Agriculture Reference
In-Depth Information
Regulation by Hormone Signals
Metabolic regulation is not the only means of controlling metabolic pathways. Plant
hormones play very important roles in many developmental processes. Recent
studies show involvement of plant hormones in regulation of different nutrient
metabolic pathways (Fig.
3.5
).
Cytokinins are adenine-derived plant hormones, which are responsible for reg-
ulation of cell division and differentiation in plants together with auxin. The best
known example of regulation of the sulfate reduction pathway by cytokinin is the
downregulation of high-affinity transporters SULTR1;1 and SULTR1;2
(Maruyama-Nakashita et al.
2004b
). Addition of cytokinins to wild type plants
decreases sulfate uptake and mRNA levels for the two transporters. It is interesting
that SULTR1;2 is much more responsive to the cytokinins than SULTR1;1. It was
suggested that in this regulation, the cytokinin response 1 (CRE1)/wooden leg
(WOL)/
Arabidopsis
histidine kinase 4 (AHK4) cytokinin receptor is involved.
The
Arabidopsis cre1-1
mutant was unable to regulate the high-affinity sulfate
transporters in response to cytokinins (Maruyama-Nakashita et al.
2004b
)
suggesting the independent regulation of high-affinity sulfate transporters by cyto-
kinin (repression) and by sulfate (induction). In contrast, Ohkama and co-workers
(
2002
) have shown that sulfur responsive genes
APR1
and
SULTR2;2
were
up-regulated by cytokinin. They have concluded that this induction is mediated
by an increase in sucrose content.
Auxin-dependent signalling in the regulation of sulfate assimilation may be
connected to the response to sulfate deficiency by indole glucosinolate hydrolysis
(Kutz et al.
2002
). During sulfate deficiency aglycon is released from indole
glucosinolates and indole acetic acid (IAA) is generated from the remaining indole
acetonitrile, catalysed by nitrilase NIT3. IAA may stimulate root growth, and
indeed an increase in length and the numbers of lateral roots is a common pheno-
type of sulfur-deficient plants (Hell and Hillebrand
2001
). Additionally, various
studies indicate positive regulation of auxin-responsive genes during sulfate star-
vation (Maruyama-Nakashita et al.
2003
; Nikiforova et al.
2003
). Decrease in
cysteine production leads to accumulation of OAS and its precursor serine and
sulfate deficiency induces tryptophan synthase. Both of these events result in
increase in tryptophan biosynthesis as in plants it is synthesised from indole and
serine through the activity of the tryptophan synthase
-subunit. In consequence,
increased biosynthesis of tryptophan increases production of auxin (Nikiforova
et al.
2003
).
Jasmonic acid (JA) is another plant hormone, which participates in regulation of
sulfate metabolism. JA is involved in response to oxidative stress and synthesis of
defence molecules. The cellular GSH content rapidly decreases during sulfur
deficiency and this may lead to oxidative stress in cells. It was shown, however,
that JA increases the expression of GSH synthesis pathway enzymes (Xiang and
Oliver
1998
). Additionally, microarray studies showed induced expression of JA
genes during sulfur limitation and in
sultr1;2
mutants (Maruyama-Nakashita
ʲ
