During their first year of growth, overwintering biennial plants transport Suc through the phloem from photosynthetic source tissues to storage tissues. In their second year, they mobilize carbon from these storage tissues to fuel new growth and reproduction. However, both the mechanisms driving this shift and the link to reproductive growth remain unclear. During vegetative growth, biennial sugar beet (Beta vulgaris) maintains a steep Suc concentration gradient between the shoot (source) and the taproot (sink). To shift from vegetative to generative growth, they require a chilling phase known as vernalization. We studied sugar beet sink-source dynamics upon vernalization and showed that before flowering, the taproot underwent a reversal from a sink to a source of carbohydrates. This transition was induced by transcriptomic and functional reprogramming of sugar beet tissue, resulting in a reversal of flux direction in the phloem. In this transition, the vacuolar Suc importers and exporters TONOPLAST SUGAR TRANSPORTER2;1 and SUCROSE TRANSPORTER4 were oppositely regulated, leading to the mobilization of sugars from taproot storage vacuoles. Concomitant changes in the expression of floral regulator genes suggest that these processes are a prerequisite for bolting. Our data will help both to dissect the metabolic and developmental triggers for bolting and to identify potential targets for genome editing and breeding.

在生长的第一年，越冬的二年生植物通过韧皮部将 Suc 从光合来源组织运输到储存组织。在它们的第二年，它们从这些储存组织中调动碳来促进新的生长和繁殖。然而，驱动这种转变的机制以及与生殖生长的联系仍不清楚。在营养生长期间，二年生甜菜（Beta vulgaris）在芽（源）和主根（库）之间保持陡峭的 Suc 浓度梯度。为了从营养生长转变为生殖生长，它们需要一个称为春化的冷却阶段。我们研究了甜菜春化过程中的库-源动态，结果表明，在开花之前，主根经历了从库到碳水化合物源的逆转。这种转变是由甜菜组织的转录组和功能重编程诱导的，导致韧皮部通量方向逆转。在此转变中，液泡 Suc 进口商和出口商 TONOPLAST SUGAR TRANSPORTER2;1 和 SUCROSE TRANSPORTER4 受到相反的调节，导致糖从主根储存液泡中动员。花调节基因表达的伴随变化表明这些过程是抽苔的先决条件。我们的数据将有助于剖析抽苔的代谢和发育触发因素，并确定基因组编辑和育种的潜在目标。