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.
Highlights
Cassava (Manihot esculenta Crantz) is a starchy root crop widely grown in Southeast Asia, Latin America, the Caribbean and subSaharan Africa (SSA) for human and livestock consumption, and as a feedstock for biofuels and other bio-based materials (Howeler et al, 2013)
We report the association of increased carotenoids with the marked reduction in the onset of postharvest physiological deterioration (PPD) of cassava storage roots
The crtB gene was linked to coding sequence of a plastid transit peptide to target the bacterial phytoene synthase to plastids, the site of carotenoid production
Summary
Cassava (Manihot esculenta Crantz) is a starchy root crop widely grown in Southeast Asia, Latin America, the Caribbean and subSaharan Africa (SSA) for human and livestock consumption, and as a feedstock for biofuels and other bio-based materials (Howeler et al, 2013). Cassava storage roots are an excellent source of dietary calories due to the high content of starch (Montagnac et al, 2009; Rickard et al, 1991) Despite these desirable qualities, cassava production in SSA is limited by its susceptibility to disease (Bull et al, 2011; Reynolds et al, 2015) and to rapid postharvest physiological deterioration (PPD) (Naziri et al, 2014; Wenham, 1995). Cassava production in SSA is limited by its susceptibility to disease (Bull et al, 2011; Reynolds et al, 2015) and to rapid postharvest physiological deterioration (PPD) (Naziri et al, 2014; Wenham, 1995) The latter restricts the crop as source of income for subsistence farmers due to difficulties in transporting rural-grown cassava to urban markets and factory locations. Biofortification of staple crops such as cassava has considerable potential for longterm impacts in correcting deficiencies of critically essential vitamins and minerals
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