Cassava (Manihot esculenta) is a deciduous woody perennial shrub that stores large amounts of carbon and water in its storage roots. Previous studies have shown that assimilate unloading into storage roots happens symplasmically once secondary anatomy is established. However, mechanisms controlling phloem loading and overall carbon partitioning to different cassava tissues remain unclear. Here, we used a combination of histological, transcriptional, and biochemical analyses on different cassava tissues and at different timepoints to better understand source–sink carbon allocation. We found that cassava likely utilizes a predominantly passive symplasmic phloem loading strategy, indicated by the lack of expression of genes coding for key players of sucrose transport, the existence of branched plasmodesmata in the companion cell/bundle sheath interface of minor leaf veins, and very high leaf sucrose concentrations. Furthermore, we showed that tissue-specific changes in anatomy and non-structural carbohydrate (NSC) contents are associated with tissue-specific modification in gene expression for sucrose cleavage/synthesis, as well as subcellular compartmentalization of sugars. Overall, our data suggest that carbon allocation during storage root filling is mostly facilitated symplasmically and is likely mostly regulated by local tissue demand and subcellular compartmentalization.

中文翻译：

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贮藏根系中的共质韧皮部负载和亚细胞运输是木薯碳分配的关键因素


木薯 （Manihot esculenta） 是一种落叶木本多年生灌木，在其储存根中储存了大量的碳和水。先前的研究表明，一旦建立了二级解剖结构，同化物卸载到储存根中就会发生共质。然而，控制韧皮部负载和向不同木薯组织的整体碳分配的机制仍不清楚。在这里，我们对不同的木薯组织和不同时间点进行了组织学、转录和生化分析的组合，以更好地了解源-汇碳分配。我们发现木薯可能主要利用被动的共质韧皮部加载策略，表现为缺乏编码蔗糖运输关键参与者的基因表达，在小叶脉的伴生细胞/束鞘界面中存在支链胞间连丝，以及非常高的叶蔗糖浓度。此外，我们表明解剖结构和非结构性碳水化合物 （NSC） 含量的组织特异性变化与蔗糖裂解/合成基因表达的组织特异性修饰以及糖的亚细胞区室化有关。总体而言，我们的数据表明，贮藏根填充过程中的碳分配主要由共质促进，并且可能主要受局部组织需求和亚细胞区室化的调节。