Instant Notes: Plant Biology

Calcium as an intracellular messenger

Cells maintain a very low concentration of Ca2+in the cytoplasm (<1μM), by

pumping it out across the plasma membrane to the apoplast using Ca2+

ATPases. They also pump Ca2+intointracellular storessuch as the vacuole and

endoplasmic reticulum. This results in a very steep Ca2+gradient across several

cell membranes. When calcium channels(Topic I3) in these membranes open,

Ca2+floods into the cytoplasm, giving a Ca2+‘wave’. Channel opening may be

regulated by a hormone receptor or by some other stimulus. This amplifies the

signal (one molecule of hormone bound can keep a channel open long enough

to permit tens of Ca2+ions to enter) and can integrate (coordinate) several

signals. The Ca2+ wave is then perceived by receptor proteins. The best

described of these is calmodulin, a protein with four Ca2+binding sites. When

calmodulin binds Ca2+, it changes conformation and activates a range of

proteins, including Ca2+-calmodulin-dependent protein kinases(CaMPKs).

F3 – Molecular action of hormones and intracellular messengers 81

Respiratory

climacteric

Respiration rate

Ethylene production

Wall

loosening

enzymes

Pigment (lycopene)

synthesis

Day 0

Green fruit

Day 10

Orange fruit

Day 20

Red fruit

MOLECULAR EVENTS

Enhanced ACC synthase and

ACC oxidase activity

Enhanced expression of gene for

nETR ethylene receptor

Burst of respiration – the

climacteric

Ethylene-induced

expression of ripening

genes

Activity

Fig. 3. Regulation of the ripening of a tomato fruit by ethylene.