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.

The cell cycle is defined by a series of complex events, finely coordinated through hormonal, developmental and environmental signals, which occur in a unidirectional manner and end up in producing two daughter cells. Accumulating evidence reveals that chromatin is not a static entity throughout the cell cycle. In fact, there are many changes that include nucleosome remodeling, histone modifications, deposition and exchange, among others. Interestingly, it is possible to correlate the occurrence of several of these chromatin-related events with specific processes necessary for cell cycle progression, e.g., licensing of DNA replication origins, the E2F-dependent transcriptional wave in G1, the activation of replication origins in S-phase, the G2-specific transcription of genes required for mitosis or the chromatin packaging occurring in mitosis. Therefore, an emerging view is that chromatin dynamics must be considered as an intrinsic part of cell cycle regulation. In this article, we review the main features of several key chromatin events that occur at defined times throughout the cell cycle and discuss whether they are actually controlling the transit through specific cell cycle stages.

Arabidopsis naturally occurring populations have allowed for the identification of considerable genetic variation remodeled by adaptation to different environments and stress conditions. Water is a key resource that limits plant growth, and its availability is initially sensed by root tissues. The root's ability to adjust its physiology and morphology under water deficit makes this organ a useful model to understand how plants respond to water stress. Here, we used hyperosmotic shock stress treatments in different Arabidopsis accessions to analyze the root cell morphological responses. We found that osmotic stress conditions reduced root growth and root apical meristem (RAM) size, promoting premature cell differentiation without affecting the stem cell niche morphology. This phenotype was accompanied by a cluster of small epidermal and cortex cells with radial expansion and root hairs at the transition to the elongation zone. We also found this radial expansion with root hairs when plants are grown under hypoosmotic conditions. Finally, root growth was less affected by osmotic stress in the Sg-2 accession followed by Ws, Cvi-0, and Col-0; however, after a strong osmotic stress, Sg-2 and Cvi-0 were the most resilience accessions. The sensitivity differences among these accessions were not explained by stress-related gene expression. This work provides new cellular insights on the Arabidopsis root phenotypic variability and plasticity to osmotic stress.Genes 2019, 10, 983 2 of 23 northern hemisphere and consequently is subjected to diverse environmental conditions, generating different natural variants called accessions [1]. The Arabidopsis accessions are an important genetic resource to identify mechanisms underlying plant development and stress tolerance as plant genotypes are constantly shaped by biotic and abiotic factors [2]. These phenotypic and genetic variations have enabled the characterization of responses in Arabidopsis natural variants using a range of different approaches [3][4][5][6][7][8].Water deficit is an abiotic stress that affects plant development and productivity. Water availability can be altered by changes in solute concentration (i.e., sugars, salt, inorganic cations and anions) during drought, cold stress, and freezing [9,10]. Some phenotypic and genetic analyses have identified tolerant accessions that can be useful to study the traits related to water deficit tolerance [11,12] (for a review see [13]).Although stress affects the whole organism [14-17], leaves and roots display different responses in order to reduce water loss and promote water foraging for survival [18]. Accordingly, plant roots act on the frontlines by sensing the water deficit, adjusting osmotic homeostasis, forcing water entrance, and avoiding water loss through the accumulation of compatible solutes inside the tissues [19,20]. This rapid root response allows increased water uptake to maintain cellular turgor and reduce the negative effects in the leaves; however, when the stress becomes more severe, the growth in all ...

Abscisic acid (ABA) is fundamental for plant development. Multiple factors have been identified that participate in the ABA signaling network, although a role of many proteins still await to be demonstrated. Here we have investigated the role of GEM (GL2 EXPRESSION MODULATOR), originally annotated as an ABA-responsive protein. GEM contains a GRAM domain, a feature shared with other eight Arabidopsis proteins for which we propose the name of GRE (GEM-RELATED) proteins. We found that (i) GEM expression responds to ABA, (ii) its promoter contains ABRE sites required for ABA response, and (iii) GEM expression depends on members of the ABA signaling pathway. This is consistent with the expression pattern of GEM during development in plant locations were ABA is known to play a direct role. We also found that GEM binds various phospholipids, e.g. mono and diphosphates and phosphatidic acid, suggesting a potential link of GEM with membrane-associated processes. Consistent with this, we found that the phosphoinositol-4-phosphate kinase PIP5K9 binds GEM in vivo. Finally, we demonstrated a role of GEM in seed dormancy. Together, our data led us to propose that GEM is an ABA-responsive protein that may function downstream of ABI5 as part of the ABA signaling pathway.

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