Calcium Homeostasis in Plants

Question 1

Explain Ca2+ levels in a normal situation and how it can lead to toxicity, growth inhibition, and defence responses.

Answer 1

The extracellular Ca2+ concentration is several magnitudes higher than intracellular Ca2+ concentration, resulting in the plant being at risk for a Ca2+ overload. Whereas low extracellular Ca2+ concentrations only prompt transient intracellular Ca2+ concentration changes, sustained elevation occurs beyond a threshold, causing toxicity, growth inhibition, and defence responses.

Question 2

What are ACAs and CAXs and what do they do?

Answer 2

Ca2+-ATPases (ACAs) and Ca2+/H+-exchangers (CAXs) are proteins that are thought to function in maintaining a low resting intracellular Ca2+ concentration in plants. CAXs also coordinate with ACAs to control plant fitness and immunity under specific conditions, although this mechanisms remains unclear.

Question 3

What does a plant do to cope with fluctuating extracellular Ca2+ concentrations?

Answer 3

Vacuolar Ca2+ sequestration mediated by CAX1/3.

Question 4

Explain the Ca2+-dependent CAX1/3 activation pathway.

Answer 4

The Ca2+ sensor CBL couples with CBL-interacting protein kinases (CIPKs) and form a complex. CBL-CIPK modules can then activate CAX1/3 by phosphorylating the conserved S-cluster in the auto-inhibitory domain. CAX1/3 may also serve as a convergent point of other Ca2+ signalling events in response to abiotic stress factors.

Question 5

Explain the Ca2+-independent CAX1/3 activation pathway.

Answer 5

Ca2+-independent activation of CAX1/3 occurs through pattern-triggered immunity (PTI). In response to MAMPs, the immune receptor complex of flagellin sensitive 2 (FLS2) and brassinosteroid insensitive 1-associated kinase 1 (BAK1) is assembled. Downstream cytoplasmic kinases BIK1 and PBL1 can then phosphorylate the conserved S-cluster in the auto-inhibitory domain of CAX1/3.

Question 6

Ca2+ is essential in plant immune responses. Why?

Answer 6

Pattern-triggered immunity (PTI) and effector-triggered immunity (ETI) both rely on Ca2+ influx through various channels.

Question 7

What is vacuolar Ca2+ sequestration?

Answer 7

Vacuolar Ca2+ sequestration is a mechanism in which Ca2+ is moved to the vacuole to prevent Ca2+ toxicity in a plant cell.