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to complete its biological function. In spite of molecular, genetic and bio-
chemical information generated through more than 100 years of studies on
auxin biological functions, the auxin-controlled signaling pathways during
rootdevelopmentaswellasthespecificrolesforintermediatemolecules
involved are not fully understood.
9.1.2
Nitric Oxide Is a New Player in Auxin-Mediated
Root Development: Summary of Its Effects
NO is a diffusible second messenger first described in mammals, where it
plays variable functions ranging from dilation of blood vessels to neuro-
transmission and defense during immune response (Gow and Ischiropou-
los 2001). Numerous investigations in the last decade have discovered new
functions for this molecule in the plant kingdom (Lamattina el al. 2003;
Neill et al. 2003). NO affects in a noticeable manner the morphology and
developmental pattern of roots. NO was shown to be involved in the promo-
tion of lateral (Correa-Aragunde et al. 2004) and adventitious (Pagnussat
et al. 2002) roots. High concentrations of NO produce root growth inhibi-
tion (Correa-Aragunde et al. 2004), while low concentrations induce root
elongation (Gouvea et al. 1997; Hu et al. 2005). The density of root hairs is
increased by exogenous application of NO in various plant species (Lom-
bardo and Lamattina, unpublished results). Recently, it was also demon-
strated that NO mediates the gravitropic bending in soybean roots (Hu
et al. 2005). Figure 9.1 shows that NO effects on plant root morphology
and physiology take place at many different locations along the root or-
gan. Interestingly, all these NO-mediated effects in roots are also under the
control of auxins. Moreover, local NO accumulation occurs in response to
auxinsduringadventitiousandLRemergenceandalsointhegravitropic
response of roots (Pagnussat et al. 2002; Correa-Aragunde et al. 2004; Hu
et al. 2005).
VariousNOsourceshavebeenidentifiedinplantsandmanyofthemhave
been described in roots. Enzymatic sources that include nitrate reductase
and NO synthase like activities are involved in NO production (Rockel
et al. 2002; Guo et al. 2003). In parallel, apoplastic synthesis of NO from
nitrite at an acidic pH might also be relevant (Bethke et al. 2004). Roots
are regularly exposed to nitrite, and acidification of root apoplast often
occurs in association with altered nutrient supply. Interestingly, cell wall
and apoplast acidification can also be induced by auxin (discussed by
Bethke et al. 2004). Additionally, a plasma membrane nitrite reductase
specific of roots has been described to catalyze the formation of NO from
nitrite (Stohr and Ulrich 2002). Finally, the potential NO source derived
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