomato plants.