木薯β-胡萝卜素的生物强化

      木薯作为非洲撒哈拉沙漠以南地区的主粮作物，富含热量物质但缺乏包括维生素A原β-胡萝卜素等重要的微量元素。近日，内布拉斯加大学林肯分校的科研人员通过马铃薯1型糖蛋白启动子驱动脱氧-D-葡萄糖-5-磷酸酯合成酶DXS基因和来源于细菌的八氢番茄红素合成酶crtB基因的共表达获得β-胡萝卜素含量显著提高的转基因木薯。田间收获转基因木薯的块根中类胡萝卜素含量最高为60μg/g DW，是非转基因木薯含量的30倍，而且85-90%的类胡萝卜素以最容易吸收的全反式胡萝卜素的形式存在。同时，β-胡萝卜素在块根的积累延迟了采后的生理恶化，而采后生理恶化是影响木薯产品利用的主要限制因子。研究人员进一步分析了富含β-胡萝卜素的转基因木薯块根的代谢产物变化，发现β-胡萝卜素的含量与干物质量呈显著的负相关，在β-胡萝卜素含量最高的转基因木薯株系中，其干物质量减少了50% 到 60%。进一步分析显示，干物质量的减少伴随着淀粉含量的减少以及脂肪酸总量、甘油三酯、可溶性糖和脱落酸的升高。类似地，转化相同基因元件的转基因马铃薯其块茎的代谢产物呈现相近的表型。转录组分析结果显示，与非转基因相比，富含β-胡萝卜素的转基因木薯其ADP-葡萄糖焦磷酸化酶等淀粉代谢相关的基因表达下调。该研究结果不仅表明富含淀粉组织的维生素A原生物强化可能会引起非预期的代谢产物变化，而且为通过代谢调控恢复淀粉产量提供了思路。
Plant Biotechnology Journal,28 November 2017
Provitamin A Biofortification of Cassava Enhances Shelf-Life but Reduces Dry Matter Content of Storage Roots Due to Altered Carbon Partitioning into Starch
Authors
Getu Beyene,Felix R. Solomon,Raj Deepika Chauhan,Eliana Gaitán-Solis,Narayanan Narayanan,Jackson Gehan,Dimuth Siritunga,Robyn L. Stevens,John Jifon,Joyce Van Eck,Edward Linsler,Malia Gehan,Muhammad Ilyas,Martin Fregene,Richard T. Sayre,Paul Anderson,Nigel J. Taylor,Edgar B. Cahoon
Summary
Storage roots of cassava, 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 non-transgenic 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 post-harvest 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% to 60% of dry matter content in the highest carotenoid accumulating storage roots of different cultivars. Further analysis confirmed 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 reduced expression of genes involved in starch biosynthesis including ADP-glucose pyrophosphorylase genes in transgenic, carotene-accumulating cassava roots relative to non-transgenic 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.