References

9 Nitric Oxide and Auxin in Root Development 127
9.2
Nitric Oxide Mediates Auxin-Induced Lateral
Root Development
LRs play a major role in taking up nutrients and water from soil and
strongly contribute to the physical support for the plant. Therefore, the
ability of plants to develop a branched root architecture greatly increases
their success in a particular environment (Malamy and Benfey 1997a).
LRs originate from differentiated nondividing pericycle cells within the
primary root. The first event during LR primordia formation occurs when
individual cells from the pericycle are induced to dedifferentiate and divide
symmetrically and asymmetrically to form the LR primordium. Finally, the
LR primordium grows principally by elongation and emerges from the parent
root (Malamy and Benfey 1997a). The initiation of LRs is dramatically
influenced by information derived from a wide range of environmental, genetic
and physiological factors (Malamy and Benfey 1997b; Casimiro et al.
2003). Hence, the plant must integrate these signals and decide whether or
not to trigger LR initiation in a specific zone of the primary root. Thereby,
several interesting questions arise: (1) how are these cues perceived and
interpreted?, (2) how are the signals transduced and (3) how is the localized
organogenesis initiated? The identification of the nature of these signals
and the understanding of how they interact to regulate LR development are
important challenges for plant biologists.
Auxinhasbeenknownforalongtimetobethemainplanthormone
involved in LR development. It was recently shown that NO is required for
auxin-mediated LR formation (Correa-Aragunde et al. 2004). The applicationoftheNOdonorsodiumnitroprussideinducesLRdevelopmentin
tomato (Lycopersicon esculentum L.) seedlings, while specific scavenging of
NOresultsinnoLRformation(Fig.9.2).TheNOeffectisdose-dependent,
displaying a typical hormone dose–response curve. Moreover, NO is able
to induce LR primordia in auxin-depleted seedlings and it was found that
auxin-induced LR formation could be prevented by application of the NO
scavenger 2-(4-carboxyphenyl)-4, 4,5,5-tetramethylimidazoline-1-oxyl-3oxide,
potassium salt (CPTIO) (Correa-Aragunde et al. 2004). These results
strongly support a lineal signal transduction cascade involving NO downstream
of auxins. NO is mainly produced in the pericycle cells that will
give place to an LR, indicating that NO is required during early stages of
LR development. According to this, depletion of endogenous NO with CP-
TIO results in the complete inhibition of LR primordia formation in the
CPTIO-treated seedlings. In parallel to the promotion of LR formation,
it was clearly demonstrated an NO dose-dependent inhibition of primary
root growth (Correa-Aragunde et al. 2004; Fig. 9.2). Microscopical analysis