Variation in growth rate between Arabidopsis ecotypes is correlated with cell division and A-type cyclin-dependent kinase activity

Abstract

We used a kinematic analysis to investigate the growth processes responsible for variation in primary root growth between 18 ecotypes of Arabidopsis. Root elongation rate differed 4-fold between the slowest (Landsberg erecta, 71 microm h(-1)) and fastest growing line (Wassilewskija [Ws]; 338 microm h(-1)). This difference was contributed almost equally by variations in mature cortical cell length (84 microm [Landsberg erecta] to 237 microm [Ws]) and rate of cell production (0.63 cell h(-1) [NW108] to 1.83 cell h(-1) [Ws]). Cell production, in turn, was determined by variation in cell cycle duration (19 h [Tsu] to 48 h [NW108]) and, to a lesser extent, by differences in the number of dividing cells (32 [Weiningen] to 61 [Ws]). We found no correlation between mature cell size and endoreduplication, refuting the hypothesis that the two are linked. However, there was a strong correlation between cell production rates and the activity of the cyclin-dependent kinase (CDKA). The level of the protein could explain 32% of the variation in CDKA. Therefore, it is likely that regulators of CDKA, such as cyclins and inhibitors, are also involved. These data provide a functional link between cell cycle regulation and whole-plant growth rate as affected by genetic differences.

Figure 1

Localization of cell division and expansion in the cortical cell file of primary root tip of Arabidopsis. Cortical cell files originate from initials directly adjacent to the quiescent center (QC). Basal to these initials is the meristematic region where new cells are produced by ongoing cell division and cell expansion, resulting in a flux of cells away from the root tip (arrows). When cells leave the meristem, they enter the elongation zone. Here, they no longer divide, but continue to elongate, resulting in a rapid increase in length as a function of position. Basal to the elongation zone, cells are of constant size and considered mature. The size of the growth zone (meristem + elongation zone) typically ranges between 1 and 2.5 mm in Arabidopsis roots grown on agar media (Beemster and Baskin, 1998, 2000; De Veylder et al., 2001b).

Figure 2

Variation in root elongation rate between 18 Arabidopsis ecotypes is correlated with cell production and mature cortical cell length. A, The complete model describing the correlation between root elongation rate (E), cell production rate (P), and mature cell length (l_mat; Eq. 3). B, Partial model of the relationship between root elongation rate (E) and cell production (P). C, Partial model of the relationship between root elongation rate (E) and mature cortical cell length (l_mat). Details of the regression parameters are listed in Table III.

Figure 3

Variation in the rate of cell production in the primary root meristem of 18 Arabidopsis ecotypes is correlated with the number of dividing cells and their average cell cycle duration. A, The complete model describing the correlation between cell production (P) in the meristem, the number of dividing cells (N_div), and mature cell length (l_mat; Eq. 4). B, Partial model of the relationship between cell production rate (P) and number of dividing cells (N_div). C, Partial model of the relationship cell production rate (P) and mature cell length (l_mat). Details of the regression parameters are listed in Table IV.

Figure 4

The absence of a relationship between endoreduplication and mature cortical cell size (A) and CDKA (B) in the roots of 18 Arabidopsis ecotypes. The fraction of endoreduplicated cells was estimated from 10 complete root systems from which the nuclei were isolated and analyzed by flow cytometry. Mature cortical cell length was determined as the average over all positions in the mature part of the root (see “Materials and Methods”).

Figure 5

Cell production rates are correlated with kinase activity and CDKA levels in the root tip of 11 Arabidopsis ecotypes. A, Relationship between kinase activity (A) and cell production (P; regression, P = −0.47 + 0.7 A; R² = 0.56, P < 0.001). B, Relationship between CDKA levels (C) and kinase activity (regression, C = 0.02 + 0.19 A; R² = 0.32, P = 0.09). Kinase activities were calculated relative to the average of all samples from each individual experiment and then averaged between two replicate experiments. Error bars denote ses (n = 2). CDKA levels were determined by measuring the integrated intensity on western blots. The regressions were obtained from multiple linear regression analysis.