Teruo Niki

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.

Serial sectioning and 3D image reconstruction methods were applied to elucidate the structures of the apices of root vascular cylinders (VCs) in taxa of the Poaceae: Zea mays “Honey Bantam”, Z. mays ssp. mexicana, Hordeum vulgare and Oryza sativa. The primary and nodal roots were investigated. Observations were performed using high-quality sectioning and 3D image-processing techniques improved and developed by the authors. We found that a quiescent uniseriate plerome was located at the most distal part of each VC. Vascular initials were located immediately basipetally to the plerome as a specific uniseriate layer that could be classified into central and peripheral initials that produced all the cells in the VC. No supplying of cells from the plerome to the vascular initials was observed. Numerical analysis revealed a “boundary point” along the root axis where the rate of increase of the vascular cell number markedly declined, and the VC diameter, number of vascular cells, and number of late-maturing metaxylem vessels (LMXs) at that point showed a similar relationship among the taxa and the types of roots examined (primary vs. nodal). The plerome and vascular initials layer can be considered independent after seed germination in these taxa. A boundary point at which procambial cell proliferation sharply declined was identified. The diameters of the VCs, number of LMXs, and number of vascular cells at the boundary point were found to be strongly related to each other.