Vallabhaneni Ratnakar, Wurtzel Eleanore T
Department of Biological Sciences, Lehman College, City University of New York, Bronx, New York 10468, USA.
Plant Physiol. 2009 Jun;150(2):562-72. doi: 10.1104/pp.109.137042. Epub 2009 Apr 3.
Enhancement of the carotenoid biosynthetic pathway in food crops benefits human health and adds commercial value of natural food colorants. However, predictable metabolic engineering or breeding is limited by the incomplete understanding of endogenous pathway regulation, including rate-controlling steps and timing of expression in carotenogenic tissues. The grass family (Poaceae) contains major crop staples, including maize (Zea mays), wheat (Triticum aestivum), rice (Oryza sativa), sorghum (Sorghum bicolor), and millet (Pennisetum glaucum). Maize carotenogenesis was investigated using a novel approach to discover genes encoding limiting biosynthetic steps in the nutritionally targeted seed endosperm. A combination of bioinformatics and cloning were first used to identify and map gene families encoding enzymes in maize and other grasses. These enzymes represented upstream pathways for isopentenyl diphosphate and geranylgeranyl diphosphate synthesis and the downstream carotenoid biosynthetic pathway, including conversion to abscisic acid. A maize germplasm collection was used for statistical testing of the correlation between carotenoid content and candidate gene transcript levels. Multiple pathway bottlenecks for isoprenoid biosynthesis and carotenoid biosynthesis were discovered in specific temporal windows of endosperm development. Transcript levels of paralogs encoding isoprenoid isopentenyl diphosphate and geranylgeranyl diphosphate-producing enzymes, DXS3, DXR, HDR, and GGPPS1, were found to positively correlate with endosperm carotenoid content. For carotenoid pathway enzymes, transcript levels for CrtISO inversely correlated with seed carotenoid content, as compared with positive correlation of PSY1 transcripts. Since zeaxanthin epoxidase (ZEP) depletes the carotenoid pool in subsequent conversion to abscisic acid, ZEP transcripts were examined. Carotenoid accumulation was found to be inversely associated with ZEP1 and ZEP2 transcript levels. Extension of the maize results using phylogenetic analysis identified orthologs in other grass species that may serve as potential metabolic engineering targets.
增强粮食作物中的类胡萝卜素生物合成途径有益于人类健康，并增加天然食用色素的商业价值。然而，由于对内源途径调控的理解不完整，包括限速步骤和类胡萝卜素生成组织中的表达时间，可预测的代谢工程或育种受到限制。禾本科包含主要的作物主食，包括玉米（Zea mays）、小麦（Triticum aestivum）、水稻（Oryza sativa）、高粱（Sorghum bicolor）和小米（Pennisetum glaucum）。利用一种新方法对玉米类胡萝卜素生成进行了研究，以发现营养目标种子胚乳中编码限制生物合成步骤的基因。首先结合生物信息学和克隆技术，鉴定并绘制了玉米和其他禾本科植物中编码酶的基因家族图谱。这些酶代表了异戊烯基二磷酸和香叶基香叶基二磷酸合成的上游途径以及下游类胡萝卜素生物合成途径，包括转化为脱落酸。利用玉米种质资源库对类胡萝卜素含量与候选基因转录水平之间的相关性进行了统计检验。在胚乳发育的特定时间窗口内发现了类异戊二烯生物合成和类胡萝卜素生物合成的多个途径瓶颈。发现编码生成类异戊二烯异戊烯基二磷酸和香叶基香叶基二磷酸的酶DXS3、DXR、HDR和GGPPS1的旁系同源物的转录水平与胚乳类胡萝卜素含量呈正相关。对于类胡萝卜素途径的酶，与PSY1转录本呈正相关相比，CrtISO的转录水平与种子类胡萝卜素含量呈负相关。由于玉米黄质环氧化酶（ZEP）在随后转化为脱落酸的过程中消耗类胡萝卜素库，因此对ZEP转录本进行了检测。发现类胡萝卜素积累与ZEP1和ZEP2转录水平呈负相关。利用系统发育分析扩展玉米研究结果，鉴定了其他禾本科物种中的直系同源物，这些直系同源物可能是潜在的代谢工程靶点。
