Glucosinolate variability between turnip organs during development

Turnip (Brassica rapa spp. rapa) is an important vegetable species, with a unique physiology. Several plant parts, including both the turnip tubers and leaves, are important for human consumption. During the development of turnip plants, the leaves function as metabolic source tissues, while the tuber first functions as a sink, while later the tuber turns into a source for development of flowers and seeds. In the present study, chemical changes were determined for two genotypes with different genetic background, and included seedling, young leaves, mature leaves, tuber surface, tuber core, stalk, flower and seed tissues, at seven different time points during plant development. As a basis for understanding changes in glucosinolates during plant development, the profile of glucosinolates was analysed using liquid chromatography (LC) coupled to mass spectrometry (MS). This analysis was complemented by a gene expression analysis, focussed on GLS biosynthesis, which could explain part of the observed variation, pointing to important roles of specific gene orthologues for defining the chemical differences. Substantial differences in glucosinolate profiles were observed between above-ground tissues and turnip tuber, reflecting the differences in physiological role. In addition, differences between the two genotypes and between tissues that were harvested early or late during the plant lifecycle. The importance of the observed differences in glucosinolate profile for the ecophysiology of the turnip and for breeding turnips with optimal chemical profiles is discussed.芜菁（Brassica rapa spp. rapa）是一类重要的蔬菜物种，具备独特的生理特征。其多个组织器官均可供人类食用，包括块根与叶片。在芜菁植株的生长发育过程中，叶片发挥代谢源（metabolic source）组织的功能；而块根最初作为代谢库（metabolic sink）存在，后续则转变为花与种子发育的代谢源。本研究针对两个遗传背景存在差异的芜菁基因型，选取植株发育过程中的7个不同时间节点，对幼苗、幼叶、成熟叶、块根表层、块根髓部、茎秆、花以及种子等组织的化学变化进行了检测。为解析植株发育过程中硫代葡萄糖苷（glucosinolates）的变化规律，本研究采用液相色谱（LC）与质谱（MS）联用技术对硫代葡萄糖苷的谱图特征进行了分析。本研究同时开展了聚焦于硫代葡萄糖苷生物合成的基因表达分析，以此作为上述化学检测的补充；该分析可解释部分观测到的变异，提示特定直系同源基因（orthologues）在决定化学特征差异中发挥了重要作用。研究发现，地上组织与芜菁块根的硫代葡萄糖苷谱存在显著差异，这一结果反映了二者生理功能的差异。此外，两个基因型之间，以及植株生育周期内不同采收时期的组织之间，亦存在硫代葡萄糖苷谱的差异。本研究最后讨论了观测到的硫代葡萄糖苷谱差异对芜菁生态生理学研究，以及培育具备最优化学特征谱的芜菁品种的重要意义。