Differential Growth

In the negatively responding organs, differential growth is due to greater elongation rate of the lower side and slower elongation rate on the upper side, whereas gravitropic curvature in roots is due to a greater elongation in the upper part than in the lower part.

Detailed studies on the Relative Elemental Rates of Elongation (RELEL) at different points along the main axis of the maize root stimulated in the horizontal position or growing in the vertical position have been carried out by Barlow and Rathfelder (1985). These authors showed that the lower side of the stimulated root was inhibited, but only in the distal part of the reaction zone, whereas the whole upper part of this zone was stimulated (Figure 6-07A).

Thus, the spatial distribution of growth during gravitropic curvature was important. This was confirmed by anatomical study at the level of the curvature of the lentil root stimulated for 2 h (Darbelley et al. 1986). The comparison of the average cortical cell length at the level of the distal zone of the meristem and the proximal zone of the cell elongation region (i.e., at the level of the curvature) demonstrated that there was a greater cell elongation in the upper side of the distal meristem and an inhibition of cell growth in the lower part of the cell elongation zone.

It has been concluded that there were two different types of cells that responded to gravistimulus: a) those which were located in the root meristem; and b) and those located at the beginning of the cell elongation zone (Darbelley et al. 1986). This difference in cell growth between the lower and the upper part of the lentil root started very close to the root cap junction. The heterogeneous nature of the zone of curvature could explain, at least partly, the controversial results published on differential growth.

Sellcer and Sievers (1987) have also analyzed the temporal component of the gravitropic bending in the Lepidium sativum root. They showed that for the first hour after tilting the root to the horizontal position, the relative extension rate of the upper side of the root was higher than that during straight growth. On the contrary, the relative growth rate of the lower side fell to near zero during this period. For the second hour, the two sides had approximately equal growth rate. At the end of the second hour, the rate of the lower side increased suddenly. For these authors, a reversal in the extension gradient is necessary to prevent the root from continuing to curve and to overshoot the direction of gravity.

In 1990, Baluska et al. pointed out the special status of cells toward the distal end of the elongation zone in maize roots and note that although most of these cells have ceased dividing they have not entered the phase of rapid elongation. They proposed the term of "postmitotic isodiametric growth zone" in reference to the shape of the cells. However, cell expansion is not isodiametric except in a very narrow region. Ishikawa and Evans (1993) proposed to refer to this region of the root as the Distal Elongation Zone (DEZ). When primary roots of maize are gravistimulated, a major factor causing downward curvature is the induction of very fast elongation in the DEZ on the top side of the root (Figure 6-07B) (Ishikawa and Evans 1995).

In Arabidopsis roots, Mullen et al. (1998) showed that after gravistimulation, the growth patterns of the root changed comparatively to vertical straight growth. Within the first hour of graviresponse, the basal limit of the DEZ and the position of the peak of Relative Elemental Growth Rate (REGR) shifted apically on the upper flank of the root. This was due to a combination of increased growth in the DEZ and growth inhibition in the central elongation zone. On the lower flank the basal limit of the DEZ shifted basipetally as the REGR decreased.

The results obtained showed that a large proportion of the initial curvature originated in the DEZ. Some evidence indicated that auxin may not be responsible for the induction of differential growth in the DEZ of gravistimulated roots. Muday and Hayworth (1994) on tomato roots and Ishikawa and Evans (1993) on maize roots found that a treatment by auxin at strong inhibiting concentrations did not suppress gravitropic response whose kinetics was similar to that of untreated roots. These results have been considered as an argument against an involvement of auxin in the differential growth. However, it has been proven recently that the lateral movement of auxin occurs in the lateral cells of the root cap even after treatments by auxins (Ottenschlager et al. 2003). Thus, the Cholodny-Went hypothesis has been nicely confirmed in the recent years and its mechanism, which is dependent upon auxin influx and efflux carriers, is now better understood.

The gravitropic signal is an asymmetrical transport of auxin, which leads to a differential growth. It is well known that this hormone induces cell wall acidification and a cell wall loosening (see review by Cosgrove 1997). Thus, gravitropic bending should be, at least in part, mediated by differential control of wall pH on the lower and upper sides of a gravistimulated organ.

Figure 6-07. Differential growth in the upper side (US) and lower side (LS) of maize roots. In A, the curve shows the relative growth (RELET, in % x min') of small root segments as a function of the distance of this segment from the root tip. Adapted from Barlow and Rathfelder (1985). In B, the graphic shows the different functional zones of the maize root. The curves show the level of the mitotic activity within the meristem (MER) and the relative rate of elongation in the elongation zone (EZ). The distal elongation zone (DEZ) is mainly responsible for the curvature of the root. Adaptedfrom Ishikawa and Evans (1995).

Figure 6-07. Differential growth in the upper side (US) and lower side (LS) of maize roots. In A, the curve shows the relative growth (RELET, in % x min') of small root segments as a function of the distance of this segment from the root tip. Adapted from Barlow and Rathfelder (1985). In B, the graphic shows the different functional zones of the maize root. The curves show the level of the mitotic activity within the meristem (MER) and the relative rate of elongation in the elongation zone (EZ). The distal elongation zone (DEZ) is mainly responsible for the curvature of the root. Adaptedfrom Ishikawa and Evans (1995).

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