Calcium in Root Hair Growth
Tatiana Bibikova and Simon Gilroy
Biology Department, 208 Mueller Laboratory, The Pennsylvania State University, University Park,
Pennsylvania 16802
Abstract.
The growth of cells as diverse as fungal hyphae, pollen tubes, algal rhizoids, and root hairs is characterized by a highly localized control of cell expansion confined to the growing tip. The concentration of cytoplasmic calcium ([Ca2+]c) has been proposed to play an important role in both structuring the apical cytoplasm of these tip growing cells as well as localizing the site of exocytosis to the expanding apex. Indeed, a tip-focused [Ca2+]c gradient is associated with the growing point of these cells and dissipating the gradient with Ca2+ buffers and Ca2+-channel blockers inhibits apical growth. In addition, responses to factors such as biotic and abiotic stresses and rhizobial nodulation factors (Nod factors) are all thought to be mediated, at least in part, by changes in root hair Ca2+ dynamics. In this chapter we will review two aspects of the involvement of Ca2+ in root hair development, the evidence for [Ca2+]c as a central determinant of localized growth, and the role of cytoplasmic Ca2+ as a second messenger in Nod factor signaling. In both these cases the combined research of many laboratories has begun to reveal [Ca2+]c as a multifunctional regulator of root hair responses. In addition, we will highlight elements of the root hair growth machinery, such as membrane trafficking and the cytoskeleton, that may be regulated by these changes in [Ca2+]c.