Abstract
Plant organ growth results from cell production and cell expansion. Deciphering the contribution of each of these processes to growth rate is an important issue in developmental biology. Here, we investigated the cellular processes governing root elongation rate, considering two sources of variation: genotype and disturbance by chemicals (NaCl, polyethylene glycol, H2O2, abscisic acid). Exploiting the adventitious rooting capacity of the Populus genus, and using time-lapse imaging under infrared-light, particle image velocimetry, histological analysis, and kinematics, we quantified the cellular processes involved in root growth variation, and analysed the covariation patterns between growth parameters. The rate of cell production by the root apical meristem and the number of dividing cells were estimated in vivo without destructive measurement. We found that the rate of cell division contributed more to the variation in cell production rate than the number of dividing cells. Regardless of the source of variation, the length of the elongation zone was the best proxy for growth rate, summarizing rates of cell production and cell elongation into a single parameter. Our results demonstrate that cell production rate is the main driver of growth rate, whereas elemental elongation rate is a key driver of short-term growth adjustments.
Highlights
Plant organ growth results from the combination of two processes: cell production and cell expansion
Variability in growth parameters across poplar genotypes In Exp1, eight poplar genotypes were grown in optimal nutrient solution and root growth was monitored independently for 75 roots.A 21-fold variation in overall root elongation rate (ORER) was found, ranging from 0.06 mm h−1 for a root of 6J29 and to 1.29 mm h−1 for a root of Flevo
To assess the contribution of the growth parameters to ORER variability, a principal component analysis was performed on the matrix of growth parameters and root diameter, using ORER as a supplementary variable (Fig. 2A, B)
Summary
Plant organ growth results from the combination of two processes: cell production and cell expansion. Understanding how cell division and cell elongation, and their balance, contribute to root elongation rate is an important issue in developmental biology (Gázquez and Beemster, 2017; Yang et al, 2017). In organs that grow linearly such as roots, growth rate is integral to the elemental elongation rate (EER) along the growth zone (Silk, 1992). In the root apical meristem (RAM), cell elongation is low, accounting for less than 15% of the total root growth, and most growth is due to rapid elongation occurring in the elongation zone (Bizet et al, 2015). Cell division rate impacts on the root elongation rate as the cells produced.
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