High-resolution transcriptomics of stem and storage root vascular cambia highlight regulatory processes for xylem parenchyma differentiation in cassava

Due to their high carbohydrate content, the storage roots of cassava are an important food source for hundreds of millions of people worldwide. In contrast to the woody stems of the plant, the xylem of the storage roots produces mainly starch-rich storage parenchyma cells and only few tracheary elements and almost no fibers. Despite these obvious differences, both stems and storage roots are formed by a vascular cambium. To find more insights into the differences in the regulation of cell division and differentiation in stems and storage roots, a cryo-sectioning approach was utilized, to generate high-resolution transcriptome profiles spanning the entire vascular cambium of both tissues. We observed that storage parenchyma formation is connected to the repression of secondary cell wall formation through a decrease in expression levels of key players in the NAC/MYB regulatory network, as well as decreases in the downstream pathways for lignin and hemicellulose biosynthesis. Additionally, the expression of MeWOX14, a transcription factor associated with GA signaling and xylem fiber differentiation, is strongly reduced in storage roots compared to stem xylem. By contrast, the expression of MeKNOX1, a well-known meristem regulator, as well as most cassava LSH genes and several ABA-related transcription factors were associated with parenchyma cells. Our data suggest that the repression of secondary cell wall formation and GA signaling, together with an active auxin and ABA signaling, as well as extended MeKNOX1 activity are associated with storage parenchyma formation in cassava storage roots.