References

9 Nitric Oxide and Auxin in Root Development 131
concentration may be involved in the signaling events leading to ARF in
cucumber. Results obtained in our laboratory indicate that Ca 2+ and Ca 2+ -
dependent protein kinase (CDPK) activity are downstream messengers in
thesignalingpathwaystriggeredbyauxinsandNO.Moreover,theavailability
of both intracellular and extracellular Ca 2+ pools seems to be required
for the action of IAA and NO to trigger ARF (Lanteri et al., submitted).
Thus, in addition to the function of Ca 2+ as a mineral nutrient modulating
the root growth (Druart 1997; Bellamine et al. 1998), evidence supports the
involvement of Ca 2+ as a second messenger linking both auxins and NO to
the activation of processes leading to ARF (Fig. 9.3).
9.3.3
Nitric Oxide Induces Cyclic GMP Independent Pathways
During Adventitious Root Formation
Diverse signal transduction cascades rely on mitogen-activated protein
kinases (MAPKs) as intermediates to regulate a variety of cellular functions
in response to extracellular stimuli. Experimental evidence from different
plant species indicates that several MAPK pathways are implicated in the
regulation of cell cycle and developmental processes, and in responses to
environmental constraints and hormone treatments (Tena et al. 2001; Jonak
et al. 2002). Evidence supports the activation of a MAPK signaling cascade
occuring in response to auxins during ARF. Interestingly, this activation
was shown to be mediated by NO. The MAPK cascade seems to be cGMPindependent,
since the NO-induced in vitro MAPK activity was not affected
by the GC inhibitor LY83583. However, the NO-induced MAPK activity
dramatically decreased when measured in the presence of the MAPK kinase
inhibitor PD098059 (Pagnussat et al. 2004).
The MAPK signaling pathway could be regulating both mitotic processes
and the expression of auxin-induced genes during the formation of new
roots. Convincing evidence on the requirement of a MAPK cascade for plant
cell division comes from experiments carried out in tobacco and Arabidopsis
cellcultures.IthasbeenshownthattheexpressionofthetobaccoMAPK
kinase kinase (MAPKKK) NPK1 (nucleus and phragmoplast-localized protein
kinase 1) is essential for phragmoplast expansion, and its absence
results in the formation of multinucleate cells (Nishihama et al. 2001).
In addition, Krysan et al. (2002) showed that members of the Arabidopsis
NPK-like protein kinase family are involved in the control of cell division in
Arabidopsis. The induction of MAPK activity in response to auxins had previously
been described by Mizoguchi et al. (1994) and Mockaitis and Howell
(2000). Interestingly, studies of tobacco have revealed that NO activates
a MAPK involved in the plant defense responses (Kumar and Klessig 2000).