Multisite evaluation of phenotypic plasticity for specialized metabolites, some involved in carrot quality and disease resistance.

Renewed consumer demand motivates the nutritional and sensory quality improvement of fruits and vegetables. Specialized metabolites being largely involved in nutritional and sensory quality of carrot, a better knowledge of their phenotypic variability is required. A metabolomic approach was used to evaluate phenotypic plasticity level of carrot commercial varieties, over three years and a wide range of cropping environments spread over several geographical areas in France. Seven groups of metabolites have been quantified by HPLC or GC methods: sugars, carotenoids, terpenes, phenolic compounds, phenylpropanoids and polyacetylenes. A large variation in root metabolic profiles was observed, in relation with environment, variety and variety by environment interaction effects in decreasing order of importance. Our results show a clear diversity structuration based on metabolite content. Polyacetylenes, ß-pinene and α-carotene were identified mostly as relatively stable varietal markers, exhibiting static stability. Nevertheless, environment effect was substantial for a large part of carrot metabolic profile and various levels of phenotypic plasticity were observed depending on metabolites and varieties. A strong difference of environmental sensitivity between varieties was observed for several compounds, particularly myristicin, 6MM and D-germacrene, known to be involved in responses to biotic and abiotic stress. This work provides useful information about plasticity in the perspective of carrot breeding and production. A balance between constitutive content and environmental sensitivity for key metabolites should be reached for quality improvement in carrot and other vegetables.

Carrot Genotypes Contrasted by Root Color and Grown under Different Conditions Displayed Differential Pharmacological Profiles in Vascular and Metabolic Cells.

Carrots' genotype and growing conditions influence their potential properties to fight against cardiovascular and metabolic diseases. The present study evaluated the influence of carrot genotypes contrasted by root color (Bolero, Presto, Karotan, Deep Purple, Kintoki and Blanche des Vosges) growing under standard, water-restricted, biotic stress (Alternaria dauci inoculation), and combined stress conditions (water restriction and A.dauci inoculation). The effect of carrots' polyphenol and carotenoid content was assessed on endothelial and smooth muscle cells, hepatocytes, adipocytes and macrophages functions (oxidative stress, apoptosis, proliferation, lipid accumulation and inflammation). Independently of varieties or growing conditions, all carrot extracts affected vascular cells' oxidative stress and apoptosis, and metabolic cells' oxidative stress and lipid accumulation. Three clusters were revealed and displayed beneficial properties mostly for adipocytes function, smooth muscle cells and hepatocytes, and endothelial cells and hepatocytes, respectively. Karotan and Presto varieties exhibited endothelial tropism while Blanche des Vosges targeted adipocytes. Carrots under biotic stress are more efficient in inducing beneficial effects, with the Bolero variety being the most effective. However, extracts from carrots which grew under combined stress conditions had limited beneficial effects. This report underscores the use of certain carrot extracts as potential effective nutraceutical supplements for metabolic diseases.

Carotenoid gene expression explains the difference of carotenoid accumulation in carrot root tissues.

Main conclusion Variations in gene expression can partially explain the difference of carotenoid accumulation in secondary phloem and xylem of fleshy carrot roots. The carrot root is well divided into two different tissues separated by vascular cambium: the secondary phloem and xylem. The equilibrium between these two tissues represents an important issue for carrot quality, but the knowledge about the respective carotenoid accumulation is sparse. The aim of this work was (i) to investigate if variation in carotenoid biosynthesis gene expression could explain differences in carotenoid content in phloem and xylem tissues and (ii) to investigate if this regulation is differentially modulated in the respective tissues by water-restricted growing conditions. In this work, five carrot genotypes contrasting by their root color were studied in control and water-restricted conditions. Carotenoid content and the relative expression of 13 genes along the carotenoid biosynthesis pathway were measured in the respective tissues. Results showed that in orange genotypes and the purple one, carotenoid content was higher in phloem compared to xylem. For the red one, no differences were observed. Moreover, in control condition, variations in gene expression explained the different carotenoid accumulations in both tissues, while in water-restricted condition, no clear association between gene expression pattern and variations in carotenoid content could be detected except in orange-rooted genotypes. This work shows that the structural aspect of carrot root is more important for carotenoid accumulation in relation with gene expression levels than the consequences of expression changes upon water restriction.

ABI5 Is a Regulator of Seed Maturation and Longevity in Legumes.

The preservation of our genetic resources and production of high-quality seeds depends on their ability to remain viable and vigorous during storage. In a quantitative trait locus analysis on seed longevity in Medicago truncatula, we identified the bZIP transcription factor ABSCISIC ACID INSENSITIVE5 (ABI5). Characterization of Mt-abi5 insertion mutant seeds revealed that both the acquisition of longevity and dormancy were severely impaired. Using transcriptomes of developing Mt-abi5 seeds, we created a gene coexpression network and revealed ABI5 as a regulator of gene modules with functions related to raffinose family oligosaccharide (RFO) metabolism, late embryogenesis abundant (LEA) proteins, and photosynthesis-associated nuclear genes (PhANGs). Lower RFO contents in Mt-abi5 seeds were linked to the regulation of SEED IMBIBITION PROTEIN1 Proteomic analysis confirmed that a set of LEA polypeptides was reduced in mature Mt-abi5 seeds, whereas the absence of repression of PhANG in mature Mt-abi5 seeds was accompanied by chlorophyll and carotenoid retention. This resulted in a stress response in Mt-abi5 seeds, evident from an increase in α-tocopherol and upregulation of genes related to programmed cell death and protein folding. Characterization of abi5 mutants in a second legume species, pea (Pisum sativum), confirmed a role for ABI5 in the regulation of longevity, seed degreening, and RFO accumulation, identifying ABI5 as a prominent regulator of late seed maturation in legumes.

Differential Pigment Accumulation in Carrot Leaves and Roots during Two Growing Periods.

Carotenoids are important secondary metabolites involved in plant growth and nutritional quality of vegetable crops. These pigments are highly accumulated in carrot root, but knowledge about the impact of environmental factors on their accumulation is limited. The purpose of this work was to investigate the impact of environmental variations on carotenoid accumulation in carrot leaves and roots. In this work, carrots were grown during two contrasting periods to maximize bioclimatic differences. In leaves, carotenoid and chlorophyll contents were lower in the less favorable growing conditions, whereas relative contents were well conserved for all genotypes, suggesting a common regulatory mechanism. The down-regulation of all genes under environmental constraints demonstrates that carotenoid accumulation is regulated at the transcriptional level. In roots, the decrease in α-carotene and lutein contents was accompanied by an increase of ß-carotene relative content. At the transcriptional level, LCYB and ZEP expression increased, whereas LCYE expression decreased, in the less favorable conditions, suggesting that carotenoid biosynthesis is switched toward the ß-branch.

Carotenoid content and root color of cultivated carrot: a candidate-gene association study using an original broad unstructured population.

Accumulated in large amounts in carrot, carotenoids are an important product quality attribute and therefore a major breeding trait. However, the knowledge of carotenoid accumulation genetic control in this root vegetable is still limited. In order to identify the genetic variants linked to this character, we performed an association mapping study with a candidate gene approach. We developed an original unstructured population with a broad genetic basis to avoid the pitfall of false positive detection due to population stratification. We genotyped 109 SNPs located in 17 candidate genes ­ mostly carotenoid biosynthesis genes ­ on 380 individuals, and tested the association with carotenoid contents and color components. Total carotenoids and ß-carotene contents were significantly associated with genes zeaxanthin epoxydase (ZEP), phytoene desaturase (PDS) and carotenoid isomerase (CRTISO) while α-carotene was associated with CRTISO and plastid terminal oxidase (PTOX) genes. Color components were associated most significantly with ZEP. Our results suggest the involvement of the couple PDS/PTOX and ZEP in carotenoid accumulation, as the result of the metabolic and catabolic activities respectively. This study brings new insights in the understanding of the carotenoid pathway in non-photosynthetic organs.