Molecular and genetic characterization of a non-climacteric phenotype in melon reveals two loci conferring altered ethylene response in fruit

Abstract

Fruit ripening and abscission are associated with an ethylene burst in several melon (Cucumis melo) genotypes. In cantaloupe as in other climacteric fruit, exogenous ethylene can prematurely induce abscission, ethylene production, and ripening. Melon genotypes without fruit abscission or without ethylene burst also exist and are, therefore, non-climacteric. In the nonabscising melon fruit PI 161375, exogenous ethylene failed to stimulate abscission, loss of firmness, ethylene production, and expression of all target genes tested. However, the PI 161375 etiolated seedlings displayed the usual ethylene-induced triple response. Genetic analysis on a population of recombinant cantaloupe Charentais x PI 161375 inbred lines in segregation for fruit abscission and ethylene production indicated that both characters are controlled by two independent loci, abscission layer (Al)-3 and Al-4. The non-climacteric phenotype in fruit tissues is attributable to ethylene insensitivity conferred by the recessive allelic forms from PI 161375. Five candidate genes (two ACO, two ACS, and ERS) that were localized on the melon genetic map did not exhibit colocalization with Al-3 or Al-4.

Figure 1

Comparison of internal ethylene concentration (A; in microliters per liter), ACS activity (B), ACC content (C), and flesh firmness (D) in melon fruit of climacteric (Védrantais, ●) and non-climacteric (PI 161375, ▴) genotypes.

Figure 2

Effect of exogenous propylene on PI 161375 melon fruit harvested 45 DAP for 1 or 6 d on ethylene production (A), flesh firmness (B), and color of the peel (C). Dashed line, Fruit in air; solid line, fruit exposed to propylene.

Figure 3

RNA analysis of genes whose expression in PI 161375 fruit is at the same level as in Védrantais fruit before the climacteric crisis. Phosphor imager data of RNA gel-blot analysis of PG2, ERS1, RM7, RM8, EIL1, and EIL2 in developing Védrantais and PI 161375 melon fruit expressed for each individual blot in terms of percentage of the maximum signal detected. A, Comparison of gene expression between Védrantais and PI 161375 melon fruit during ripening on the vine. Tissues were from seven stages of fruit development (30, 35, 36, 37, 38, 39, and 41 DAP) for Védrantais climacteric line or from 6 stages (30, 35, 40, 45, 50, and 60 DAP) for the non-climacteric PI 161375 line. B, Comparison of gene expression in harvested PI 161375 fruit (45 DAP) treated with air or 2,000 μL L⁻¹ of propylene for 1 to 6 d.

Figure 4

RNA analysis of genes whose expression in PI 161375 fruit is lower than in Védrantais fruit before the climacteric crisis. Legend is identical to Figure 3.

Figure 5

Comparison of ethylene-induced triple response between Védrantais (A) and PI 161375 (B) etiolated seedlings. An enlargement of the apical region in ethylene-treated seedlings is shown in the right corners of A and B.

Figure 6

Mapping of the genes ech, Al-3, and Al-4; the QTLs eth1.1, eth2.1, eth3.1, and eth11.1; and some candidate genes (in bold on the left side of each LG) on the composite map of melon. All of them were mapped on a population of RILs generated between Védrantais, a climacteric line, and PI 161375, a non-climacteric line, with the exception of the ERS1 locus, which was mapped on a RIL population derived between Védrantais and PI 414723.