Evaluation of Metaxylem Vessel Histogenesis and the Occurrence of Vessel Collapse during Early Development in Primary Roots of Zea mays ssp. mexicana : A Result of Premature Programmed Cell Death?

Root apical meristem histological organization in Zea mays has been carefully studied previously. Classical histology describes its system as having a "closed organization" and a development of xylem that conforms to predictable rules. Among the first cell types to begin differentiation are late-maturing metaxylem (LMX) vessels. As part of a larger study comparing domestic maize root development to a wild subspecies of Z. mays (teosinte), we encountered a metaxylem development abnormality in a small percentage of our specimens that begged further study, as it interrupted normal maturation of LMX. Primary root tips of young seedlings of Zea mays ssp. mexicana were fixed, embedded in appropriate resins, and sectioned for light and transmission electron microscopy. Longitudinal and serial transverse sections were analyzed using computer imaging to determine the position and timing of key xylem developmental events. We observed a severe abnormality of LMX development among 3.5% of the 227 mexicana seedlings we screened. All LMX vessel elements in these abnormal roots collapsed and probably became non-functional shortly after differentiation began. Cytoplasm and nucleoplasm in the abnormal LMX elements became condensed and subdivided into irregularly-shaped "macrovesicles" as their cell walls collapsed inward. We propose that these seedlings possibly suffered from a mutation that affected the timing of the programmed cell death (PCD) that is required to produce functional xylem vessels, such that autolysis of the cytoplasm was prematurely executed, i.e., prior to the development and lignification of secondary walls.