Boosting carotenoid content in Malus domestica var. Fuji by expressing AtDXR through an Agrobacterium-mediated transformation method

Overview on Current Selectable Marker Systems and Novel Marker Free Approaches in Fruit Tree Genetic Engineering

Selectable marker genes are useful for recognizing which cells have integrated specific sequences in their genome after genetic transformation processes. They are especially important for fruit trees genetic transformation to individuate putatively genetically modified events, because most of the protocols used to genetic engineer these species are often unsuccessful or with low efficiency. Traditional selectable marker genes, mainly of bacterial origin, confer antibiotics/herbicides-resistance or metabolic advantages to transformed cells. Genes that allow the visual recognition of engineered tissues without using any selective agent, such as morphogenic regulators and reporter genes, are also used as selection tools to in vitro identify genetically modified regenerated lines. As final step, genetic engineered plants should be tested in field conditions, where selectable marker genes are no longer necessary, and strongly unpopular especially for the commercial development of the new products. Thus, different approaches, mainly based on the use of site-specific recombinases and/or editing nucleases, are being now used to recover marker-free fruit crops. This review describes and comments the most used and suitable selection tools of interest, particularly for fruit tree genetic engineering. Lastly, a spotlight highlights the biosafety aspects related to the use of selectable marker genes exploited for fruit species genetic engineering.

Plant terpenoid biosynthetic network and its multiple layers of regulation

Terpenoids constitute one of the largest and most chemically diverse classes of primary and secondary metabolites in nature with an exceptional breadth of functional roles in plants. Biosynthesis of all terpenoids begins with the universal five‑carbon building blocks, isopentenyl diphosphate (IPP) and its allylic isomer dimethylallyl diphosphate (DMAPP), which in plants are derived from two compartmentally separated but metabolically crosstalking routes, the mevalonic acid (MVA) and methylerythritol phosphate (MEP) pathways. Here, we review the current knowledge on the terpenoid precursor pathways and highlight the critical hidden constraints as well as multiple regulatory mechanisms that coordinate and homeostatically govern carbon flux through the terpenoid biosynthetic network in plants.

MdTPR16, an apple tetratricopeptide repeat (TPR)-like superfamily gene, positively regulates drought stress in apple

The Tetratricopeptide repeat (TPR)-like superfamily with TPR conserved domains is widely involved in the growth and abiotic stress in many plants. In this report, the gene MdTPR16 belongs to the TPR family in apple (Malus domestica). Promoter analysis reveal that MdTPR16 incorporated various stress response elements, including the drought stress response elements. And different abiotic stress treatments, drought especially, significantly induce the response of MdTPR16. Overexpression of MdTPR16 result in better drought tolerance in apple and Arabidopsis by up-regulating the expression levels of drought stress-related genes, achieving a higher chlorophyll content level, more material accumulation, and overall better growth compared to WT in the drought. Under drought stress, the overexpressed MdTPR16 also mitigate the oxidative damage in cells by reducing the electrolyte leakage, malondialdehyde content, and the H2O2 and O2- accumulation in apples and Arabidopsis. In conclusion, MdTPR16 act as a beneficial regulator of drought stress response by regulating the expression of related genes and the cumulation of reactive oxygen species (ROS).

Carrot DcALFIN4 and DcALFIN7 Transcription Factors Boost Carotenoid Levels and Participate Differentially in Salt Stress Tolerance When Expressed in Arabidopsis thaliana and Actinidia deliciosa

ALFIN-like transcription factors (ALs) are involved in several physiological processes such as seed germination, root development and abiotic stress responses in plants. In carrot (Daucus carota), the expression of DcPSY2, a gene encoding phytoene synthase required for carotenoid biosynthesis, is induced after salt and abscisic acid (ABA) treatment. Interestingly, the DcPSY2 promoter contains multiple ALFIN response elements. By in silico analysis, we identified two putative genes with the molecular characteristics of ALs, DcAL4 and DcAL7, in the carrot transcriptome. These genes encode nuclear proteins that transactivate reporter genes and bind to the carrot DcPSY2 promoter in yeast. The expression of both genes is induced in carrot under salt stress, especially DcAL4 which also responds to ABA treatment. Transgenic homozygous T3 Arabidopsis thaliana lines that stably express DcAL4 and DcAL7 show a higher survival rate with respect to control plants after chronic salt stress. Of note is that DcAL4 lines present a better performance in salt treatments, correlating with the expression level of DcAL4, AtPSY and AtDXR and an increase in carotenoid and chlorophyll contents. Likewise, DcAL4 transgenic kiwi (Actinidia deliciosa) lines show increased carotenoid and chlorophyll content and higher survival rate compared to control plants after chronic salt treatment. Therefore, DcAL4 and DcAL7 encode functional transcription factors, while ectopic expression of DcAL4 provides increased tolerance to salinity in Arabidopsis and Kiwi plants.

Overexpression of PSY1 increases fruit skin and flesh carotenoid content and reveals associated transcription factors in apple (Malus × domestica)

Knowledge of the transcriptional regulation of the carotenoid metabolic pathway is still emerging and here, we have misexpressed a key biosynthetic gene in apple to highlight potential transcriptional regulators of this pathway. We overexpressed phytoene synthase (PSY1), which controls the key rate-limiting biosynthetic step, in apple and analyzed its effects in transgenic fruit skin and flesh using two approaches. Firstly, the effects of PSY overexpression on carotenoid accumulation and gene expression was assessed in fruit at different development stages. Secondly, the effect of light exclusion on PSY1-induced fruit carotenoid accumulation was examined. PSY1 overexpression increased carotenoid content in transgenic fruit skin and flesh, with beta-carotene being the most prevalent carotenoid compound. Light exclusion by fruit bagging reduced carotenoid content overall, but carotenoid content was still higher in bagged PSY fruit than in bagged controls. In tissues overexpressing PSY1, plastids showed accelerated chloroplast to chromoplast transition as well as high fluorescence intensity, consistent with increased number of chromoplasts and carotenoid accumulation. Surprisingly, the expression of other carotenoid pathway genes was elevated in PSY fruit, suggesting a feed-forward regulation of carotenogenesis when this enzyme step is mis-expressed. Transcriptome profiling of fruit flesh identified differentially expressed transcription factors (TFs) that also were co-expressed with carotenoid pathway genes. A comparison of differentially expressed genes from both the developmental series and light exclusion  treatment revealed six candidate TFs exhibiting strong correlation with carotenoid accumulation. This combination of physiological, transcriptomic and metabolite data sheds new light on plant carotenogenesis and TFs that may play a role in regulating apple carotenoid biosynthesis.

Mining of Potential Gene Resources for Breeding Nutritionally Improved Maize

Maize is one of the leading food crops and its kernel is rich in starch, lipids, protein and other energy substances. In addition, maize kernels also contain many trace elements that are potentially beneficial to human health, such as vitamins, minerals and other secondary metabolites. However, gene resources that could be applied for nutrient improvement are limited in maize. In this review, we summarized 107 genes that are associated with nutrient content from different plant species and identified 246 orthologs from the maize genome. In addition, we constructed physical maps and performed a detailed expression pattern analysis for the 246 maize potential gene resources. Combining expression profiles and their potential roles in maize nutrient improvement, genetic engineering by editing or ectopic expression of these genes in maize are expected to improve resistant starch, oil, essential amino acids, vitamins, iron, zinc and anthocyanin levels of maize grains. Thus, this review provides valuable gene resources for maize nutrient improvement.

Phytochemical Content and Antioxidant Activity of Malus domestica Borkh Peel Extracts

Apple is an important dietary source of carotenoids and phenolic compounds, and its regular consumption is associated with several health benefits. The aim of this study was to evaluate the phytochemical composition of fresh peels of four red-skinned (“Champion”, “Generos”, “Idared”, “Florina”) and two yellow-skinned (“Golden Delicious”, “Reinette Simirenko”) apple varieties. Antioxidant activity of apple peel extracts was determined by ferric reducing antioxidant power (FRAP) and ABTS radical scavenging capacity assays. Total carotenoid and polyphenolic contents were determined spectrophotometrically, while the profile of individual carotenoids and anthocyanins (in red-skinned varieties) was analyzed using high-performance liquid chromatography coupled to a photodiode array detector (HPLC-PDA). Carotenoid composition was specific for each variety, and total carotenoid content was slightly higher in yellow-skinned apple peels compared to red-skinned varieties. In contrast, total phenolic content was higher in the peels of red-skinned cultivars. Anthocyanin profile was predominated by cyanidin-3-O-galactoside. Antioxidant potential followed the trend of the total polyphenolic content, being highest in “Florina”, as measured by both FRAP and ABTS assays. Our results demonstrated apple peels have high phytochemical content with diverse compositions, and their regular consumption can be an excellent source of antioxidants.

Daucus carota DcPSY2 and DcLCYB1 as Tools for Carotenoid Metabolic Engineering to Improve the Nutritional Value of Fruits

Carotenoids are pigments with important nutritional value in the human diet. As antioxidant molecules, they act as scavengers of free radicals enhancing immunity and preventing cancer and cardiovascular diseases. Moreover, α-carotene and β-carotene, the main carotenoids of carrots (Daucus carota) are precursors of vitamin A, whose deficiency in the diet can trigger night blindness and macular degeneration. With the aim of increasing the carotenoid content in fruit flesh, three key genes of the carotenoid pathway, phytoene synthase (DcPSY2) and lycopene cyclase (DcLCYB1) from carrots, and carotene desaturase (XdCrtI) from the yeast Xanthophyllomyces dendrorhous, were optimized for expression in apple and cloned under the Solanum chilense (tomatillo) polygalacturonase (PG) fruit specific promoter. A biotechnological platform was generated and functionally tested by subcellular localization, and single, double and triple combinations were both stably transformed in tomatoes (Solanum lycopersicum var. Microtom) and transiently transformed in Fuji apple fruit flesh (Malus domestica). We demonstrated the functionality of the S. chilense PG promoter by directing the expression of the transgenes specifically to fruits. Transgenic tomato fruits expressing DcPSY2, DcLCYB1, and DcPSY2-XdCRTI, produced 1.34, 2.0, and 1.99-fold more total carotenoids than wild-type fruits, respectively. Furthermore, transgenic tomatoes expressing DcLCYB1, DcPSY2-XdCRTI, and DcPSY2-XdCRTI-DcLCYB1 exhibited an increment in β-carotene levels of 2.5, 3.0, and 2.57-fold in comparison with wild-type fruits, respectively. Additionally, Fuji apple flesh agroinfiltrated with DcPSY2 and DcLCYB1 constructs showed a significant increase of 2.75 and 3.11-fold in total carotenoids and 5.11 and 5.84-fold in β-carotene, respectively whereas the expression of DcPSY2-XdCRTI and DcPSY2-XdCRTI-DcLCYB1 generated lower, but significant changes in the carotenoid profile of infiltrated apple flesh. The results in apple demonstrate that DcPSY2 and DcLCYB1 are suitable biotechnological genes to increase the carotenoid content in fruits of species with reduced amounts of these pigments.