Provitamin A biofortification of cassava enhances shelf life but reduces dry matter content of storage roots due to altered carbon partitioning into starch

Abstract

SummaryStorage roots of cassava (Manihot esculenta Crantz), a major subsistence crop of sub‐Saharan Africa, are calorie rich but deficient in essential micronutrients, including provitamin A β‐carotene. In this study, β‐carotene concentrations in cassava storage roots were enhanced by co‐expression of transgenes for deoxy‐d‐xylulose‐5‐phosphate synthase (DXS) and bacterial phytoene synthase (crtB), mediated by the patatin‐type 1 promoter. Storage roots harvested from field‐grown plants accumulated carotenoids to ≤50 μg/g DW, 15‐ to 20‐fold increases relative to roots from nontransgenic plants. Approximately 85%–90% of these carotenoids accumulated as all‐trans‐β‐carotene, the most nutritionally efficacious carotenoid. β‐Carotene‐accumulating storage roots displayed delayed onset of postharvest physiological deterioration, a major constraint limiting utilization of cassava products. Large metabolite changes were detected in β‐carotene‐enhanced storage roots. Most significantly, an inverse correlation was observed between β‐carotene and dry matter content, with reductions of 50%–60% of dry matter content in the highest carotenoid‐accumulating storage roots of different cultivars. Further analysis confirmed a concomitant reduction in starch content and increased levels of total fatty acids, triacylglycerols, soluble sugars and abscisic acid. Potato engineered to co‐express DXS and crtB displayed a similar correlation between β‐carotene accumulation, reduced dry matter and starch content and elevated oil and soluble sugars in tubers. Transcriptome analyses revealed a reduced expression of genes involved in starch biosynthesis including ADP‐glucose pyrophosphorylase genes in transgenic, carotene‐accumulating cassava roots relative to nontransgenic roots. These findings highlight unintended metabolic consequences of provitamin A biofortification of starch‐rich organs and point to strategies for redirecting metabolic flux to restore starch production.

The Cassava Source–Sink project: opportunities and challenges for crop improvement by metabolic engineering

“…For example, previous work on complex traits such as starch content in storage organs has mostly focused on enzymes or metabolite transporters involved in starch metabolism. Considering that storage root starch content based on dry matter may already be as high as 85% in modern cassava varieties (Beyene et al ., 2018), further increases in overall dry yield may be difficult to achieve by focusing on starch metabolism alone. It would therefore be useful to identify potential feedback mechanisms in cassava and other crops that could trigger storage organ growth when starch content increases.…”

Excellence in Cassava Breeding: Perspectives for the Future

“…No transgenic cassava has been grown commercially, partly due to the regulatory problems for this technology but also because of the difficulty in developing a transgenic cassava varieties that offer real advantages for producers and processors. Enhanced expression of phytoene synthase resulted in increased levels of carotenoids in the roots, but was reached at the expense of a drastic reduction in starch content [56]. Genetic transformation has been used to create resistance to CBSD.…”

Genetic Engineering for Global Food Security: Photosynthesis and Biofortification

“…Crops such as wheat (Triticum aestivum), rice (Oryza sativa), cassava (Manihot esculenta) and potato (Solanum tuberosum), which make up a large part of the diets of poorer communities, contain no significant levels of carotenoids or carotenoid derived products. Increasing the pro-vitamin A content of these staple crops through genetic engineering of carotenoid biosynthesis has resulted in high carotenoid varieties of tomato (Solanum lycopersicum) [124][125][126], maize (Zea mays) [127,128], wheat [129], canola (Brassica napus) [130], potato (Solanum tuberosum) [131][132][133], flaxseed (Linum usitatissimum) [134,135], cassava (Manihot esculenta) [132] and Sorghum [136,137] ( Table 2). Early efforts to increase pro-vitamin A content in rice (Oryza sativa) resulted in the generation of the β-carotene enriched "golden rice" [138,139], firstly over-expressing the rate limiting enzyme phytoene synthase (PSY) and subsequently by over-expressing multiple enzymatic steps [139].…”

“…The reductions in tocopherol have been addressed in Sorghum, where carotenoid biosynthetic genes were over-expressed along with homogentisic acid geranylgeranyl transferase (HGGT), which catalyses the committed step of tocotrienol biosynthesis has previously been shown to enhance total vitamin E antioxidants (see Section 3.5) (tocotrienols plus tocopherols) content by 10-to 15-fold [152]. Furthermore, in cassava and potato, increasing content resulted in a~25% decrease in the dry matter content of the tubers, and a decrease in starch content, whist sucrose, glucose, total fatty acid, triacylglycerols increased [132].…”