ROOT CAP MUCILAGE AND EXTRACELLULAR CALCIUM AS MODULATORS OF CELLULARGROWTH IN POSTMITOTIC GROWTH ZONES OF THE MAIZE ROOT APEX

The control of maize root growth by root cap mucilage and extracellula r calcium (Ca) was examined. Special attention was paid to the influen ce of these factors on cellular aspects of root growth, such as cell s hape and organization of the microtubular (MT) cytoskeleton. Externall y supplied Ca impaired the transition of early post-mitotic cells from a more-or-less apolar mode of expansion to a strictly anisotropic mod e of elongation accompanied by their more rapid growth. However, this inhibitory effect of Ca was not associated with any re-arrangement of the cortical MTs, their transverse arrays, with respect to the root ax is, being maintained under these conditions. Root mucilage, collected from donor root caps and placed around root tips, exerted a similar ef fect on cell shapes as did externally supplied Ca. In contrast, roots grown in a medium of low Ca content, or from which the root cap mucila ge was continually removed, had more elongated cell shapes in their po st-mitotic growth regions when compared to the control roots. These fi ndings are consistent with a notion that Ca is present in the root cap mucilage in physiologically relevant amounts and can mediate growth r esponses in both the PIG region and the apical part of the elongation zone. Integrating several known effects of Ca ions on growth at the ro ot apex, a hypothesis is proposed that a Ca-mediated and MT-independen t control of cell growth in the Pie region might be involved in morpho genetic root movements (e.g. gravitropism), and that root growth respo nses could be initiated by an asymmetric distribution of extracellular calcium, or root cap slime, around the growing root tip.