<i>Arabidopsis</i> sucrose transporter 4 (AtSUC4) is involved in high sucrose-mediated inhibition of root elongation

Sucrose transporters (SUCs/SUTs) play crucial roles in apoplast transport and long-distance distribution of sucrose throughout the whole plant. However, whether and how the <i>Arabidopsis</i> AtSUC4 modulates sucrose import from apoplast to cytosol remains unclear. In the present study, we found that AtSUC4 protein was localized to the plasma membrane in the root. Expression of <i>AtSUC4</i> in roots was gradually induced with the increasing sucrose concentrations (0%, 2%, 4% and 6%). When feeding high concentrations (4% and 6%) of sucrose, the primary root growth of seedling was inhibited. Interestingly, <i>atsuc4</i> mutants exhibited longer primary root than the wild type under these conditions, indicating that <i>atsuc4</i> mutants were less sensitive to excess sucrose. Moreover, the root of <i>atsuc4</i> mutants accumulated less sucrose and abscisic acid (ABA) and more indole-3-acetic acid (IAA) on 4% and 6% sucrose supplementation. Transcriptomic analysis revealed that numerous genes associated with sugar transport and metabolism, as well as ABA signalling were down-regulated, whereas many IAA signaling-related genes were up-regulated in mutant plants relative to the wild type under 6% sucrose treatment. Collectively, our finding demonstrated that the deficiency of <i>AtSUC4</i> reduced the inhibition of primary root growth under high sucrose condition, probably through reducing the sucrose transportation and metabolism, and subsequent alteration in IAA and ABA signalling.蔗糖转运蛋白（Sucrose transporters, SUCs/SUTs）在质外体运输以及整株植物体内蔗糖的长距离分配中发挥关键作用。然而，拟南芥（Arabidopsis）AtSUC4是否以及如何调控蔗糖从质外体向细胞质的导入，目前仍不明晰。本研究中，我们发现AtSUC4蛋白定位于根系的质膜（plasma membrane）。随着蔗糖浓度梯度升高（0%、2%、4%与6%），AtSUC4在根系中的表达量逐渐被诱导上调。当以高浓度（4%和6%）蔗糖进行饲喂处理时，幼苗主根生长受到显著抑制。值得注意的是，在上述高蔗糖培养条件下，atsuc4突变体的主根长度显著长于野生型，提示atsuc4突变体对过量蔗糖的敏感性显著降低。此外，在添加4%和6%蔗糖的培养基中，atsuc4突变体根系内的蔗糖与脱落酸（abscisic acid, ABA）积累量更低，而吲哚-3-乙酸（indole-3-acetic acid, IAA）积累量更高。转录组分析结果显示：相较于野生型，在6%蔗糖处理条件下，突变体植株中大量参与糖转运、糖代谢以及ABA信号通路的基因表达下调，而众多与IAA信号通路相关的基因表达上调。综上，本研究证实AtSUC4功能缺失可缓解高蔗糖环境下主根生长的抑制效应，这一过程可能通过削弱蔗糖转运与代谢过程，进而改变IAA与ABA的信号通路得以实现。