Quantitative analysis of cell division in leaves: methods, developmental patterns and effects of environmental conditions

In planta quantitative studies of cell cycle are necessary for examining th e role of cell division in the response of plants to environmental conditio ns and to analyse the behaviour of transformed plants in this context. We p resent and discuss non-intrusive kinematic methods which allow estimating t he duration of cell cycle with a high spatial resolution in the leaf. Diffe rent methods are proposed and discussed for monocotyledons and dicotyledons , and compared with methods involving the use of chemicals. In monocotyledo n leaves, cell division is restricted to a limited zone near the leaf inser tion point, twice as long in the mesophyll as in the epidermis. In dicotyle dons, cell division occurs in the whole leaf with a uniform and constant ce ll cycle duration for a determinate number of cell cycles, representing abo ut half of leaf development. Over several experiments, this number is well conserved in a given leaf zone in the absence of stresses, but larger near the leaf base than near the leaf tip. After that, cell cycle duration incre ases because cells are progressively blocked in G1 while the durations of S -G2-M phases do not change with time. Leaf temperature affects neither the distribution of nuclei in each phase of the cycle nor the number of cell cy cles in a leaf. Water or light deficits both cause a partial blockage of nu clei in G1 during the stress only, thereby increasing cell cycle duration a nd decreasing final cell number. These results suggest that a strong develo pmental programme drives cell division in leaves, so a simple framework all ows analysis of temporal patterns, of spatial gradients and of the effect o f environmental conditions.