Instant Notes: Plant Biology

increased pumping of protons (H+) into the cell wall by the plasma membrane

proton ATPase(proton pump; Topic I3). The acid growth theorystates that the

following stages occur:

Auxin

Increased proton pumping

H+to acidify cell wall

Cell wall loosens (enzymes break bonds in wall matrix)

Turgor pressure pushes loosened wall and growth occurs

Proteins (expansins) present in the cell wall catalyze pH-dependent wall

extension growth, by loosening bonds between components of the wall matrix.

How does auxin increase the acidification of the cell wall? Two possibilities

exist: the first is the activation of proton ATPase already present at the plasma

membrane; the second is that auxin causes an increase in the amount of proton

ATPase present by upregulating gene expression. In addition to the effect of

auxin on the proton pump, other auxin-responsive mRNAshave been found,

some of which also increase during elongation. Table 2summarizes some of

these and their possible functions.

F3 – Molecular action of hormones and intracellular messengers 79

DNA-binding protein

+ Gibberellic acid

Gibberellic acid

response element

Gibberellic acid

responsive gene

Transcribed to mRNA Protein

Fig. 2. Model of regulation of a gene by a hormone (gibberellic acid). The grey hatching is the

gibberellic acid response element (GARE), regulated by a gibberellic acid-responsive DNA-

binding protein.