Impaired phloem loading in zmsweet13a,b,c sucrose transporter triple knock-out mutants in Zea mays

Crop yield depends on efficient allocation (配给) of sucrose from leaves to seeds. In Arabidopsis, phloem loading (韧皮部装载) is mediated by a combination of SWEET sucrose effluxers and subsequent uptake by SUT1/SUC2 sucrose/H+ symporters (协同转运子). ZmSUT1 is essential for carbon allocation in maize, but the relative contribution to apoplasmic phloem loading and retrieval (收回) of sucrose leaking from the translocation path (输导途径) is not known. Here we analysed the contribution of SWEETs to phloem loading in maize. We identified three leaf-expressed SWEET sucrose transporters as key components of apoplasmic phloem loading in Zea mays L. ZmSWEET13 paralogues (a, b, c) are among the most highly expressed genes in the leaf vasculature (维管). Genome-edited triple knock-out mutants were severely stunted (发育不良). Photosynthesis of mutants was impaired and leaves accumulated high levels of soluble sugars (可溶性糖) and starch. RNA-seq revealed profound transcriptional deregulation (反常) of genes associated with photosynthesis and carbohydrate metabolism (碳水化合物代谢). Genome-wide association study (GWAS) analyses may indicate that variability in ZmSWEET13s correlates with agronomical traits (农艺性状), especifically flowering time and leaf angle (叶倾角). This work provides support for cooperation of three ZmSWEET13s with ZmSUT1 in phloem loading in Z. mays.

作物产量取决于来自于叶片的糖向种子的配给。在拟南芥中，韧皮部装载过程由SWEET蛋白糖外排及随后SUT1/SUC2糖/H+协同转运子糖吸收共同介导。在玉米中，ZmSUT1对于碳配置是必要的，然而我们对于质外体的韧皮部装载及对于糖输导途径中的泄漏的糖的回收利用还不是很清楚。本文分析了玉米中的SWEET蛋白对于韧皮部装载的作用。作者鉴定了3个在玉米叶片中表达的SWEET糖转运子，这些转运子可能作为玉米中质外体韧皮部装载的主要组分。ZmSWEET13的三个旁系同源（a，b，c）是叶片维管组织中表达量最高的基因。基因组编辑获得的三突变体材料表现出了严重的发育不良。这些突变体的光合作用有缺陷，叶片积累了高水平的可溶性糖和淀粉。RNA-seq揭示了一些参与光合及碳水化合物代谢的基因出现表达反常。全基因组关联分析显示ZmSWEET13s的基因可变性与一些农艺性状相关，尤其是开花时间与叶片倾角。本文的工作揭示了三个ZmSWEET13蛋白与ZmSUT1一起共同调控玉米中的韧皮部装载过程。

通讯：Wolf B. Frommer (http://www.molecular-physiology.hhu.de/en/our-team/curriculum-vitae.html)

研究方向：植物糖是如何从叶片转运到种子中去的；植物的叶片和种子之间如何协调供给和需求；糖转运蛋白在病原菌侵染时所起的作用。

doi: 10.1111/nph.15021