The 2-C-methylerythritol 4-phosphate pathway in melon is regulated by specialized isoforms for the first and last steps.

Montserrat Saladié,Louwrance P Wright,Michael A Phillips,Manuel Rodriguez-Concepcion,Jordi Garcia-Mas

doi:10.1093/jxb/eru275

Abstract

The 2-C-methyl-d-erythritol-4-phosphate (MEP) pathway provides the precursors for the biosynthesis of plastidial isoprenoids, which include the carotenoid pigments of many fruits. We have analysed the genes encoding the seven enzymes of the MEP pathway in melon (Cucumis melo L.) and determined that the first one, 1-deoxyxylulose 5-phosphate synthase (DXS), and the last one, 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate reductase (HDR), are represented in the genome as a small gene family and paralogous pair, respectively. In the case of DXS, three genes encode functional DXS activities which fall into previously established type I (CmDXS1) and II (CmDXS2a and CmDXS2b) categories, while a fourth DXS-like gene belonging to the type III group did not encode a protein with DXS activity. Their expression patterns and phylogenies suggest that CmDXS1 is functionally specialized for developmental and photosynthetic processes, while CmDXS2a and CmDXS2b are induced in flowers and ripening fruit of orange- (but not white-) fleshed varieties, coinciding with β-carotene accumulation. This is the first instance connecting type II DXS genes to specialized isoprenoid biosynthesis in the fruit of an agronomically important species. Two HDR paralogues were shown to encode functional enzymes, although only CmHDR1 was highly expressed in the tissues and developmental stages tested. Phylogenetic analysis showed that in cucurbits such as melon, these HDR paralogues probably arose through individual gene duplications in a common angiosperm ancestor, mimicking a prior division in gymnosperms, while other flowering plants, including apple, soy, canola, and poplar, acquired HDR duplicates recently as homoeologues through large-scale genome duplications. We report the influence of gene duplication history on the regulation of the MEP pathway in melon and the role of specialized MEP-pathway isoforms in providing precursors for β-carotene production in orange-fleshed melon varieties.

Highlights

Melon (Cucumis melo L.) is an economically important species in the family Cucurbitaceae, which includes cucumber (C. sativus), squash (Cucurbita spp.), and watermelon (Citrullus lanatus)
We report that small-scale gene duplications in the MEP pathway have led to specialized isoforms in melon which play an important role in providing carotenoid precursors during fruit ripening, including the first report of type II deoxyxylulose 5-phosphate synthase (DXS) proteins involved in this process
Individual sequencing of isolated cDNAs from the orangefleshed Ved variety indicated that all MEP pathway genes matched their sequences with the melon reference genome DHL92

Summary

Introduction

Melon (Cucumis melo L.) is an economically important species in the family Cucurbitaceae, which includes cucumber (C. sativus), squash (Cucurbita spp.), and watermelon (Citrullus lanatus). Non-climacteric white-fleshed varietals such as ‘Piel de Sapo’ (PS) generally lack carotenoid pigments, whereas β-carotene, a nutritionally important carotenoid that serves as the main precursor for vitamin A synthesis, accumulates in climacteric orange-fleshed varieties such as ‘Vedrantais’ (Ved) and Dulce (Dul), resulting in their characteristic orange hue. These carotenoid-rich and -deficient melon strains provide a convenient comparative system for understanding the molecular regulation of carotenoid formation in fruit. An investigation of the upstream steps providing carotenoid precursors has not previously been undertaken

Methods

Results

Conclusion