Environmental and Experimental Botany 43 2000 239 – 251
Diversity of cell lengths in terminal portions of roots: implications to cell proliferation
Lance S. Evans
Laboratory of Plant Morphogenesis, Biological Sciences Research Laboratories, Manhattan College, The Bronx, New York, NY,
10471
, USA Received 8 September 1999; received in revised form 8 November 1999; accepted 12 November 1999
Abstract
Terminal meristems are responsible for all primary growth of roots. It has been asserted that all cells of root meristems are actively dividing no cells cycle slowly or arrest in the cycle and stem cell populations expand
exponentially. Because cells do not slide relative to each other in roots, relative cell lengths may be used to determine relative cell cycle durations andor proportions of cells actively dividing in root tissues. If all cells are cycling, no
interphase cells should be longer than critical length length of longest mitotic cell in the meristem and cells should exhibit an exponential cell – age distribution. Lengths of all cells were obtained radially across entire median
longitudinal root sections at 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mm from the founder cellroot cap boundary for five plant species to estimate percentages of cells longer than critical length. For example, up to 15 and 90 of all interphase
cells were longer than critical length in 0.5 and 2.0 mm tissues, respectively, indicating that slow-cycling andor non-proliferative cells are present in such tissues. In order to determine if the distribution of cell lengths in 0.5 mm
segments approximated an exponential cell – age distribution, lengths of interphase cells less than critical length were determined. Such interphase cells were placed into ten groups according to cell length and percentages of cells in each
group were compared with percentages of cells in groups calculated from an exponential cell – age distribution. Percentages of cells were significantly different from predicted percentages of between 6 and 9 out of ten groups —
cell lengths were not distributed exponentially. Because there are significant numbers of interphase cells longer than critical length and since lengths of interphase cells shorter than critical length do not resemble an exponential cell – age
distribution, it must be concluded that not all cells in root segments from 0.5 to 3.0 mm root segments are actively dividing. Heretofore, no databases of cell lengths have been used to test these assertions. © 2000 Elsevier Science B.V.
All rights reserved.
Keywords
:
Root meristems; Cell lengths; Critical cell length; Exponential cell – age distribution www.elsevier.comlocateenvexpbot
1. Introduction
Plant root meristems are complex tissues com- posed of many types of cells that have many
different functions Rost et al., 1998. Researchers
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have attempted to simplify the linear growth of roots Erickson and Sax, 1956 and to minimize
the variability of the types of cells that compose the terminal root area Webster, 1980. Mathe-
matical models and functions have helped re- searchers understand cell cycle kinetics and how
roots grow Erickson and Sax, 1956; Webster and MacLeod, 1980; Bertaud et al., 1986; Ivanov and
Dubrovsky, 1997. By their nature, mathematical models and functions simplify otherwise complex
processes. Over-simplification of otherwise com- plex processes may lead to false conclusions. This
paper addresses the conclusion that all cells of root terminals are proliferative Webster and
MacLeod, 1980; Ivanov and Dubrovsky, 1997. Our approach used data of cell lengths from six
locations from terminal root portions from five plant species to determine percentages of cells
greater than critical length the length of the longest cell in mitosis in the terminal portion and
the distribution of cell lengths that are shorter than critical length. If all cells shorter than critical
length are proliferative, then they should approxi- mate an exponential cell-age distribution. To date,
data of cell lengths in roots have not been used to address these issues.
If the assertion that all cells are dividing at the same rate so there are no slow cycling or non-cy-
cling cells Webster and MacLeod, 1980; Ivanov and Dubrovsky, 1997 is true and if cells do not
slide relative to their neighbors Sinnot and Bloch, 1939; Brumfield, 1942 then all cells of root meris-
tems should have a maximum cell length equal to cells in prophase. Green 1976, Webster and
MacLeod 1980 argued that if cells become non- proliferative and miss n cycles, such cells should
be 2n times longer than cells which remain prolif- erative at an equivalent position along roots.
Ivanov 1971 described a critical cell length for mitosis in roots of Zea mays, in which he stated
that practically all cells in a tissue enter mitosis upon reaching ‘critical length’. Brumfield 1942
also cited research by Abele 1936 that supported the idea of a critical length. From all the above
information, lengths of cells can be used to deter- mine relative numbers of cell cycles among neigh-
boring cells. To date, few data with only limited numbers of cells have been used to determine
diversities of cell lengths within terminal root portions. Therefore, the first hypothesis addressed
herein was: because all cells of root meristems are proliferative and because all proliferative cells are
shorter than critical length in root meristems, there are no interphase cells longer than critical
length.
Webster and MacLeod 1980, Ivanov and Du- brovsky 1997 concluded that all cells in root
meristems are dividing and that the population of proliferative cells is expanding exponentially.
Since the cell population is expanding exponen- tially, it should have an exponential cell – age dis-
tribution in which there are two cells beginning interphase for every cell terminating interphase.
In addition, there should be an exponential decay formula for cells between these two stages. Al-
though an exponential condition has been as- sumed Webster and MacLeod, 1980; Ivanov and
Dubrovsky, 1997, few databases from terminal root portions have been compared with an expo-
nential cell – age distribution. The second aspect assumed that interphase cells with less than criti-
cal length in terminal root portions are all prolif- erative and that cell populations are growing
exponentially Webster and MacLeod, 1980; Ivanov and Dubrovsky, 1997. In this manner, the
second hypothesis was: cells in terminal root por- tions that are less than critical length will have an
exponential cell – age distribution based upon cell lengths.
2. Materials and methods
