Iodine biofortification of crops: agronomic biofortification, metabolic engineering and iodine bioavailability

Iodine deficiency is a widespread micronutrient malnutrition problem, and the addition of iodine to table salt represents the most common prophylaxis tool. The biofortification of crops with iodine is a recent strategy to further enrich the human diet with a potentially cost-effective, well accepted and bioavailable iodine source. Understanding how iodine functions in higher plants is key to establishing suitable biofortification approaches. This review describes the current knowledge regarding iodine physiology in higher plants, and provides updates on recent agronomic and metabolic engineering strategies of biofortification. Whereas the direct administration of iodine is effective to increase the iodine content in many plant species, a more sophisticated genetic engineering approach seems to be necessary for the iodine biofortification of some important staple crops

Alternative Splicing in the Anthocyanin Fruit Gene Encoding an R2R3 MYB Transcription Factor Affects Anthocyanin Biosynthesis in Tomato Fruits

Abstract Tomato (Solanum lycopersicum L.) fruits are typically red at ripening, with high levels of carotenoids and a low content in flavonoids. Considerable work has been done to enrich the spectrum of their health-beneficial phytochemicals, and interspecific crosses with wild species have successfully led to purple anthocyanin-colored fruits. The Aft (Anthocyanin fruit) tomato accession inherited from Solanum chilense the ability to accumulate anthocyanins in fruit peel through the introgression of loci controlling anthocyanin pigmentation, including four R2R3 MYB transcription factor encoding genes. Here, we carried out a comparative functional analysis of these transcription factors in wild type and Aft plants, testing their ability to take part in the transcriptional complexes that regulate the biosynthetic pathway and their efficiency in inducing anthocyanin pigmentation. Significant differences emerged for SlAN2like, both in the expression level and protein functionality, with splicing mutations determining a complete loss-of-function of the wild type protein. This transcription factor thus appears to play a key role in the anthocyanin fruit pigmentation. Our data provide new clues to the long-awaited genetic bases of the Aft phenotype and also contribute to clarify why domesticated tomato fruits display a homogeneous red coloration without the typical purple streaks observed in wild tomato species

Tomato fruits: a good target for iodine biofortification

Iodine is a trace element that is fundamental for human health: its deficiency affects about two billion people worldwide. Fruits and vegetables are usually poor sources of iodine; however, plants can accumulate iodine if it is either present or exogenously administered to the soil. The biofortification of crops with iodine has therefore been proposed as a strategy for improving human nutrition. A greenhouse pot experiment was carried out to evaluate the possibility of biofortifying tomato fruits with iodine. Increasing concentrations of iodine supplied as KI or KIO3 were administered to plants as root treatments and the iodine accumulation in fruits was measured. The influences of the soil organic matter content or the nitrate level in the nutritive solution were analyzed. Finally, yield and qualitative properties of the biofortified tomatoes were considered, as well as the possible influence of fruit storage and processing on the iodine content. Results showed that the use of both the iodized salts induced a significant increase in the fruit’s iodine content in doses that did not affect plant growth and development. The final levels ranged from a few mg up to 10 mg iodine kg-1 fruit fresh weight and are more than adequate for a biofortification program, since 150 μg iodine per day is the recommended dietary allowance for adults. In general, the iodine treatments scarcely affected fruit appearance and quality, even with the highest concentrations applied. In contrast, the use of KI in plants fertilized with low doses of nitrate induced moderate phytotoxicity symptoms. Organic matter-rich soils improved the plant’s health and production, with only mild reductions in iodine stored in the fruits. Finally, a short period of storage at room temperature or a 30-min boiling treatment did not reduce the iodine content in the fruits, if the peel was maintained. All these results suggest that tomato is a particularly suitable crop for iodine biofortification programs

Misexpression of a chloroplast aspartyl protease leads to severe growth defects and alters carbohydrate metabolism in Arabidopsis

The crucial role of carbohydrate in plant growth and morphogenesis is widely recognized. In this study, we describe the characterization of nana, a dwarf Arabidopsis (Arabidopsis thaliana) mutant impaired in carbohydrate metabolism. We show that the nana dwarf phenotype was accompanied by altered leaf morphology and a delayed flowering time. Our genetic and molecular data indicate that the mutation in nana is due to a transfer DNA insertion in the promoter region of a gene encoding a chloroplast-located aspartyl protease that alters its pattern of expression. Overexpression of the gene (oxNANA) phenocopies the mutation. Both nana and oxNANA display alterations in carbohydrate content, and the extent of these changes varies depending on growth light intensity. In particular, in low light, soluble sugar levels are lower and do not show the daily fluctuations observed in wild-type plants. Moreover, nana and oxNANA are defective in the expression of some genes implicated in sugar metabolism and photosynthetic light harvesting. Interestingly, some chloroplast-encoded genes as well as genes whose products seem to be involved in retrograde signaling appear to be down-regulated. These findings suggest that the NANA aspartic protease has an important regulatory function in chloroplasts that not only influences photosynthetic carbon metabolism but also plastid and nuclear gene expression

Anthocyanins from Purple Tomatoes as Novel Antioxidants to Promote Human Health

Anthocyanins are plant secondary metabolites belonging to the class of polyphenols, whose beneficial roles in the prevention and treatment of several important human diseases have been demonstrated in many epidemiological studies. Their intake through diet strictly depends on the eating habits, as anthocyanins are contained in red and purple fruit and vegetables as well as in some processed foods and beverages, such as red wine. Genetic engineering and breeding programs have been recently carried out to increase the content of anthocyanins in candidate plant species which cannot offer satisfactory levels of these precious compounds. Tomato (Solanum lycopersicum) is a vegetable commodity where these strategies have resulted in success, leading to the production of new anthocyanin-rich fruit varieties, some of which are already marketed. These varieties produce purple fruits with a high nutraceutical value, combining the health benefits of the anthocyanins to the other classical tomato phytochemicals, particularly carotenoids. The antioxidant capacity in tomato purple fruits is higher than in non-anthocyanin tomatoes and their healthy role has already been demonstrated in both in vitro and in vivo studies. Recent evidence has indicated a particular capacity of tomato fruit anthocyanins to act as scavengers of harmful reactive chemical species and inhibitors of proliferating cancer cells, as well as anti-inflammatory molecules

Fruit Colour and Novel Mechanisms of Genetic Regulation of Pigment Production in Tomato Fruits

Fruit colour represents a genetic trait with ecological and nutritional value. Plants mainly use colour to attract animals and favour seed dispersion. Thus, in many species, fruit colour coevolved with frugivories and their preferences. Environmental factors, however, represented other adaptive forces and further diversification was driven by domestication. All these factors cooperated in the evolution of tomato fruit, one of the most important in human nutrition. Tomato phylogenetic history showed two main steps in colour evolution: the change from green-chlorophyll to red-carotenoid pericarp, and the loss of the anthocyanic pigmentation. These events likely occurred with the onset of domestication. Then spontaneous mutations repeatedly occurred in carotenoid and phenylpropanoid pathways, leading to colour variants which often were propagated. Introgression breeding further enriched the panel of pigmentation patterns. In recent decades, the genetic determinants underneath tomato colours were identified. Novel evidence indicates that key regulatory and biosynthetic genes undergo mechanisms of gene expression regulation that are much more complex than what was imagined before: post-transcriptional mechanisms, with RNA splicing among the most common, indeed play crucial roles to fine-tune the expression of this trait in fruits and offer new substrate for the rise of genetic variables, thus providing further evolutionary flexibility to the character

The atroviolacea gene encodes an R3-MYB protein repressing anthocyanin synthesis in tomato plants

The anthocyanin biosynthetic pathway is well characterized in plants. However, in tomato (Solanum lycopersicum L.) an exhaustive knowledge of its regulation is still lacking. Tomato mutants showing higher levels of anthocyanins in fruits or vegetative tissues, such as Anthocyanin fruit (Aft) or atroviolacea (atv), have been extensively exploited in the attempt to clarify the process. Nevertheless, only candidate genes have been proposed as responsible for such phenotypes. The recessive atv mutation likely represents an allelic variant of a gene introgressed in tomato from wild Solanum species. We performed genome sequencing of atv/atv plants followed by candidate gene analysis, and identified a mutated gene encoding an R3-MYB protein. When overexpressed, this protein abolished anthocyanin production in tomato seedlings and plants, by silencing key regulators and biosynthetic genes of the pathway. The functional analysis of the protein clearly showed that it can negatively interfere with the activation of the anthocyanin biosynthetic pathway mediated by the endogenous MYB-bHLH-WDR (MBW) complexes. In particular, this R3-MYB protein can directly bind the bHLH factors which are part of the MBW complexes, therefore acting as a competitive inhibitor. The R3-MYB protein here described is therefore involved in a feedback mechanism that dampens the production of anthocyanins once activated by endogenous or exogenous stimuli. The atv mutation causes the production of a truncated version of the R3-MYB factor that cannot retain the full potential to inhibit the MBW complexes, thus leading to a constitutively higher production of anthocyanins