Instant Notes: Plant Biology

Plants also have a range of Ca2+-dependent(but calmodulin independent)

protein kinases. Protein kinases are important in signal transduction pathways

as they phosphorylate other proteins at specific sites, altering their activity.

Inositol trisphosphate

Theinositol triphosphate(IP 3 ) pathway begins with the conversion of a plasma

membrane lipid, phosphatidyl inositolintophosphatidyl inositol bisphos-

phate (PIP 2 ) by kinase enzymes in the plasma membrane. PIP 2 is then

hydrolyzed by phospholipase C(PLC) to give IP 3 anddiacylglycerol(DAG).

PLC activity is regulated by a signal transduction pathway initiated by the

binding of a hormone to a receptor, so IP 3 and DAG levels respond to stimuli.

IP 3 causes Ca2+channels to open at the tonoplast (vacuole) and endoplasmic

reticulum, resulting in a Ca2+signal being initiated (see above). DAG activates

protein kinase Cthat regulates other processes. Figure 4summarizes the Ca2+

and inositol trisphosphate signaling pathways.

82 Section F – Growth and development

Hormone receptor Protein kinase C

Cell membrane

G-protein Phospholipase C

PIP 2

DAG

Ca2+ Ca

2+

Ca2+Ca

2+

Ca2+

Ca2+

Ca2+

Ca2+

Ca2+

Ca2+

Ca2+

Ca2+

Ca2+

Ca2+

Protein phosphorylation

(leads to altered cell function)

Endoplasmic reticulum

Vacuole

IP 3 -sensitive calcium channels

IP 3

IP 3 IP 3

IP 3 IP^3

Fig. 4. Intracellular messengers. Release of Ca2+from the endoplasmic reticulum (ER) and

vacuole results in a transient rise in cytoplasmic Ca2+concentration. The Ca2+binds with

proteins like calmodulin- and Ca2+-dependent protein kinases to alter cell function.