Anatomical Assessment of Root Formation and Tuberization in Cassava (Manihot esculenta Crantz)

Formation of storage root organs in cassava (Manihot esculenta) is poorly understood, but considered to occur when a subset of fibrous roots receive unknown signals to undergo secondary thickening. Large amounts of secondary xylem parenchyma are then produced in which starch is synthesized and stored. Anatomical studies were undertaken to examine rhizogenesis from greenhouse-grown cassava stem cuttings. Root formation was observed from the stem cut end (basal) and from close to the buried nodes (nodal) 5-10 days after planting. Transverse sectioning of the stem provided evidence that the basal roots were initiated from the cambium, while the nodal-derived roots developed from tissues deeper within the stem, at the boundary of the xylem and pith. Basal root anatomy remained constant with age, with minimal development of metaxylem. No tuberization was seen to occur from the basal roots. In contrast, nodal roots produced significant amounts of metaxylem and subsequently secondary xylem to form a large central stele. Further development established the storage organ in which secondary xylem parenchyma, tracheids and vessels were produced from the cambium. As a result, the nodal-derived roots were seen as precursors of the storage organs. It is proposed that nodal-derived and basal-derived fibrous roots are fundamentally different organs, that they originate through different rhizogenic processes and are committed to their respective developmental fates from the earliest stages of their initiation. These anatomical investigations offer new insight into root tuberization in cassava and should guide better focused studies into the underlying molecular and developmental control mechanisms.