Taste has been the subject of human selection in the evolution of agricultural crops, and acidity is one of the three major components of fleshy fruit taste, together with sugars and volatile flavour compounds. We identify a family of plant-specific genes with a major effect on fruit acidity by map-based cloning of C. melo PH gene (CmPH) from melon, Cucumis melo taking advantage of the novel natural genetic variation for both high and low fruit acidity in this species. Functional silencing of orthologous PH genes in two distantly related plant families, cucumber and tomato, produced low-acid, bland tasting fruit, showing that PH genes control fruit acidity across plant families. A four amino-acid duplication in CmPH distinguishes between primitive acidic varieties and modern dessert melons. This fortuitous mutation served as a preadaptive antecedent to the development of sweet melon cultigens in Central Asia over 1,000 years ago.
The exact transport characteristics of the vacuolar dicarboxylate transporter tDT from Arabidopsis are elusive. To overcome this limitation, we combined a range of experimental approaches comprising generation/analysis of tDT overexpressors, 13 CO 2 feeding and quantification of 13 C enrichment, functional characterization of tDT in proteoliposomes and electrophysiological studies on vacuoles. tdt knock-out plants showed decreased malate and increased citrate concentrations in leaves during the diurnal light-dark rhythm and after onset of drought, when compared to wild types. Interestingly, under latter two conditions tDT overexpressors exhibited malate-and citrate levels opposite to tdt knock-out plants. Highly purified tDT protein transports malate and citrate in a 1:1 antiport mode. The apparent affinity for malate decreased with decreasing pH, while citrate affinity increased. This observation indicates that tDt exhibits a preference for dianion substrates, which is supported by electrophysiological analysis on intact vacuoles. tDT also accepts fumarate and succinate as substrates, but not α-ketoglutarate, gluconate, sulphate or phosphate. Taking tDT as an example we demonstrated that it is possible to reconstitute a vacuolar metabolite transporter functionally in proteoliposomes. The displayed, so far unknown counter-exchange properties of tDT now explains the frequently observed reciprocal concentration changes of malate and citrate in leaves from various plant species. tDT from Arabidopsis is the first member of the wellknown and widely present SLC13 group of carrier proteins, exhibiting an antiport mode of transport.
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