Transcriptional Analysis of Carotenoids Accumulation and Metabolism in a Pink-Fleshed Lemon Mutant.

Giuseppe Lana,Gaetano Distefano,Jaime Zacarias-Garcia,María J Rodrigo,Lorenzo Zacarias,Alessandra Gentile

doi:10.3390/genes11111294

Abstract

Pink lemon is a spontaneous bud mutation of lemon (Citrus limon, L. Burm. f) characterized by the production of pink-fleshed fruits due to an unusual accumulation of lycopene. To elucidate the genetic determinism of the altered pigmentation, comparative carotenoid profiling and transcriptional analysis of both the genes involved in carotenoid precursors and metabolism, and the proteins related to carotenoid-sequestering structures were performed in pink-fleshed lemon and its wild-type. The carotenoid profile of pink lemon pulp is characterized by an increased accumulation of linear carotenoids, such as lycopene, phytoene and phytofluene, from the early stages of development, reaching their maximum in mature green fruits. The distinctive phenotype of pink lemon is associated with an up-regulation and down-regulation of the genes upstream and downstream the lycopene cyclase, respectively. In particular, 9-cis epoxycarotenoid dioxygenase genes were overexpressed in pink lemon compared with the wild-type, suggesting an altered regulation of abscisic acid biosynthesis. Similarly, during early development of the fruits, genes of the carotenoid-associated proteins heat shock protein 21, fibrillin 1 and 2 and orange gene were overexpressed in the pulp of the pink-fleshed lemon compared to the wild-type, indicating its increased capacity for sequestration, stabilization or accumulation of carotenes. Altogether, the results highlighted significant differences at the transcriptomic level between the pink-fleshed lemon and its wild-type, in terms of carotenoid metabolism and the capacity of stabilization in storage structures between the two accessions. Such changes may be either responsible for the altered carotenoid accumulation or in contrast, a metabolic consequence.

Highlights

Carotenoids are isoprenoids-derived molecules that carry out essential functions in plant cells; they are part of the photosynthetic system and participate in light capture
The reddish coloration of the pulp at early stages moved to clearer shades, probably due to the dilution effect caused by the substantial growth of the pulp along the maturation process (Figure 2A)
Our results suggest that β-carotene hydroxylase (β-CHX) is not a limiting factor for the accumulation of carotenoids in the pink lemon (PL), the levels of the transcripts are very low compared with other citrus fruits [15,29]

Summary

Introduction

Carotenoids are isoprenoids-derived molecules that carry out essential functions in plant cells; they are part of the photosynthetic system and participate in light capture. Carotenoids play an important role in photo-protection, increasing tolerance to light and heat stresses while preventing membranes from lipid peroxidation. These pigments are precursors of important phytohormones such as abscisic acid (ABA) and strigolactones; carotenoids constitute the substrates for the formation of apocarotenoids-derived volatiles. Carotenoids are responsible for the attractive color of flowers, fruits and other organs in many plant species, but they are known for their benefits to human health.

Objectives

Methods

Results

Discussion

Conclusion