A chemical genetic roadmap to improved tomato flavor.

Modern commercial tomato varieties are substantially less flavorful than heirloom varieties. To understand and ultimately correct this deficiency, we quantified flavor-associated chemicals in 398 modern, heirloom, and wild accessions. A subset of these accessions was evaluated in consumer panels, identifying the chemicals that made the most important contributions to flavor and consumer liking. We found that modern commercial varieties contain significantly lower amounts of many of these important flavor chemicals than older varieties. Whole-genome sequencing and a genome-wide association study permitted identification of genetic loci that affect most of the target flavor chemicals, including sugars, acids, and volatiles. Together, these results provide an understanding of the flavor deficiencies in modern commercial varieties and the information necessary for the recovery of good flavor through molecular breeding.

A 13-lipoxygenase, TomloxC, is essential for synthesis of C5 flavour volatiles in tomato.

C5 volatile compounds, derived from fatty acids, are among the most important contributors to consumer liking of fresh tomatoes. Despite their important roles in flavour, the genes responsible for C5 volatile synthesis have yet to be identified. This work shows that their synthesis is catalysed in part by a 13-lipoxygenase (LOX), TomloxC, the same enzyme responsible for synthesis of C6 volatiles. C5 synthesis is independent of hydroperoxide lyase (HPL); moreover, HPL knockdown significantly increased C5 volatile synthesis. This LOX-dependent, HPL-independent pathway functions in both fruits and leaves. Synthesis of C5 volatiles increases in leaves following mechanical wounding but does not increase in response to infection with Xanthomonas campestris pv. vesicatoria. Large reductions in C5 and C6 volatiles in antisense TomloxC knockdown plants were observed but those reductions did not alter the development of disease symptoms, indicating that these volatiles do not have an important defensive function against this bacterial pathogen.

The chemical interactions underlying tomato flavor preferences.

Although human perception of food flavors involves integration of multiple sensory inputs, the most salient sensations are taste and olfaction. Ortho- and retronasal olfaction are particularly crucial to flavor because they provide the qualitative diversity so important to identify safe versus dangerous foods. Historically, flavor research has prioritized aroma volatiles present at levels exceeding the orthonasally measured odor threshold, ignoring the variation in the rate at which odor intensities grow above threshold. Furthermore, the chemical composition of a food in itself tells us very little about whether or not that food will be liked. Clearly, alternative approaches are needed to elucidate flavor chemistry. Here we use targeted metabolomics and natural variation in flavor-associated sugars, acids, and aroma volatiles to evaluate the chemistry of tomato fruits, creating a predictive and testable model of liking. This nontraditional approach provides novel insights into flavor chemistry, the interactions between taste and retronasal olfaction, and a paradigm for enhancing liking of natural products. Some of the most abundant volatiles do not contribute to consumer liking, whereas other less abundant ones do. Aroma volatiles make contributions to perceived sweetness independent of sugar concentration, suggesting a novel way to increase perception of sweetness without adding sugar.

Pod shatter-resistant Brassica fruit produced by ectopic expression of the FRUITFULL gene.

Arabidopsis has proven to be extremely useful as a reference organism for studies in plant biology, and huge efforts have been employed to unravel various mechanisms of Arabidopsis growth. A major challenge now is to demonstrate that this wealth of knowledge can be used for global agricultural and environmental improvement. Brassica species are closely related to Arabidopsis and represent ideal candidates for model-to-crop approaches as they include important crop plants, such as canola. Brassica plants normally disperse their seeds by a pod-shattering mechanism. Although this mechanism is an advantage in nature, unsynchronized pod shatter constitutes one of the biggest problems for canola farmers. Here, we show that ectopic expression of the Arabidopsis FRUITFULL gene in Brassica juncea is sufficient to produce pod shatter-resistant Brassica fruit and that the genetic pathway leading to valve margin specification is conserved between Arabidopsis and Brassica. These studies demonstrate a genetic strategy for the control of seed dispersal that should be generally applicable to diverse Brassica crop species to reduce seed loss.

Green light adjusts the plastid transcriptome during early photomorphogenic development.

During the transition from darkness to light, a suite of light sensors guides gene expression, biochemistry, and morphology to optimize acclimation to the new environment. Ultraviolet, blue, red, and far-red light all have demonstrated roles in modulating light responses, such as changes in gene expression and suppression of stem growth rate. However, green wavebands induce stem growth elongation, a response not likely mediated by known photosensors. In this study, etiolated Arabidopsis (Arabidopsis thaliana) seedlings were treated with a short, dim, single pulse of green light comparable in fluence and duration to that previously shown to excite robust stem elongation. Genome microarrays were then used to monitor coincident changes in gene expression. As anticipated, phytochrome A-regulated, nuclear-encoded transcripts were induced, confirming proper function of the sensitive phytochrome system. In addition, a suite of plastid-encoded transcripts decreased in abundance, including several typically up-regulated after phytochrome and/or cryptochrome activation. Further analyses using RNA gel-blot experiments demonstrated that the response is specific to green light, fluence dependent, and detectable within 30 min. The response obeys reciprocity and persists in the absence of known photosensors. Plastid transcript down-regulation was also observed in tobacco (Nicotiana tabacum) with similar temporal and fluence-response kinetics. Together, the down-regulation of plastid transcripts and increase in stem growth rate represent a mechanism that tempers progression of early commitment to the light environment, helping tailor seedling development during the critical process of establishment.

Expressed sequence tags (ESTs) and simple sequence repeat (SSR) markers from octoploid strawberry (Fragaria x ananassa).

BACKGROUND: Cultivated strawberry (Fragaria x ananassa) represents one of the most valued fruit crops in the United States. Despite its economic importance, the octoploid genome presents a formidable barrier to efficient study of genome structure and molecular mechanisms that underlie agriculturally-relevant traits. Many potentially fruitful research avenues, especially large-scale gene expression surveys and development of molecular genetic markers have been limited by a lack of sequence information in public databases. As a first step to remedy this discrepancy a cDNA library has been developed from salicylate-treated, whole-plant tissues and over 1800 expressed sequence tags (EST's) have been sequenced and analyzed. RESULTS: A putative unigene set of 1304 sequences--133 contigs and 1171 singlets--has been developed, and the transcripts have been functionally annotated. Homology searches indicate that 89.5% of sequences share significant similarity to known/putative proteins or Rosaceae ESTs. The ESTs have been functionally characterized and genes relevant to specific physiological processes of economic importance have been identified. A set of tools useful for SSR development and mapping is presented. CONCLUSION: Sequences derived from this effort may be used to speed gene discovery efforts in Fragaria and the Rosaceae in general and also open avenues of comparative mapping. This report represents a first step in expanding molecular-genetic analyses in strawberry and demonstrates how computational tools can be used to optimally mine a large body of useful information from a relatively small data set.