G-Proteins as Molecular Switches in Signal Transduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 193

3.22 Interplay Between G-Proteins and Auxin

Signaling Systems

The auxin signaling system plays an important role in plant innate immunity
(Fu and Wang 2011 ). The auxin indole-3-acetic acid activates small monomeric
G-proteins (Rac-like GTPases) and they in turn stimulate auxin-responsive gene
expression (Tao et al. 2002 ; Xu et al. 2010 ; Wu et al. 2011 ). Addition of exogenous
auxin to tobacco seedlings stimulates activation of endogenous Rac/Rops (Tao et al.
2002 ). It has been demonstrated that overexpressing a wild-type tobacco Rac/Rop
GTPase, NtRac1, and its constitutively active mutant form activates auxin-
responsive gene expression (Tao et al. 2002 ). On the other hand, overexpressing
dominant-negative NtRac1and Rac-negative regulators, or reducing the endogenous
NtRac1 level, suppresses auxin -induced gene expression (Tao et al. 2002 ).
GH3 and DR5 are natural and synthetic auxin-inducible promoters, respectively.
GH3-GUS ( -glucuronidase) and DR5-GUS were found to be induced by naphtha-

lene acetic acid (NAA) in transfected protoplasts (Tao et al. 2002 ). Coexpression of
NtRac1 activated expression of the auxin-responsive genes. The Rac-GTPase regu-
lators Rac-GAP (GTP-ase activating protein) and Rac-GD1 (guanine nucleotide
dissociation inhibitor) downregulate the activity of the Rac GTPases by maintaining
them predominantly in the GDP-bound inactive state. Expression of these negative
regulators in transfected protoplasts counteracted considerably the ability of coex-
pressed NtRac1 to activate auxin -responsive promoters. These results suggest that
active NtRac1 upregulates a subset of auxin-responsive promoters in transfected
protoplasts (Tao et al. 2002 ).
It has been reported that the tobacco RAC-like GTPase Ntrac1 activates auxin-
responsive gene expression in the absence of auxin (Tao et al. 2002 ). It suggests a
signaling role for auxin-regulated gene expression for this small GTPase. The
dominant-negative mutant forms of NtRac1(NtRac1[DN], with either a D121A or a
T20N conversion rendering them constitutively in the inactive GDP-bound state)
and the negative Rac-GTPase regulators Rac GAP and Rac-GDI block auxin-
induced gene expression in the presence of exogenous auxin, indicating that endog-
enous active NtRacs are important for mediating the auxin signal to responsive
genes. Further it has been shown that auxin activates NtRacs (Tao et al. 2002 ).
Collectively these results suggest the existence of a signaling pathway whereby
auxin-activated NtRacs stimulate downstream responsive gene expression.

β

3.23 G-Proteins Activate Defense-Related Enzymes

G-proteins may regulate expression of several defense-related enzymes. OsRac1,
the rice small G-protein, induced several enzymes associated with the phenylpro-
panoid pathway, including caffeic acid- O -methyltransferase (the enzyme involved in
lignin biosynthesis) and isofl avone reductase (the enzyme involved in phytoalexin