Fruit ripening in tomato (Solanum lycopersicum) requires the coordination of both developmental cues as well as the plant hormone ethylene. Although the role of ethylene in mediating climacteric ripening has been established, knowledge regarding the developmental regulators that modulate the involvement of ethylene in tomato fruit ripening is still lacking. Here, we show that the tomato APETALA2a (AP2a) transcription factor regulates fruit ripening via regulation of ethylene biosynthesis and signaling. RNA interference (RNAi)-mediated repression of AP2a resulted in alterations in fruit shape, orange ripe fruits, and altered carotenoid accumulation. Microarray expression analyses of the ripe AP2 RNAi fruits showed altered expression of genes involved in various metabolic pathways, such as the phenylpropanoid and carotenoid pathways, as well as in hormone synthesis and perception. Genes involved in chromoplast differentiation and other ripening-associated processes were also differentially expressed, but softening and ethylene biosynthesis occurred in the transgenic plants. Ripening regulators RIPENING-INHIBITOR, NON-RIPENING, and COLORLESS NON-RIPENING (CNR) function upstream of AP2a and positively regulate its expression. In the pericarp of AP2 RNAi fruits, mRNA levels of CNR were elevated, indicating that AP2a and CNR are part of a negative feedback loop in the regulation of ripening. Moreover, we demonstrated that CNR binds to the promoter of AP2a in vitro.

Genome-wide association studies have been successful in identifying genes involved in polygenic traits and are valuable for crop improvement. Tomato (Solanum lycopersicum) is a major crop and is highly appreciated worldwide for its health value. We used a core collection of 163 tomato accessions composed of S. lycopersicum, S. lycopersicum var cerasiforme, and Solanum pimpinellifolium to map loci controlling variation in fruit metabolites. Fruits were phenotyped for a broad range of metabolites, including amino acids, sugars, and ascorbate. In parallel, the accessions were genotyped with 5,995 single-nucleotide polymorphism markers spread over the whole genome. Genome-wide association analysis was conducted on a large set of metabolic traits that were stable over 2 years using a multilocus mixed model as a general method for mapping complex traits in structured populations and applied to tomato. We detected a total of 44 loci that were significantly associated with a total of 19 traits, including sucrose, ascorbate, malate, and citrate levels. These results not only provide a list of candidate loci to be functionally validated but also a powerful analytical approach for finding genetic variants that can be directly used for crop improvement and deciphering the genetic architecture of complex traits.

The Arabidopsis thaliana L. genome contains 58 membrane proteins belonging to the mitochondrial carrier family. Two mitochondrial carrier family members, here named AtNDT1 and AtNDT2, exhibit high structural similarities to the mitochondrial nicotinamide adenine dinucleotide (NAD ؉ ) carrierScNDT1 from bakers' yeast. Expression of AtNDT1 or AtNDT2 restores mitochondrial NAD ؉ transport activity in a yeast mutant lacking ScNDT. Localization studies with green fluorescent protein fusion proteins provided evidence that AtNDT1 resides in chloroplasts, whereas only AtNDT2 locates to mitochondria. Heterologous expression in Escherichia coli followed by purification, reconstitution in proteoliposomes, and uptake experiments revealed that both carriers exhibit a submillimolar affinity for NAD ؉ and transport this compound in a counterexchange mode. Among various substrates ADP and AMP are the most efficient counter-exchange substrates for NAD ؉ .Atndt1-and Atndt2-promoter-GUS plants demonstrate that both genes are strongly expressed in developing tissues and in particular in highly metabolically active cells. The presence of both carriers is discussed with respect to the subcellular localization of de novo NAD ؉ biosynthesis in plants and with respect to both the NAD ؉ -dependent metabolic pathways and the redox balance of chloroplasts and mitochondria.Nucleotides are metabolites of enormous importance for all living cells. They are the essential building blocks for DNA and RNA synthesis, energize most anabolic and many catabolic reactions, and fulfill critical functions in intracellular signal transduction (1, 2). Moreover, nucleotides serve as cofactors for a wide number of enzymes and are, with water, the most highly connected compounds within the metabolic network (3). Among these co-factors nicotinamide adenine dinucleotides are widely used for reductive/oxidative processes, playing important roles in the operation and control of a wide range of dehydrogenase activities. Accordingly, nucleotides are essential in nearly all cell organelles, and transport of these solutes into mitochondria, plastids, the endoplasmic reticulum, the Golgi apparatus, and peroxisomes has been observed (4 -7).Two types of nucleotide transport proteins have been identified to date at the molecular level: nucleotide transporter (NTT) 2 type carriers and members of the mitochondrial carrier family. The former transporters occur in plastids from all plants (8) and in a limited number of intracellular pathogenic bacteria (9). Most NTT-type carrier proteins catalyze an ATP/ADPϩP i counter-exchange mode of transport (10 -13), but several bacterial NTT proteins mediate either H ϩ /nucleotide transport or NAD ϩ /ADP counter-exchange (12,14,15). With the exception of the bacterial NAD ϩ /ADP carrier (14), all NTT proteins exhibit 12 predicted trans-membrane domains, whereas none of the NTT proteins share structural or domain similarities to members of the mitochondrial carrier family (11).Carriers belonging to the mitochondrial carrier family (MC...