References

130 M.L. Lanteri et al.
9.3.1
Nitric Oxide Acts Downstream of Auxins to Induce
Adventitious Root Formation
The auxin IAA is synthesized in the shoot apical meristem of the seedlings
and is basipetally transported via the polar transport system (Jones 1998).
When the apical IAA production was disrupted by decapitation of cucumber
explants and basipetal transport of auxins was inhibited by treatment
with 1-naphthylphthalamic acid, ARF was significantly reduced (Pagnussat
et al. 2003). Interestingly, this inhibitory effect due to blockage of auxin production
and transport could be reversed by NO, suggesting that IAA and
NO might be acting through a lineal signaling pathway. Accordingly, the
auxin-induced ARF was prevented by the application of the NO scavenger
CPTIO. Altogether, these results indicate that NO operates downstream of
auxins triggering ARF.
9.3.2
Nitric Oxide Activates Cyclic GMP Dependent Pathways
During Adventitious Root Formation
In mammalian systems, one of the most studied targets of NO is the enzyme
guanylate cyclase (GC). GC, together with the enzyme phosphodiesterase
(PDE), regulates the endogenous concentration of the cellular messenger
cGMP. NO is able to transiently activate GC and increase the level of cGMP,
whilePDEactivityisresponsibleforthebreakdownofcGMPinto5 ′ GMP.
cGMP is an important signaling molecule involved in mechanisms that
sense extracellular stimuli and transduce the signals into metabolic responses
(Reggiani 1997). The involvement of cGMP in different plant physiological
processes has been assessed by the use of inhibitors of the enzyme
GC like 6-anilino-5, 8-quinilinedione (LY83583; Mulsch et al. 1988; Donaldson
et al. 2004). The GC inhibitor LY83583 was able to reduce ARF in both
IAA- and NO-treated cucumber explants. This inhibition was reversed by
the treatment with the permeable cGMP analog 8-Br-cGMP. In addition, the
PDE inhibitor sildenafil citrate mimicked the NO effect on ARF (Pagnussat
et al. 2003). This evidence strongly indicates that NO operates downstream
of IAA inducing ARF through the GC-catalyzed synthesis of cGMP.
A potential target for cGMP is a cGMP-dependent protein kinase (Pk-G).
Although no plant Pk-G has been cloned yet, biochemical evidence for
a Pk-G activity has recently been reported in the plant morning glory
(Szmidt-Jaworska et al. 2003). cGMP can also act via cyclic ADP ribose
(cADPR). cADPR was reported to regulate cytosolic Ca 2+ concentration in
various plant systems (Sanders et al. 1999). Consequently, variations in Ca 2+