Magdalena Graziano

Nitric Oxide Improves Internal Iron Availability in Plants

Iron deficiency impairs chlorophyll biosynthesis and chloroplast development. In leaves, most of the iron must cross several biological membranes to reach the chloroplast. The components involved in the complex internal iron transport are largely unknown. Nitric oxide (NO), a bioactive free radical, can react with transition metals to form metal-nitrosyl complexes. Sodium nitroprusside, an NO donor, completely prevented leaf interveinal chlorosis in maize (Zea mays) plants growing with an iron concentration as low as 10 μmFe-EDTA in the nutrient solution.S-Nitroso-N-acetylpenicillamine, another NO donor, as well as gaseous NO supply in a translucent chamber were also able to revert the iron deficiency symptoms. A specific NO scavenger, 2-(4-carboxy-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, blocked the effect of the NO donors. The effect of NO treatment on the photosynthetic apparatus of iron-deficient plants was also studied. Electron micrographs of mesophyll cells from iron-deficient maize plants revealed plastids with few photosynthetic lamellae and rudimentary grana. In contrast, in NO-treated maize plants, mesophyll chloroplast appeared completely developed. NO treatment did not increase iron content in plant organs, when expressed in a fresh matter basis, suggesting that root iron uptake was not enhanced. NO scavengers 2-(4-carboxy-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and methylene blue promoted interveinal chlorosis in iron-replete maize plants (growing in 250 μm Fe-EDTA). Even though results support a role for endogenous NO in iron nutrition, experiments did not establish an essential role. NO was also able to revert the chlorotic phenotype of the iron-inefficient maize mutants yellow stripe1 and yellow stripe3, both impaired in the iron uptake mechanisms. All together, these results support a biological action of NO on the availability and/or delivery of metabolically active iron within the plant.

Nitric oxide functions as a positive regulator of root hair development.

The root epidermis is composed by two cell types: trichoblasts (or hair cells) and atrichoblasts (or non-hair cells). In lettuce (Lactuca sativa cv. Grand Rapids var. Rapidmor oscura) plants grown hydroponically in water, the root epidermis did not form root hairs. The addition of 10 µM sodium nitroprusside (SNP), a nitric oxide (NO) donor, resulted in almost all rhizodermal cells differentiated into root hairs. Treatment with the synthetic auxin 1-naphthyl acetic acid (NAA) displayed a significant increase of root hair formation (RHF) that was prevented by the specific NO scavenger carboxy-PTIO (cPTIO). In Arabidopsis, two mutants have been shown to be defective in NO production and to display altered phenotypes in which NO is implicated. Arabidopsis nos1 has a mutation in an NO synthase structural gene (NOS1), and the nia1 nia2 double mutant is null for nitrate reductase (NR) activity. We observed that both mutants were affected in their capacity of developing root hairs. Root hair elongation was significantly reduced in nos1 and nia1 nia2 mutants as well as in cPTIO-treated wild type plants. A correlation was found between endogenous NO level in roots detected by the fluorescent probe DAF-FM DA and RHF. In Arabidopsis, as well as in lettuce, cPTIO blocked the NAA-induced root hair elongation. Taken together, these results indicate that: (i) NO is a critical molecule in the process leading to RHF, and (ii) NO is involved in the auxin-signaling cascade leading to RHF.

Nitric Oxide Functions as Intermediate in Auxin, Abscisic Acid, and Lipid Signaling Pathways

Nitric oxide (NO) is a chemical messenger that actively operates in the plant kingdom. In recent years, NO has been shown to be involved in many and diverse growth, developmental, and physiological processes in plants. It has been shown that NO takes part in different hormone signaling pathways and also acts in concert with well-characterized second messengers. In this chapter, we discuss findings that contribute to the understanding of the role/s of NO during root organogenesis and stomatal movement, focusing on the interrelations between NO and the phytohormones auxin and abscisic acid. We emphasize the requirement of calcium as an essential intermediate present in the NO-mediated responses. Finally, novel data concerning the cross-talk between NO and phosphatidic acid signaling pathways in response to (a)biotic stresses are presented and discussed.

Decoding plant responses to iron deficiency: Is nitric oxide a central player?

Plants respond to iron deprivation by inducing a series of physiological and morphological responses to counteract the nutrient deficiency. These responses include: (i) the acidification of the extracellular medium, (ii) the reduction of ferric ion and (iii) the increased transport of ferrous ion inside of root cells. This iron transport system is present in strategy I plants and is strictly regulated; at low iron concentration the responses are induced whereas upon iron supply they are repressed. The mechanisms related with this process has been extensively studied, however, the specific cellular effectors involved in sensing iron deficiency, the cascade of components participating in signal transduction, and the way iron is metabolized and delivered, are yet poorly understood. Recently, it has been proposed nitric oxide (NO) as a signaling molecule required for plant responses to iron deficiency. NO is produced rapidly in the root epidermis of tomato plants that are growing under iron deficient conditions. Furthermore, it was demonstrated that NO is required for the expression and activity of iron uptake components in roots during iron deprivation. Here we propose and discuss a working hypothesis to understand the way NO is acting in plants responses to iron deficiency. We specifically highlight the cross talk between NO and plant hormones, and the interaction between NO, iron and glutathione for the formation of dinitrosyl iron complexes (DNICs). Finally, a potential role of DNICs in iron mobilization is proposed. Addendum to: Graziano M, Lamattina L. Nitric oxide accumulation is required for molecular and physiological responses to iron deficiency in tomato roots. Plant J 2007; 52:949-60.

Nitric oxide and dinitrosyl iron complexes: roles in plant iron sensing and metabolism

Publisher Summary This chapter aims to review the role of nitric oxide (NO) and dinitrosyl iron complexes (DNICs) in plants. NO is a signaling and physiologically active molecule in plants. However, the molecular mechanism/s involved in transducing the NO signal between cells and tissues is/are still unknown. The formation of low-molecular weight DNICs from internal NO sources has been recently demonstrated in plants. In addition, S-nitrosoglutathione (GSNO) has been shown to be a biologically active compound in plants. Both DNICs and GSNO are candidates for NO storage and/or mobilization between plant tissues and cells. NO has been shown to have a role in plant iron nutrition; therefore, it is proposed that DNICs and GSND may have roles in NO-mediated improvement of iron nutrition in plants growing under iron deficient conditions. It is suggested that the formation of DNICs constitutes a key process in plant iron sensing and metabolism. An interconversion between DNICs and GSNO based on the iron and NO status of the plant cell might be the core of a metabolic process leading plant iron homeostasis.

From Cell Division to Organ Shape: Nitric Oxide Is Involved in Auxin-Mediated Root Development

Roots are plant organs that mainly function to acquire water and nutrients from soil. Root development is under the control of a regulated cell proliferation and morphogenesis, and auxin is the central plant hormone that governs those processes. In this review we discuss new aspects of the mechanisms that operate during root organogenesis. We particularly emphasize the analyses of downstream signals involved in the auxin control of root development. Nitric oxide (NO), an emerging chemical messenger that plays a significant role in a broad spectrum of plant developmental processes, is a key component in the signal transduction pathways that determine root architecture. Lateral root development as well as adventitious root formation are strictly NO-dependent processes in the auxin-promoted root organogenesis.

Distribution of Endogenous NO Regulates Early Gravitropic Response and PIN2 Localization in Arabidopsis Roots

High-resolution and automated image analysis of individual roots demonstrated that endogenous nitric oxide (NO) contribute significantly to gravitropism of Arabidopsis roots. Lowering of endogenous NO concentrations strongly reduced and even reversed gravitropism, resulting in upward bending, without affecting root growth rate. Notably, the asymmetric accumulation of NO along the upper and lower sides of roots correlated with a positive gravitropic response. Detection of NO by the specific DAF-FM DA fluorescent probe revealed that NO was higher at the lower side of horizontally-oriented roots returning to initial values 2 h after the onset of gravistimulation. We demonstrate that NO promotes plasma membrane re-localization of PIN2 in epidermal cells, which is required during the early root gravitropic response. The dynamic and asymmetric localization of both auxin and NO is critical to regulate auxin polar transport during gravitropism. Our results collectively suggest that, although auxin and NO crosstalk occurs at different levels of regulation, they converge in the regulation of PIN2 membrane trafficking in gravistimulated roots, supporting the notion that a temporally and spatially coordinated network of signal molecules could participate in the early phases of auxin polar transport during gravitropism.