Root Cap Mucilage and Extracellular Calcium as Modulators of Cellular Growth in Postmitotic Growth Zones of the Maize Root Apex*

Abstract

The control of maize root growth by root cap mucilage and extracellular calcium (Ca) was examined. Special attention was paid to the influence of these factors on cellular aspects of root growth, such as cell shape and organization of the microtubular (MT) cytoskeleton. Externally supplied Ca impaired the transition of early post-mitotic cells from a more-or-Iess apolar mode of expansion to a strictly anisotropic mode 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 axis, being maintained under these conditions. Root mucilage, collected from donor root caps and placed around root tips, exerted a similar effect 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 mucilage was continually removed, had more elongated cell shapes in their post-mitotic growth regions when compared to the control roots. These findings are consistent with a notion that Ca is present in the root cap mucilage in physiologically relevant amounts and can mediate growth responses in both the PIG region and the apical part of the elongation zone.Integrating several known effects of Ca ions on growth at the root apex, a hypothesis is proposed that a Ca-mediated and MTindependent control of cell growth in the PIG region might be involved in morphogenetic root movements (e.g. gravitropism), and that root growth responses could be initiated by an asymmetric distribution of extracellular calcium, or root cap slime, around the growing root tip.

Cellular specifiCity of the gravitropic motor response in roots

“…Prevention of the development of gravi-induced calcium asymmetry by application of the mobile calcium-binding site, BAPTA [1, 2-bis(2-amino-5-nitro-phenoxy)ethane-N,N, N¢N¢-tetraacetic acid] to maize root tips prevents gravitropic curvature (BjoÈ rkman and Cleland 1991b). BalusÏ ka et al (1996b) examined the eect of applied calcium on the transition of cells from the DEZ to the main elongation zone in maize roots. They found that exogenous calcium delayed the transition of cells from the slow and more-or-less apolar mode of expansion in the DEZ to the pattern of rapid anisotropic expansion characteristic of the elongation zone.…”

Impacts of Aluminum on the Cytoskeleton of the Maize Root Apex. Short-Term Effects on the Distal Part of the Transition Zone1

“…Recent findings on the cell wall-PM-cytoskeleton continuum (Wyatt and Carpita, 1993;Miller et al, 1997) call for a reassessment of this debate. In maize (Zea mays L.), detailed information on the arrangement of CMTs and actin MFs was presented for growing and developing cells of intact roots under diverse experimental conditions (Baluška et al, 1992(Baluška et al, , 1993a(Baluška et al, , 1993b(Baluška et al, , 1995(Baluška et al, , 1996a(Baluška et al, , 1996cBlancaflor and Hasenstein, 1993, 1995a, 1995bBaluška and Hasenstein, 1997).…”

Identification and characterization of a rhizosphere β-galactosidase from Pisum sativum L.

“…However, inhibiting its activity in plants has not previously been reported to be deleterious to growth and development. The root cap mucilage layer has been theorized to serve as a conduit for root signalling and growth (Baluska et al 1996;Moore and Fondren 1988). The discovery of a complex of enzymes secreted from the root cap may in part explain how such signalling can operate outside the cellular environment (Wen et al 2007).…”