From signal to form: Aspects of the cytoskeleton-plasma membrane-cell wall continuum in root hair tips

Abstract

Root hairs are excellent cells for the study of the exocytotic process that leads to growth in higher plants, because the exocytotic event takes place locally and because the cells are directly accessible for signals, drugs, fixatives, microinjection, and microscopic observation. Well-characterized lipochitooligosaccharides, signal molecules excreted by Rhizobium bacteria, induce root hair growth which can be recorded microscopically in a root hair deformation assay developed for Vicia sativa L. Root hair deformation is a morphogenetic process involving swelling of the hair tip and subsequent new hair outgrowth from that swelling. This response to the signal occurs at a specific developmental stage, namely when hairs are terminating growth. Thus, since polar growth can be triggered intentionally, the system allows the study of growth phenomena in higher plants at the cellular level. Furthermore, important advances are being made with molecular genetics that will allow the unravelling of the signal transduction pathways in root hair morphogenesis leading to growth. This paper first discusses cytological phenomena involved in the process of polar growth, such as cytoplasmic polarity, cytoplasmic streaming and the organization of actin filaments, the location of a spectrin-like antigen, the distribution of intracellular calcium, cortical microtubules and cell wall texture, endocytosis by means of coated pits, and physical aspects of the incorporation of exocytotic vesicles into the plasma membrane. In the second part, changes are discussed that occur in some of these phenomena when growth is influenced by growth regulators and mutations.