ABSTRACT
Calcium is an essential plant macronutrient (Maathuis 2009). Intracellularly, Ca2+ regulates the activity of a host of enzymes and regulatory proteins (Dodd et al. 2010), and in the apoplast, this ion has long been recognized as a key structural component of the pectin-rich regions of the cell wall. Here it plays a role in coordinating between the acidic groups on pectin to increase wall rigidity (Jones and Lunt 1967). Pectins make up to 35% of the primary cell wall polymers in dicots (2%–10% in the grasses) and approximately 5% in woody tissues (Mohnen 2008). us, pectin structure has an important impact on the mechanical properties of the wall. Roots do show a great deal of variability in pectin content related to both genotypic and environmental factors. For example, closely related buckwheat and maize cultivars can exhibit signicant dierences in amounts of pectin in the root that change in response to stresses such as the presence of the toxic metal Al3+ (Dejene et al. 2005; Eticha et al. 2005; Yang et al. 2011). Such regulation of the amount and placement of pectin provides the plant with one mechanism to control wall properties. However, variations in the availability of apoplastic Ca2+ and in the masking of the carboxylic groups on the pectin polymer by esterication and de-esterication (pectin methylesterases) provide another avenue for the plant to modulate the rigidity of the wall. e dynamic nature of such changes in Ca2+ levels and esterication makes these Ca2+-pectin interactions an important feature in the remodeling of wall properties essential for the regulation of turgor-driven growth (Wolf et al. 2009). Not
surprisingly therefore, mutants in this regulatory system, such as those in the pectin methylesterases, have defects on growth and developmental patterning (e.g., Jiang et al. 2005; Bosch and Hepler 2006; Tian et al. 2006; Peaucelle et al. 2008).
