Keep your balance!

Like animals, plant cells are exposed to fluctuating extracellular levels of Ca²⁺ (Ca_o²⁺), but somehow manage to keep their internal Ca²⁺ levels on a relatively even keel. This implies that there is a mechanism for maintaining Ca²⁺ homeostasis, but until now, what this was and how it operated was elusive. But a large clue has come from the work of Shengcheng Han et al., which is reported in Nature.

Ca²⁺ has pleiotropic roles in plants, but Ca_o²⁺ in particular can increase the concentration of cytosolic Ca²⁺ ([Ca²⁺]_i) in guard cells (which surround stomatal pores), thereby promoting closure of the stomata. The authors reasoned that this so-called Ca_o²⁺-induced [Ca²⁺]_i increase (CICI) in guard cells might correspond to a means by which Ca_o²⁺ is sensed. So, first they verified that Ca_o²⁺ did indeed increase [Ca²⁺]_i in guard cells. Next, on the basis that inositol-1,4,5-trisphosphate — through the actions of phospholipase C (PLC) — can often mediate receptor-induced Ca²⁺ release, they inhibited PLC. This inhibited guard-cell CICI and stomatal closure, implicating receptor-mediated Ca²⁺ sensing in CICI.

So the authors screened complementary DNA libraries from Arabidopsis, measuring [Ca²⁺]_i increases using a mammalian cell line, and identified a 1.4-kb cDNA that they named CAS (for Ca²⁺ sensing). Homologues of the 387-amino-acid CAS protein were identified in other plants, but not in animals. The amino terminus of CAS contains many acidic amino acids — which can bind Ca²⁺ with low affinity — rather than high-affinity Ca²⁺-binding sites of calmodulin and other Ca²⁺ sensors. Binding studies subsequently showed there to be low-affinity/high-capacity Ca²⁺-binding sites in the amino terminus of CAS.

Han et al. then showed that CAS is mainly expressed in leaf shoots — including guard cells — and used an antisense approach to study the requirement of CAS in guard-cell Ca_o²⁺ signalling. CICI and Ca_o²⁺-induced stomatal closure were impaired following the introduction of an antisense transgene. Furthermore, and in line with the known role of Ca²⁺ in several developmental processes, CAS-antisense-transgenic plants were severely defective in bolting — the rapid upward growth at the transition to seed production — although they did eventually bolt at reduced Ca²⁺ concentrations.

The pathway by which CAS mobilizes Ca²⁺ has yet to be determined, but being able to manipulate CAS should greatly help in clarifying the mechanism of [Ca²⁺]_i control, and in assessing the molecular function of Ca_o²⁺ in plants.