Nitric oxide (NO) is a small highly diffusible gas and a ubiquitous bioactive molecule. Its chemical properties make NO a versatile signal molecule that functions through interactions with cellular targets via either redox or additive chemistry. In plants, NO plays a role in a broad spectrum of pathophysiological and developmental processes. Although nitric oxide synthase (NOS)-dependent NO production has been reported in plants, no gene, cDNA, or protein has been isolated to date. In parallel, precise and regulated NO production can be measured from the activity of the ubiquitous enzyme nitrate reductase (NR). In addition to endogenous NO formation, high NO emissions are observed from fertilized soils, but their effects on the physiology of plants are largely unknown. Many environmental and hormonal stimuli are transmitted either directly or indirectly by NO signaling cascades. The ability of NO to act simultaneously on several unrelated biochemical nodes and its redox homeostatic properties suggest that it might be a synchronizing molecule in plants.

Seed germination, greening of etiolated plants and inhibition of hypocotyl elongation are stimulated by light, which is sensed by various types of photoreceptor. Nitric oxide (NO) has proven to be a bioactive molecule, especially in mammalian cells and, most recently, in plants. Like some phytochrome-dependent processes, many NO-mediated ones are accomplished through increases in cGMP levels. Given these similarities, we proposed that NO could take part in light-mediated events in plants. Here we show that NO promotes seed germination and de-etiolation, and inhibits hypocotyl and internode elongation, processes mediated by light. Two NO donors, sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine induced germination of lettuce (Lactuca sativa L. cv. Grand Rapids) seeds in conditions in which this process is dependent on light (e.g. 26 degrees C). This was a dose-dependent response and was arrested by addition of an NO scavenger, carboxy-PTIO. In addition, nitrite and nitrate, two NO-decomposition products were ineffective in stimulating germination. Wheat seedlings sprayed with SNP and grown in darkness contained 30-40% more chlorophyll than control seedlings. Nitric-oxide-mediated partial greening was increased by light pulses, wounding and biotic stress. Arabidopsis thaliana (L.) Heynh. (ecotype Columbia) and lettuce seedlings grown in the dark had 20%-shorter hypocotyls in NO treatments than in control ones. On the other hand, internode lengths of potato plants growing under low light intensity and sprayed with 100 microM SNP were also 20% shorter than control ones. These results implicate NO as a stimulator molecule in plant photomorphogenesis, either dependent on or independent of plant photoreceptors.

In this report, we demonstrate that nitric oxide (NO) mediates the auxin response leading the adventitious root formation. A transient increase in NO concentration was shown to be required and to be part of the molecular events involved in adventitious root development induced by indole acetic acid (IAA).The discovery of signal molecules involved in the intricate network that triggers root formation remains a major goal for a large number of biotechnological procedures. Adventitious rooting involves the development of a meristematic tissue after removal of the primary root system. The plant hormone auxins promote this process through the dedifferentiation of cells to reestablish the new apical meristem. Although a variety of components of auxin transport and signal transduction were identified, the molecular mechanism underlying the initiation of new root meristems is poorly understood (Doerner, 2000; Berleth and Sachs, 2001).NO is a diffusible multifunctional second messenger first described in mammals, where it plays variable functions ranging from dilation of blood vessels to neurotransmission and defense during immune response (Gow and Ischiropoulos, 2001). Several researches have shown the presence of NO in plants and have attributed novel roles to this gas in the plant kingdom (Beligni and Lamattina, 2001a and refs. therein).Of late, and contemporary to genomics and proteomics, it is interesting to note the revival of pharmacological and surgical techniques in the field of plant developmental biology (Nemhauser et al., 2000; Reinhardt et al., 2000). In this communication, we demonstrate through pharmacological and surgical approaches that NO is required for root organogenesis.Two NO donors, sodium-nitroprussiate (SNP) and S-nitroso, N-acetyl penicillamine (SNAP), applied to hypocotyl cuttings (primary roots removed) of cucumber (Cucumis sativus) were able to mimic the effect of the auxin IAA in inducing de novo root organogenesis (Fig. 1). In addition, NO-and IAAinduced roots presented similar anatomic structure when they were analyzed by optic microscopy (not shown). This NO-mediated effect was prevented when the specific NO-scavenger carboxy-PTIO (cPTIO) was added with SNP or SNAP (Fig. 1). Result of treatments performed with different SNP concentrations confirmed that the effect was dose dependent, with a maximal biological response at 10 m SNP (Fig. 2). Within 3 d after removal of the primary root system, adventitious root development was detected in the explants treated with IAA, SNP, or SNAP. Two parameters of root growth were considered, and length and number of adventitious roots exhibit similar behavior among these treatments (Fig. 3, t test, P Ͻ 0.05). In control experiments, when hypocotyl cuttings were kept in water or in NO 2 Ϫ / NO 3 Ϫ (normal products of NO decomposition, not shown), adventitious roots emerged 4 d after primary root removal, and they reached only 22% of the length obtained from NO-or IAA-treated explants (Fig. 3). The treatment of hypocotyls with SNP or SNAP plus IAA resu...