Annals of Botany

Tipped for growth

Root hairs are an excellent subject for the study of tip growth. The existence of mutants and the use of real-time microscopic techniques on living cells have recently added significantly to our knowledge, including features of behaviour of the cytoskeleton. However, there are still many details to be elucidated. This has led Xue He et al. (Beijing, China, pp. 49–55) to look at the relationships between the distributions of actin and of calcium in relation to growth of wheat (Triticum aestivum) root hairs. Instead of using living material, they fixed whole seedlings (which provided root hairs at different stages of development) and used specific antibody staining techniques to examine actin distribution. In root hairs that were still growing at the time of fixation, F-actin was organized in bundles along the axis of growth. However, the bundles were absent from the apical 5–10 mm, the actual growing zone, but there were some very fine F-actin filaments. By contrast, G-actin was at its highest concentration in this zone, organized as a tip-focused gradient. There was no tip-located G-actin in hairs that had finished growing. Instead, the F-actin bundles extended right to the tip, suggesting that the apical location of F-actin bundles was related to the cessation of growth and that G-actin was involved in active growth. To investigate the relationship between actin dynamics and Ca²⁺, roots were treated with BAPTA-AM, a disrupter of cellular Ca²⁺ distribution known to inhibit root hair growth. In these experiments it inhibited the tip-focused Ca²⁺ gradient characteristic of growing hairs and also inhibited the tip-focused gradient of G-actin. At the same time it caused an accumulation of a dense mesh of F-actin in the tip, more characteristic of hairs that have ceased growing. All this suggests an intimate relationship between Ca²⁺ distribution, actin dynamics and root hair growth.

Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk