Origin and Development of Phloem and Xylem as Revealed in 3D Models of the Vascular Cylinder in Primary Roots of Oryza sativa L. cv. Hitomebore

Xylem and phloem are the defining features of vascular plants and are central to their evolutionary success. The origin and development of phloem and xylem within the vascular cylinder of rice primary roots (Oryza sativa L.) were investigated using serial sectioning and three-dimensional (3D) image processing techniques improved and developed by the authors. Protophloem mother cells and metaphloem sieve tube members were derived from vascular initials at about 20 μm from the apex of the vascular cylinder (AVC). Protophloem tissue and companion cells were generated through two successive divisions of protophloem mother cells 60–95 μm from the AVC of a 20 mm-long primary root. Metaxylem and late-metaxylem were initiated by the division of vascular initials at about 20 μm from the AVC of a 20 mm long root. Protoxylem was formed basipetally from secondary initials produced through periclinal division of pericycle cells at distances ranging 175–560 μm from the AVC. Protoxylem vessels could be clearly identified with a thickened cell wall preceding the thickening of metaxylem cell walls. Around 15 mm from the AVC of the 20 mm-long root, three kinds of xylem vessels (protoxylem, metaxylem and late metaxylem) were confirmed. The histogenesis of protophloem and protoxylem in rice primary roots was carefully analyzed and discussed in this study. In particular, the view has been confirmed regarding the pericycle origin of protoxylem in rice.