Nitric oxide (NO) is considered a key regulator of plant developmental processes and defense, although the mechanism and direct targets of NO action remain largely unknown. We used phenotypic, cellular, and genetic analyses in Arabidopsis thaliana to explore the role of NO in regulating primary root growth and auxin transport. Treatment with the NO donors S-nitroso-N-acetylpenicillamine, sodium nitroprusside, and S-nitrosoglutathione reduces cell division, affecting the distribution of mitotic cells and meristem size by reducing cell size and number compared with NO depletion by 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO). Interestingly, genetic backgrounds in which the endogenous NO levels are enhanced [chlorophyll a/b binding protein underexpressed 1/NO overproducer 1 (cue1/nox1) mirror this response, together with an increased cell differentiation phenotype. Because of the importance of auxin distribution in regulating primary root growth, we analyzed auxin-dependent response after altering NO levels. Both elevated NO supply and the NO-overproducing Arabidopsis mutant cue1/nox1 exhibit reduced expression of the auxin reporter markers DR5pro:GUS/GFP. These effects were accompanied by a reduction in auxin transport in primary roots. NO application and the cue1/nox1 mutation caused decreased PIN-FORMED 1 (PIN1)-GFP fluorescence in a proteasome-independent manner. Remarkably, the cue1/nox1-mutant root phenotypes resemble those of pin1 mutants. The use of both chemical treatments and mutants with altered NO levels demonstrates that high levels of NO reduce auxin transport and response by a PIN1-dependent mechanism, and root meristem activity is reduced concomitantly.cell division and elongation | plant growth regulator | root development N itric oxide (NO) is a signaling molecule involved in a variety of physiological processes during plant growth and development and also is an important modulator of disease resistance. Extensive research has shown that NO is involved in the promotion of seed germination, photomorphogenesis, mitochondrial activity, leaf expansion, root growth, stomatal closure, fruit maturation, senescence, and iron metabolism (as reviewed in ref. 1). NO also is important for defense response, playing key roles in the activation of defense genes (e.g., pathogenesis-related protein 1), in phytoalexin production, and in modulation of programmed cell death (1-3). The mechanism for NO signal transduction, plant resistance to pathogens and cell death, cellular transport, basic metabolism, and photosynthesis frequently occurs through an NO-induced change in transcription (4).Additionally, NO is produced in plant tissues by two major pathways, one enzymatic and the other nonenzymatic (5). The enzymatic pathway of NO production is being studied thoroughly, and much information about the type and subcellular localization of the enzymes involved is available. Different enzymes have been identified that catalyze the synthesis of NO from two different substrates, nitrate and argi...

During the past two decades, nitric oxide (NO) has evolved from a mere gaseous free radical to become a new messenger in plant biology with an important role in a plethora of physiological processes. This molecule is involved in the regulation of plant growth and development, pathogen defence and abiotic stress responses, and in most cases this is achieved through its interaction with phytohormones. Understanding the role of plant growth regulators is essential to elucidate how plants activate the appropriate set of responses to a particular developmental stage or a particular stress. The first task to achieve this goal is the identification of molecular targets, especially those involved in the regulation of the crosstalk. The nature of NO targets in these growth and development processes and stress responses remains poorly described. Currently, the molecular mechanisms underlying the effects of NO in these processes and their interaction with other plant hormones are beginning to unravel. In this review, we made a compilation of the described interactions between NO and phytohormones during early plant developmental processes (i.e. seed dormancy and germination, hypocotyl elongation and root development).

Nitric oxide (NO) is a unique reactive nitrogen molecule with an array of signaling functions that modulates plant developmental processes and stress responses. To explore the mechanisms by which NO modulates root development, we used a pharmacological approach and NO-deficient mutants to unravel the role of NO in establishing auxin distribution patterns necessary for stem cell niche homeostasis. Using the NO synthase inhibitor and Arabidopsis (Arabidopsis thaliana) NO biosynthesis mutants (nitric oxide-associated1 [noa1], nitrate reductase1 [nia1] and nia2, and nia1 nia2 noa1), we show that depletion of NO in noa1 reduces primary root elongation and increases flavonol accumulation consistent with elevated reactive oxygen species levels. The elevated flavonols are required for the growth effect, because the transparent testa4 mutation reverses the noa1 mutant root elongation phenotype. In addition, noa1 and nia1 nia2 noa1 NO-deficient mutant roots display small root meristems with abnormal divisions. Concomitantly, auxin biosynthesis, transport, and signaling are perturbed. We further show that NO accumulates in cortex/endodermis stem cells and their precursor cells. In endodermal and cortical cells, the noa1 mutant acts synergistically to the effect of the wuschel-related homeobox5 mutation on the proximal meristem, suggesting that NO could play an important role in regulating stem cell decisions, which has been reported in animals.Root growth enables plants to explore their substrate and extract nutrients and water from larger areas and greater depths while also providing anchorage to the substrate. To properly regulate primary root elongation, multiple intrinsic and extrinsic signals must be integrated. Plant hormones are important long-distance signals, and auxin in particular regulates many aspects of root development. Auxin regulates primary root growth in three important ways: by positioning the stem cell niche, affecting divisions in a transit-amplifying zone known as the meristem, and increasing cell volume in the elongation zone (for review, see Bennett and Scheres, 2010). These processes are dependent on the establishment of auxin gradients. Models generated on the basis of available information predict a maximum auxin concentration in the quiescent center (QC) and a steep auxin gradient in the proximal meristem, which decreases with distance to the QC (Grieneisen et al., 2007;Kramer et al., 2008;Laskowski et al., 2008).Interestingly, reactive oxygen species (ROS) and reactive nitrogen species (RNS) have been found to play an important role in the response to intrinsic and extrinsic growth-modulating signals. Research over the last decades has shown several crucial roles of nitric oxide (NO; an RNS) during plant development. NO is involved in the promotion of seed germination and alleviation of seed dormancy, the shaping of root architecture, the repression of floral transition, and the regulation of mitochondrial respiratory complexes, stomatal closure, fruit maturation, senescence, and iron metaboli...

Summary Root branching in plants relies on the de novo formation of lateral roots (LRs). These are initiated from founder cells, triggering new formative divisions that generate lateral root primordia (LRP). The LRP size and shape depends on the balance between positive and negative signals that control cell proliferation.The mechanisms controlling proliferation potential of LRP cells remains poorly understood. We found that Arabidopsis thaliana MYB36, which have been previously shown to regulate genes required for Casparian strip formation and the transition from proliferation to differentiation in the primary root, plays a new role in controlling LRP development at later stages.We found that MYB36 is a novel component of LR development at later stages. MYB36 was expressed in the cells surrounding LRP where it controls a set of peroxidase genes, which maintain ROS balance. This was required to define the transition between proliferating and arrested cells inside the LRP, coinciding with the change from flat to dome-shaped primordia. Reducing the levels of hydrogen peroxide (H2O2) in myb36-5 significantly rescues the mutant phenotype.Our results uncover a role for MYB36 outside the endodermis during LRP development through a mechanism analogous to regulating the proliferation/differentiation transition in the root meristem.