Regulation of Growth Anisotropy in Well-Watered and Water-Stressed Maize Roots (I. Spatial Distribution of Longitudinal, Radial, and Tangential Expansion Rates)

Abstract

As a system to study the regulation of growth anisotropy, we studied thinning of the primary root of maize (Zea mays L.) occurring developmentally or induced by water stress. Seedlings were transplanted into vermiculite at a water potential of approximately -0.03 MPa (well-watered) or -1.6 MPa (water-stressed). The diameter of roots in both treatments decreased with time after transplanting; the water-stressed roots became substantially thinner than well-watered roots at steady state, showing that root thinning is a genuine response to water stress. To analyze the thinning responses we quantified cell numbers and the spatial profiles of longitudinal, radial, and tangential expansion rates separately for the cortex and stele. The results showed that there was no zone of isotropic expansion and the degree of anisotropy varied greatly with position and treatment. Thinning over time in well-watered roots was caused by rates of radial and tangential expansion being too low to maintain the shape of the root. I n response to low water potential, cell number in both tissues was unchanged radially but increased tangentially, which shows that thinning was caused wholly by reduced cell expansion. Water stress substantially decreased rates of tangentia1 and radial expansion in both the stele and cortex, but only in the apical 5 mm of the root; basal to this, rates were similar in wellwatered and water-stressed roots. By contrast, as in previous studies, longitudinal expansion was identical between the treatments in the apical 3 mm but in water-stressed roots was inhibited at more basal locations. The results show that expansion i n longitudinal and radial directions can be regulated independently.The beautiful and adaptive forms of plants arise through the growth of component cells. Not only must plant cells control their overall rate of expansion but they must also control the rate of expansion in different directions. When

Disorganization of Cortical Microtubules Stimulates Tangential Expansion and Reduces the Uniformity of Cellulose Microfibril Alignment among Cells in the Root of Arabidopsis

“…Importantly, the term is essentially the same at all positions in the growth zone. Spatial constancy was also reported for the time-dependent change in the diameter of the well-watered maize root (Liang et al, 1997). Therefore, neglecting the time-dependent term underestimates tangential rate inconsequentially.…”

“…We quantified the spatial distribution of relative expansion kinematically (Liang et al, 1997;Beemster and Baskin, 1998). Sample images from the analysis are shown in Figure 3.…”

“…The degree of anisotropy was hypothesized by Green (1964) to be proportional to the degree of alignment among microfibrils; that is, the more parallel cellulose microfibrils are laid down, the greater the difference between maximal and minimal expansion rates. This hypothesis was tested for the maize root, which grows more anisotropically under water deficit (Liang et al, 1997) and falsified; despite there being a more than 10-fold difference in the degree of anisotropy, there was no difference in microfibril alignment (Baskin et al, 1999). Likewise, decreased anisotropy occurs in several Arabidopsis mutants without detectable disorganization among microfibrils (Wiedemeier et al, 2002;Sugimoto et al, 2003).…”

“…The spatial profile of diameter was also smoothed and interpolated by the same algorithm. Relative tangential expansion rates were calculated from the spatial profiles of velocity and diameter (Silk and Abou Haidar, 1986;Liang et al, 1997). Longitudinal and tangential profiles were obtained for each root and then averaged over the roots in the treatment.…”

“…Surprisingly, there have been few published measurements of growth anisotropy in plant organs. Those that have been made show that the degree of anisotropy differs with position, species, and treatment (Erickson, 1966;Silk and Abou Haidar, 1986;Liang et al, 1997). One of our aims here was to quantify the spatial profiles of expansion in length and radius and compare them to the underlying profile of microfibril alignment.…”