Fruit-localized phytochromes regulate lycopene accumulation independently of ethylene production in tomato

Abstract

We show that phytochromes modulate differentially various facets of light-induced ripening of tomato fruit (Solanum lycopersicum L.). Northern analysis demonstrated that phytochrome A mRNA in fruit accumulates 11.4-fold during ripening. Spectroradiometric measurement of pericarp tissues revealed that the red to far-red ratio increases 4-fold in pericarp tissues during ripening from the immature-green to the red-ripe stage. Brief red-light treatment of harvested mature-green fruit stimulated lycopene accumulation 2. 3-fold during fruit development. This red-light-induced lycopene accumulation was reversed by subsequent treatment with far-red light, establishing that light-induced accumulation of lycopene in tomato is regulated by fruit-localized phytochromes. Red-light and red-light/far-red-light treatments during ripening did not influence ethylene production, indicating that the biosynthesis of this ripening hormone in these tissues is not regulated by fruit-localized phytochromes. Compression analysis of fruit treated with red light or red/far-red light indicated that phytochromes do not regulate the rate or extent of pericarp softening during ripening. Moreover, treatments with red or red/far-red light did not alter the concentrations of citrate, malate, fructose, glucose, or sucrose in fruit. These results are consistent with two conclusions: (a) fruit-localized phytochromes regulate light-induced lycopene accumulation independently of ethylene biosynthesis; and (b) fruit-localized phytochromes are not global regulators of ripening, but instead regulate one or more specific components of this developmental process.

Figure 1

PHYA mRNA abundance increases in tomato fruit during ripening. Fruit (cv UC-82B) at five different developmental stages were harvested on the same day from plants in the greenhouse. mRNA was isolated from these fruit, separated electrophoretically, and blotted to nylon. RNA blots were subsequently hybridized with a ³²P-labeled probe derived from a conserved region of PHYA, washed at high stringency, and exposed to a phosphor plate. Images were developed using a phosphor imager (Molecular Dynamics, Sunnyvale, CA). The lower panel shows the absolute abundance of PHYA mRNA. IG, Immature-green; MG, mature-green; B, breaker; T, turning/orange; RR, red-ripe.

Figure 2

Fruit-localized phytochromes mediate light-induced lycopene biosynthesis in fruit pericarp. Mature-green fruit (cv UC-82B) were harvested 45 DPA and, except for the indicated treatments, fully ripened in the dark (23°C, 80% relative humidity). After harvest, fruit received one of three light treatments daily: R, 5 min of red light followed immediately by a mock treatment of 15 min of far-red light (fruits were placed under the far-red-light source for 15 min without irradiation); R/FR, 5 min of red light followed immediately by 15 min of far-red light; D, mock treatment of 5 min of red light (fruits were placed under the red-light source for 5 min without irradiation) followed immediately by a mock treatment of 15 min of far-red light. Carotenoids were extracted from red-ripe fruit and quantified via RP-HPLC. se bars are shown (n = 10).

Figure 3

Fruit-localized phytochromes do not regulate ethylene biosynthesis in cv UC-82B or cv MoneyMaker fruit. Mature-green fruit were harvested 45 (cv UC-82B) or 39 (cv MoneyMaker) DPA and, except for the indicated treatments, fully ripened in the absence of light (23°C, 80% relative humidity). During ripening, fruit received one of three light treatments daily: R (black-dashed line, ▪), 5 min of red light followed immediately by a mock treatment of 15 min of far-red light (fruits were placed under the far-red-light source for 15 min without irradiation); R/FR (gray line, ♦), 5 min of red light followed immediately by 15 min of far-red light; D (black line, ▴), mock treatment of 5 min of red light (fruits were placed under the red-light source for 5 min without irradiation) followed immediately by a mock treatment of 15 min of far-red light. Ethylene production was assayed approximately every 12 h and quantified by GC. se bars are shown (n = 16). A, cv UC-82B. B, cv MoneyMaker.

Figure 4

Fruit-localized phytochromes do not regulate fruit softening. Mature-green fruit (cv UC-82B) were harvested 45 DPA and, except for the indicated treatments, ripened in the absence of light (23°C, 80% relative humidity). During ripening, fruit received one of three light treatments daily: R (stippled bars), 5 min of red light followed immediately by a mock treatment of 15 min of far-red light (fruits were placed under the far-red-light source for 15 min without irradiation); R/FR (gray bars), 5 min of red light followed immediately by 15 min of far-red light; D (black bars), mock treatment of 5 min of red light (fruits were placed under the red-light source for 5 min without irradiation) followed immediately by a mock treatment of 15 min of far-red light. Fruit were analyzed for pericarp resistance at various stages during ripening. After removal of the epidermal tissue, pericarp resistance was measured using a penetrometer attached to a motorized drill press. Mature-green fruit have an average pericarp resistance of 11.9 ± 0.82 kg cm⁻² (data not shown). se bars are shown (n = 8–16).