Comparison of Promeristem Structure and Ontogeny of Procambium in Primary Roots of Zea mays ssp. Mexicana and Z. mays 'Honey Bantam' with Emphasis on Metaxylem Vessel Histogenesis.

Susumu Saito,Daniel Gladish,Teruo Niki

doi:10.3390/plants8060162

Susumu Saito, Daniel Gladish + Show 1 more

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https://doi.org/10.3390/plants8060162

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Abstract

Classical histology describes the histological organization in Zea mays as having a “closed organization” that differs from Arabidopsis with the development of xylem conforming to predictable rules. We speculated that root apical meristem organization in a wild subspecies of Z. mays (a teosinte) would differ from a domestic sweetcorn cultivar (‘Honey Bantam’). Careful comparison could contribute to understanding how evolutionary processes and the domestication of maize have affected root development. Root tips of seedlings were prepared and sectioned for light microscopy. Most sections were treated with RNase before staining to increase contrast between the walls and cytoplasm. Longitudinal and serial transverse sections were analyzed using computer imaging to determine the position and timing of key xylem developmental events. Metaxylem development in mexicana teosinte differed from sweetcorn only in that the numbers of late-maturing metaxylem vessels in the latter are typically two-fold greater and the number of cells in the transverse section of procambium were greater in the latter, but parenchymatous cell sizes were not statistically different. Promeristems of both were nearly identical in size and organization, but did not operate quite as previously described. Mitotic activity was rare in the quiescent centers, but occasionally a synchronized pulse of mitoses was observed there. Our reinterpretation of histogen theory and procambium development should be useful for future detailed studies of regulation of development, and perhaps its evolution, in this species.

Highlights

Maize is believed to have been domesticated beginning about 9000 years ago from one of several species of large grasses, commonly known as “teosinte”, native to what is central Mexico [1,2].“Teosinte” is no longer considered to be one species separate from maize.This common name is understood to have been traditionally applied by indigenous people to three non-maize species (Z. perennis, Z. diploperennis, and Z. luxurians) and three subspecies of Z. mays, all of which have older natural histories than domesticated cultivars of Z. mays and are more similar to each other structurally than they are to domesticated maize
The objectives of the present study were (1) to use a new approach to slide preparation that more clearly reveals structural patterns at the tissue and cell level to enhance precision thin-sectioning and high magnification analysis of root apical and primary meristem zones (RAMs) structure, procambium development, and late-maturing metaxylem vessel initiation in an undomesticated Z. mays subspecies and a modern, highly-derived commercial cultivar (Z. mays ‘Honey Bantam’); and (2) to evaluate the anatomical differences that that may contribute to our understanding of the factors that cause dimensional differences apparent in their primary root tips
We found that treating tissue sections with ribonuclease A (RNase) to remove cytoplasmic RNA before staining with toluidine blue O, which heavily stains cytoplasmic RNA, simplified evaluation of cell wall patterns and mitotic figure recognition by making cell walls and chromatin in nuclei more clearly visible

Summary

Introduction

Maize is believed to have been domesticated beginning about 9000 years ago from one of several species of large grasses, commonly known as “teosinte”, native to what is central Mexico [1,2].“Teosinte” is no longer considered to be one species separate from maize (i.e., domesticated Z. mays).This common name is understood to have been traditionally applied by indigenous people to three non-maize species (Z. perennis, Z. diploperennis, and Z. luxurians) and three subspecies of Z. mays (ssp. huehuetenangensis, mexicana, and parviglumis), all of which have older natural histories than domesticated cultivars of Z. mays and are more similar to each other structurally than they are to domesticated maize. “Teosinte” is no longer considered to be one species separate from maize (i.e., domesticated Z. mays) This common name is understood to have been traditionally applied by indigenous people to three non-maize species (Z. perennis, Z. diploperennis, and Z. luxurians) and three subspecies of Z. mays Huehuetenangensis, mexicana, and parviglumis), all of which have older natural histories than domesticated cultivars of Z. mays and are more similar to each other structurally than they are to domesticated maize The inclusion of the latter three into species Z. mays was done by virtue of the “reproductive criterion” Several morphological and anatomical parameters of mature root systems were recently shown to differ significantly between wild teosintes and domesticated

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