Identification and Characterization of the Lipoxygenase Gene Family during Development of Pepino (Solanum muricatum Aiton)

Pepino is a diploid (2n=24) subtropical species, also known as melon pear, melon shrub or sweet cucumber, native species from South America specifically from The Andes area of Peru and Chile. Pepino fruit belongs to the Solanaceae family, which includes many important crops such as tomato and potato. Many aroma volatiles in fresh fruit are produced via the lipoxygenase (LOX) pathway. LOX genes are classified according to their function and grouped in the 13-LOX and the 9-LOX pathways, which generate C6 and C9 aroma compounds, respectively. Most of the LOX-derived compounds found in fruit aroma profiles are derived from the 13-LOX branch. The aims of this study were (i) to characterize the LOX gene family throughout development in flesh and peel of pepino fruit, and (ii) to investigate the relationship between LOX gene expression and the aroma volatiles emitted by the fruit. Two different studies were performed: (i) Six phenological stages were evaluated from immature fruit (2 days after fruit set) to senescent fruit (65 days after fruit set), and (ii) fruit of two maturity stages were collected (M1 = green background and M2= white background) at commercial harvest, ripened at 20 ºC and evaluated every two days for 15 days. All fruit were collected from Ovalle (IV Region) North of Chile, and analyzed for respiration rate, internal ethylene production, and quality parameters. Nine candidate LOX genes were identified using heterologous and degenerated primers designed from the closest family members (tomato and tobacco). We isolated RNA from pepino peel and flesh of the two different experiments, made cDNA and performed quantitative PCR for all LOXs. Interestingly, most of the LOXs were higher expressed in pulp rather than peel. Three LOXs showed a higher expression in peel: SmLOXC, SmLOXD (13-LOXs), and SmLOX5-like2 (9-LOX). Although no differences were found in quality parameters between M1 and M2, important differences were found at transcript level. LOX genes were higher expressed in M1, in contrast to M2 which showed a general decline of gene expression probably due to its advanced ripening stage. The volatile analyses of these experiments correlated to the gene expression will be discussed.