From Cell Division to Organ Shape: Nitric Oxide Is Involved in Auxin-Mediated Root Development - Communication in Plants: Neuronal Aspects of Plant Life - page 129

cells named founder cells. Moreover, auxin negatively regulates the expres-
sion of two CDK inhibitors: Kip-related protein 1 (KRP1) and 2 (KRP2),
involved in the inhibition of G1-to-S phase transition in Arabidopsis (Hi-
manen et al. 2002). Interestingly, NO was able to induce G1-to-S phase
transition through the expression of cyclins and the downregulation of
KRP2duringLRformationintomato(Correa-Aragundeetal.,submitted).
Results clearly show that NO is involved in the control of LR initiation
and development. Whether or not NO is the signal that integrates environ-
mental, physiological and genetic factors that conduce to LR formation and
hence modifies root architecture is a yet unanswered question. It remains to
be explored if any factor that stimulates LR development could also induce
NO production in roots. For example, nutrients such as nitrate have an
important impact on LR development. In soils or media with patchy nitrate
distribution, LRs preferentially proliferate in the nitrate-rich zone (Granato
and Raper 1989; Zhang and Forde 2000). Since enzymatic and nonenzy-
matic nitrate and nitrite reduction could produce NO in roots (Rockel et
al. 2002; Stohr and Ullrich 2002; Bethke et al. 2004), an interesting point to
analyze is whether NO could also be the signal that announces the presence
of nitrate in the soil and triggers the formation of LRs in the nutrient-rich
zone. Additionally, the nitrification and denitrification reactions produced
by soil microorganisms are probably the most important NO sources in
the soil. Thus, the NO released by microorganisms could have a significant
impact on root architecture. Recently, it has been reported that the rhi-
zobacterium Azospirillum brasilense producesNOandthatitisinvolvedin
the Azospirillum -mediated LR promotion in tomato (Creus et al. 2005).
9.3
Nitric Oxide Is Required for Adventitious Root Formation
Auxin is the hormone responsible for the induction of ARF, a process that
involves cell division and root primordia development. Auxin promotes
dedifferentiation of parenchyma cells and the entrance to cell division to
form a new root meristem (De Klerk et al. 1995; Fujita and Syôno 1996).
AfewyearsagoitwasdemonstratedthatNOisinvolvedintheauxin
response during ARF in cucumber (Pagnussat et al. 2002). A transient
increase in endogenous NO concentration after IAA treatment was reported
in the basal region of the hypocotyl, where the new root meristems develop
(Pagnussat et al. 2002). More recently, it was proposed that convergent
and complex cyclic GMP (cGMP) dependent and independent signaling
pathways are orchestrating the formation of a new root system when the
primary root is removed (Pagnussat et al. 2003, 2004).