Salinity impairs photosynthetic capacity and enhances carotenoid-related gene expression and biosynthesis in tomato (Solanum lycopersicum L. cv. Micro-Tom).

Carotenoids are essential components of the photosynthetic antenna and reaction center complexes, being also responsible for antioxidant defense, coloration, and many other functions in multiple plant tissues. In tomato, salinity negatively affects the development of vegetative organs and productivity, but according to previous studies it might also increase fruit color and taste, improving its quality, which is a current agricultural challenge. The fruit quality parameters that are increased by salinity are cultivar-specific and include carotenoid, sugar, and organic acid contents. However, the relationship between vegetative and reproductive organs and response to salinity is still poorly understood. Considering this, Solanum lycopersicum cv. Micro-Tom plants were grown in the absence of salt supplementation as well as with increasing concentrations of NaCl for 14 weeks, evaluating plant performance from vegetative to reproductive stages. In response to salinity, plants showed a significant reduction in net photosynthesis, stomatal conductance, PSII quantum yield, and electron transport rate, in addition to an increase in non-photochemical quenching. In line with these responses the number of tomato clusters decreased, and smaller fruits with higher soluble solids content were obtained. Mature-green fruits also displayed a salt-dependent higher induction in the expression of PSY1, PDS, ZDS, and LYCB, key genes of the carotenoid biosynthesis pathway, in correlation with increased lycopene, lutein, ß-carotene, and violaxanthin levels. These results suggest a key relationship between photosynthetic plant response and yield, involving impaired photosynthetic capacity, increased carotenoid-related gene expression, and carotenoid biosynthesis.

Differential role of the two ζ-carotene desaturase paralogs in carrot (Daucus carota): ZDS1 is a functional gene essential for plant development and carotenoid synthesis.

Daucus carota is a biennale crop that develops an edible storage root. Orange carrots, the most consumed cultivar worldwide, accumulate high levels of ß-carotene and α-carotene in the storage root during secondary growth. Genes involved in ß-carotene synthesis have been identified in carrots and unlike most species, D. carota has two ζ-carotene desaturase genes, named ZDS1 and ZDS2, that share 91.3 % identity in their coding regions. ZDS1 expression falls during leaf, but not root development, while ZDS2 is induced in leaves and storage roots of a mature plant. In this work, by means of post-transcriptional gene silencing, we determined that ZDS1 is essential for initial carrot development. The suppression of the expression of this gene by RNAi triggered a reduction in the transcript levels of ZDS2 and PSY2 genes, with a concomitant decrease in the carotenoid content in both, leaves and storage roots. On the contrary, transgenic lines with reduced ZDS2 transcript abundance maintain the same levels of expression of endogenous ZDS1 and PSY2 and carotenoid profile as wild-type plants. The simultaneous silencing of ZDS1 and ZDS2 resulted in lines with a negligible leaf and root development, as well as significantly lower endogenous PSY2 expression. Further functional analyses, such as a plastidial subcellular localization of ZDS1:GFP and the increment in carotenoid content in transgenic tobacco plants overexpressing the carrot ZDS1, confirmed that ZDS1 codifies for a functional enzyme. Overall, these results lead us to propose that the main ζ-carotene desaturase activity in carrot is encoded by the ZDS1 gene and ZDS2 gene has a complementary and non essential role.

Agrobacterium tumefaciens-Mediated Stable Transformation of Daucus carota.

Daucus carota L. (carrot) is one of the ten most important vegetables cultivated and consumed worldwide and is a main source of provitamin A. Carrot storage root is rich in dietary fiber, antioxidants, and other nutrients but especially in carotenoids. It has been also used as plant model for studding embryogenesis, as well as the genetic and genomic evolution of carrots and for carotenoid synthesis regulation, among others. Research in carrot often needs genetic transformation. Here we describe a step-by-step protocol on the nuclear and stable transformation of carrot through Agrobacterium tumefaciens and somatic embryogenesis in vitro culture. Somatic embryos, induced by supplementation of Murashige-Skoog medium with the 2,4D hormone, develop into seedlings after 6 months approximately when plants are ready to be transferred to a greenhouse. The protocol has over 85% of transformation efficiency.