We tested the hypothesis that the degree of anisotropic expansion of plant tissues is controlled by the degree of alignment of cortical microtubules or cellulose microfibrils. Previously, for the primary root of maize (Zea mays L.), we quantified spatial profiles of expansion rate in length, radius, and circumference and the degree of growth anisotropy separately for the stele and cortex, as roots became thinner with time from germination or in response to low water potential (B.M. Liang, A.M. Dennings, R.E. Sharp, T.I. Baskin [1997] Plant Physiol 115:101-111). Here, for the same material, we quantified microtubule alignment with indirect immunofluorescence microscopy and microfibril alignment throughout the cell wall with polarized-light microscopy and from the innermost cell wall layer with electron microscopy. Throughout much of the growth zone, mean orientations of microtubules and microfibrils were transverse, consistent with their parallel alignment specifying the direction of maximal expansion rate (i.e. elongation). However, where microtubule alignment became helical, microfibrils often made helices of opposite handedness, showing that parallelism between these elements was not required for helical orientations. Finally, contrary to the hypothesis, the degree of growth anisotropy was not correlated with the degree of alignment of either microtubules or microfibrils. The mechanisms plants use to specify radial and tangential expansion rates remain uncharacterized.

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

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