Phytochrome coordinates Arabidopsis shoot and root development

The phytochrome family of photoreceptors are potent regulators of plant development, affecting a broad range of responses throughout the plant life cycle, including hypocotyl elongation, leaf expansion and apical dominance. The plant hormone auxin has previously been linked to these phytochrome-mediated responses; however, these studies have not identified the molecular mechanisms that underpin such extensive phytochrome and auxin cross-talk. In this paper, we show that phytochrome regulates the emergence of lateral roots, at least partly by manipulating auxin distribution within the seedling. Thus, shoot-localized phytochrome is able to act over long distances, through manipulation of auxin, to regulate root development. This work reveals an important role for phytochrome as a coordinator of shoot and root development, and provides insights into how phytochrome is able to exert such a powerful effect on growth and development. This new link between phytochrome and auxin may go some way to explain the extensive overlap in responses mediated by these two developmental regulators.     

sas1, an Arabidopsis mutant overaccumulating sodium in the shoot, shows deficiency in the control of the root radial transport of sodium

A recessive mutation of Arabidopsis designated sas1 (for sodium overaccumulation in shoot) that was mapped to the bottom of chromosome III resulted in a two- to sevenfold overaccumulation of Na(+) in shoots compared with wild-type plants. sas1 is a pleiotropic mutation that also caused severe growth reduction. The impact of NaCl stress on growth was similar for sas1 and wild-type plants; however, with regard to survival, sas1 plants displayed increased sensitivity to NaCl and LiCl treatments compared with wild-type plants. sas1 mutants overaccumulated Na(+) and its toxic structural analog Li(+), but not K(+), Mg(2)+, or Ca(2)+. Sodium accumulated preferentially over K(+) in a similar manner for sas1 and wild-type plants. Sodium overaccumulation occurred in all of the aerial organs of intact sas1 plants but not in roots. Sodium-treated leaf fragments or calli displayed similar Na(+) accumulation levels for sas1 and wild-type tissues. This suggested that the sas1 mutation impaired Na(+) long-distance transport from roots to shoots. The transpiration stream was similar in sas1 and wild-type plants, whereas the Na(+) concentration in the xylem sap of sas1 plants was 5.5-fold higher than that of wild-type plants. These results suggest that the sas1 mutation disrupts control of the radial transport of Na(+) from the soil solution to the xylem vessels.     

Salt sensitivity in chickpea is determined by sodium toxicity

Main conclusion

Salt sensitivity in chickpea is determined by Na⁺ toxicity, whereas relatively high leaf tissue concentrations of Cl^? were tolerated, and the osmotic component of 60-mM NaCl was not detrimental.Chickpea (Cicer arietinum L.) is sensitive to salinity. This study dissected the responses of chickpea to osmotic and ionic components (Na⁺ and/or Cl^?) of salt stress. Two genotypes with contrasting salt tolerances were exposed to osmotic treatments (?0.16 and ?0.29 MPa), Na⁺-salts, Cl^?-salts, or NaCl at 0, 30, or 60 mM for 42 days and growth, tissue ion concentrations and leaf gas-exchange were assessed. The osmotic treatments and Cl^?-salts did not affect growth, whereas Na⁺-salts and NaCl treatments equally impaired growth in either genotype. Shoot Na⁺ and Cl^? concentrations had markedly increased, whereas shoot K⁺ had declined in the NaCl treatments, but both genotypes had similar shoot concentrations of each of these individual ions after 14 and 28 days of treatments. Genesis836 achieved higher net photosynthetic rate (64–84 % of control) compared with Rupali (35–56 % of control) at equivalent leaf Na⁺ concentrations. We conclude that (1) salt sensitivity in chickpea is determined by Na⁺ toxicity, and (2) the two contrasting genotypes appear to differ in ‘tissue tolerance’ of high Na⁺. This study provides a basis for focus on Na⁺ tolerance traits for future varietal improvement programs for salinity tolerance in chickpea.

     

Abscisic acid alleviates iron deficiency by promoting root iron reutilization and transport from root to shoot in Arabidopsis

Abscisic acid (ABA) has been demonstrated to be involved in iron (Fe) homeostasis, but the underlying mechanism is largely unknown. Here, we found that Fe deficiency induced ABA accumulation rapidly (within 6 h) in the roots of Arabidopsis. Exogenous ABA at 0.5 μM decreased the amount of root apoplastic Fe bound to pectin and hemicellulose, and increased the shoot Fe content significantly, thus alleviating Fe deficiency‐induced chlorosis. Exogenous ABA promoted the secretion of phenolics to release apoplastic Fe and up‐regulated the expression of AtNRAMP3 to enhance reutilization of Fe stored in the vacuoles, leading to a higher level of soluble Fe and lower ferric–chelate reductase (FCR) activity in roots. Treatment with ABA also led to increased Fe concentrations in the xylem sap, partially because of the up‐regulation of AtFRD3, AtYSL2 and AtNAS1, genes related to long‐distance transport of Fe. Exogenous ABA could not alleviate the chlorosis of abi5 mutant resulting from the significantly low expression of AtYSL2 and low transport of Fe from root to shoot. Taken together, our data support the conclusion that ABA is involved in the reutilization and transport of Fe from root to shoot under Fe deficiency conditions in Arabidopsis.     

A molecular framework for the inhibition of Arabidopsis root growth in response to boron toxicity

Boron is an essential micronutrient for plants and is taken up in the form of boric acid (BA). Despite this, a high BA concentration is toxic for the plants, inhibiting root growth and is thus a significant problem in semi-arid areas in the world. In this work, we report the molecular basis for the inhibition of root growth caused by boron. We show that application of BA reduces the size of root meristems, correlating with the inhibition of root growth. The decrease in meristem size is caused by a reduction of cell division. Mitotic cell number significantly decreases and the expression level of key core cell cycle regulators is modulated. The modulation of the cell cycle does not appear to act through cytokinin and auxin signalling. A global expression analysis reveals that boron toxicity induces the expression of genes related with abscisic acid (ABA) signalling, ABA response and cell wall modifications, and represses genes that code for water transporters. These results suggest that boron toxicity produces a reduction of water and BA uptake, triggering a hydric stress response that produces root growth inhibition.     

Disentangling the intertwined genetic bases of root and shoot growth in Arabidopsis

Root growth and architecture are major components of plant nutrient and water use efficiencies and these traits are the matter of extensive genetic analysis in several crop species. Because root growth relies on exported assimilate from the shoot, and changes in assimilate supply are known to alter root architecture, we hypothesized (i) that the genetic bases of root growth could be intertwined with the genetic bases of shoot growth and (ii) that the link could be either positive, with alleles favouring shoot growth also favouring root growth, or negative, because of competition for assimilates. We tested these hypotheses using a quantitative genetics approach in the model species Arabidopsis thaliana and the Bay-0 × Shahdara recombinant inbred lines population. In accordance with our hypothesis, root and shoot growth traits were strongly correlated and most root growth quantitative trait loci (QTLs) colocalized with shoot growth QTLs with positive alleles originating from either the same or the opposite parent. In order to identify regions that could be responsible for root growth independently of the shoot, we generated new variables either based on root to shoot ratios, residuals of root to shoot correlations or coordinates of principal component analysis. These variables showed high heritability allowing genetic analysis. They essentially all yielded similar results pointing towards two regions involved in the root--shoot balance. Using Heterogeneous Inbred Families (a kind of near-isogenic lines), we validated part of the QTLs present in these two regions for different traits. Our study thus highlights the difficulty of disentangling intertwined genetic bases of root and shoot growth and shows that this difficulty can be overcome by using simple statistical tools.     

Cytokinin receptors are involved in alkamide regulation of root and shoot development in Arabidopsis

Alkamides and N-acilethanolamides are a class of lipid compounds related to animal endocannabinoids of wide distribution in plants. We investigated the structural features required for alkamides to regulate plant development by comparing the root responses of Arabidopsis (Arabidopsis thaliana) seedlings to a range of natural and synthetic compounds. The length of the acyl chain and the amide moiety were found to play a crucial role in their biological activity. From the different compounds tested, N-isobutyl decanamide, a small saturated alkamide, was found to be the most active in regulating primary root growth and lateral root formation. Proliferative-promoting activity of alkamide treatment was evidenced by formation of callus-like structures in primary roots, ectopic blades along petioles of rosette leaves, and disorganized tumorous tissue originating from the leaf lamina. Ectopic organ formation by N-isobutyl decanamide treatment was related to altered expression of the cell division marker CycB1:uidA and an enhanced expression of the cytokinin-inducible marker ARR5:uidA both in roots and in shoots. The involvement of cytokinins in mediating the observed activity of alkamides was tested using Arabidopsis mutants lacking one, two, or three of the putative cytokinin receptors CRE1, AHK2, and AHK3. The triple cytokinin receptor mutant was insensitive to N-isobutyl decanamide treatment, showing absence of callus-like structures in roots, the lack of lateral root proliferation, and absence of ectopic outgrowths in leaves under elevated levels of this alkamide. Taken together our results suggest that alkamides and N-acylethanolamides may belong to a class of endogenous signaling compounds that interact with a cytokinin-signaling pathway to control meristematic activity and differentiation processes during plant development.     

Nitrate‐dependent shoot sodium accumulation and osmotic functions of sodium in Arabidopsis under saline conditions

Improving crop plants to be productive in saline soils or under irrigation with saline water would be an important technological advance in overcoming the food and freshwater crises that threaten the world population. However, even if the transformation of a glycophyte into a plant that thrives under seawater irrigation was biologically feasible, current knowledge about Na⁺ effects would be insufficient to support this technical advance. Intriguingly, crucial details about Na⁺ uptake and its function in the plant have not yet been well established. We here propose that under saline conditions two nitrate‐dependent transport systems in series that take up and load Na⁺ into the xylem constitute the major pathway for the accumulation of Na⁺ in Arabidopsis shoots; this pathway can also function with chloride at high concentrations. In nrt1.1 nitrate transport mutants, plant Na⁺ accumulation was partially defective, which suggests that NRT1.1 either partially mediates or modulates the nitrate‐dependent Na⁺ transport. Arabidopsis plants exposed to an osmotic potential of ?1.0 MPa (400 mOsm) for 24 h showed high water loss and wilting in sorbitol or Na/MES, where Na⁺ could not be accumulated. In contrast, in NaCl the plants that accumulated Na⁺ lost a low amount of water, and only suffered transitory wilting. We discuss that in Arabidopsis plants exposed to high NaCl concentrations, root Na⁺ uptake and tissue accumulation fulfil the primary function of osmotic adjustment, even if these processes lead to long‐term toxicity.     

Stevioside biosynthesis by callus,root, shoot and rooted-shoot cultures in vitro

Leaf explants of Stevia rebaudiana Bertoni (Compositae), an herb which produces the sweet ent-kaurene glycoside stevioside, were cultured in Murashige and Skoog medium with vitamins, sucrose (30 g l^–1), agar (0.9% w/v) and supplemented with naphthaleneacetic acid (NAA, 0.5 mg l^–1) and benzylaminopurine (BAP, 0.5 mg l^–1). These conditions yielded friable callus cultures. Differentiation of the callus tissue was then achieved by eliminating the agar and modulating the medium's hormone concentrations. Thus, medium containing increased auxin concentration (1.0 mg l^–1) and no cytokinin or increased cytokinin (1.0 mg l^–1) and no auxin yielded root or shoot cultures respectively. Supplementation of the shoot medium with NAA (1.0 mg ml^–1) induced shoot cultures to grow roots thereby differentiating into rooted-shoot cultures. Only the rooted-shoot cultures tasted sweet. Feedings of [2-¹⁴C]acetic acid to callus, shoot or rooted-shoot cultures demonstrated that only the rooted-shoot cultures are capable of de novo biosynthesis of the aglycone moiety of stevioside (steviol). In addition, [methyl-³H(N)steviol feedings to shoot or rooted-shoot cultures illustrated that both types of cultures are capable of the glycosylation reaction. The ability of these tissues to glycosylate steviol to stevioside was also demonstrated employing crude enzyme preparations derived from shoot or rooted-shoot cultures. These results suggest that stevioside biosynthesis is a function of tissue differentiation since both roots and leaves are required for cultured S. rebaudiana to biosynthesize stevioside from acetate, while the final biosynthetic steps can be performed at all levels of differentiation.     

Two ways to skin a plant: The analysis of root and shoot epidermal development in Arabidopsis

The post-embryonic architecture of higher plants is derived from the activity of two meristems that are formed in the embryo: the shoot meristem and the root meristem. The epidermis of the shoot is derived from the outermost layer of cells covering the shoot meristem through repeated anticlinal divisions. By contrast, the epidermis of the root is derived from an internal ring of cells, located at the centre of the root meristem, by a precise series of both periclinal and anticlinal divisions. Each epidermis has an independent origin. In Arabidopsis the mature shoot epidermis is composed of a small number of cell types: hair cells (trichomes), stomatal guard cells and other epidermal cells. In shoots, hairs take the form of branched trichomes that are surrounded at their base by a ring of accessory cells in a sheet of epidermal cells. The root epidermis is composed of two cell types: trichoblasts that form root hair cells and atrichoblasts that form non-hair cells. Mutations affecting both the patterning and the morphogenesis of cells in both shoot and root epidermis have recently been described. Most of these mutations affect development in a single epidermis, but at least one, ttg, is involved in development in both epidermal systems.     

Aberrant temporal growth pattern and morphology of root and shoot caused by a defective circadian clock in Arabidopsis thaliana

Circadian clocks synchronized with the environment allow plants to anticipate recurring daily changes and give a fitness advantage. Here, we mapped the dynamic growth phenotype of leaves and roots in two lines of Arabidopsis thaliana with a disrupted circadian clock: the CCA1 over‐expressing line (CCA1ox) and the prr9 prr7 prr5 (prr975) mutant. We demonstrate leaf growth defects due to a disrupted circadian clock over a 24 h time scale. Both lines showed enhanced leaf growth compared with the wild‐type during the diurnal period, suggesting increased partitioning of photosynthates for leaf growth. Nocturnal leaf growth was reduced and growth inhibition occurred by dawn, which may be explained by ineffective starch degradation in the leaves of the mutants. However, this growth inhibition was not caused by starch exhaustion. Overall, these results are consistent with the notion that the defective clock affects carbon and energy allocation, thereby reducing growth capacity during the night. Furthermore, rosette morphology and size as well as root architecture were strikingly altered by the defective clock control. Separate analysis of the primary root and lateral roots revealed strong suppression of lateral root formation in both CCA1ox and prr975, accompanied by unusual changes in lateral root growth direction under light–dark cycles and increased lateral extension of the root system. We conclude that growth of the whole plant is severely affected by improper clock regulation in A. thaliana, resulting not only in altered timing and capacity for growth but also aberrant development of shoot and root architecture.     

Gene silencing in Arabidopsis spreads from the root to the shoot, through a gating barrier, by template-dependent, nonvascular, cell-to-cell movement

Upward long-distance mobile silencing has been shown to be phloem mediated in several different solanaceous species. We show that the Arabidopsis (Arabidopsis thaliana) seedling grafting system and a counterpart inducible system generate upwardly spreading long-distance silencing that travels not in the phloem but by template-dependent reiterated short-distance cell-to-cell spread through the cells of the central stele. Examining the movement of the silencing front revealed a largely unrecognized zone of tissue, below the apical meristem, that is resistant to the silencing signal and that may provide a gating or protective barrier against small RNA signals. Using a range of auxin and actin transport inhibitors revealed that, in this zone, alteration of vesicular transport together with cytoskeleton dynamics prevented or retarded the spread of the silencing signal. This suggests that small RNAs are transported from cell to cell via plasmodesmata rather than diffusing from their source in the phloem.     

Brassinosteroid negatively regulates jasmonate inhibition of root growth in Arabidopsis

Jasmonate (JA) inhibits root growth of Arabidopsis thaliana seedlings. The mutation in COI1, that plays a central role in JA signaling, displays insensitivity to JA inhibition of root growth. To dissect JA signaling pathway, we recently isolated one mutant named psc1, which partially suppresses coi1 insensitivity to JA inhibition of root growth. As we identified the PSC1 gene as an allele of DWF4 that encodes a key enzyme in brassinosteroid (BR) biosynthesis, we hypothesized and demonstrated that BR is involved in JA signaling and negatively regulates JA inhibition of root growth. In our Plant Physiology paper, we analyzed effects of psc1 or exogenous BR on the inhibition of root growth by JA. Here we show that treatment with brassinazole (Brz), a BR biosynthesis inhibitor, increased JA sensitivity in both coi1-2 and wild type, which further confirms that BR negatively regulates JA inhibition of root growth. Since effects of psc1, Brz and exogenous BR on JA inhibition of root growth were mild, we suggests that BR negatively finely regulates JA inhibition of root growth in Arabidopsis.Key words: jasmonate signaling, root growth, brassinosteroid, brassinazole, arabidopsisJasmonate (JA) regulates many plant developmental processes and stress responses.^1,2 COI1 plays a central role in JA signaling and is required for all JA responses in Arabidopsis.^3,4 coi1-1, a strong mutation in COI1, is male sterile and exhibits loss of all JA responses tested to date, such as JA inhibition of root growth, the expression of JA-induced genes, and susceptibility to insect attack and pathogen infection, and coi1-2, a weak mutant of COI1, shows similar JA responses to coi1-1 except for partially fertile that makes it able to produce a small quantity of seeds.⁵To investigate COI1-mediated JA responses and dissect JA signaling pathway, we conducted genetic screens for suppressors of coi1-2. Previously, we identified cos1 that completely suppresses coil-2 insensitive to JA.⁶ Recently, we isolated the psc1 mutant that partially suppresses coi1-2 insensitivity to JA, and found that PSC1 is an allele of DWF4.⁷Since the DWF4 gene encodes a key enzyme in brassinosteroid (BR) biosynthesis,⁸ we hypothesized that BR is involved in JA signaling. By physiological analysis, we showed that psc1 partially restored JA inhibition of root growth in coi1-2 background and displayed JA hypersensitivity in wild-type COI1 background, the effects of psc1 were eliminated by exogenous BR, and that exogenous BR could attenuated JA inhibition of root growth in wild type. These findings demonstrated that BR is involved in JA signaling and indicated that BR negatively regulates JA inhibition of root growth.BR is a family of polyhydroxylated steroid hormones involved in many aspects of plant growth and development. The BR-deficient mutants exhibited severely retarded growth that was able to be rescued by exogenous BR.⁹ Brassinazole (Brz) is a BR biosynthesis inhibitor. The Arabidopsis seedlings treated with Brz displayed a BR deficient-mutant-like phenotype, which could be elimilated by exogenous BR.¹⁰To determine wether treatment with Brz affects JA inhibition of root growth, the seedlings of wild type and coi1-2 were grown in MS medium supplemented with MeJA and/or Brz. As shown in Figure 1, the relative root length was obviously reduced in both coi1-2 and wild type when treated with Brz relative to without Brz, indicating that the repression of BR biosynthesis by Brz could increase JA sensitivity. These results further confirm BR negatively regulates JA inhibition of root growth.Open in a separate windowFigure 1Effect of Brz on JA inhibition of root growth. Brz increased JA inhibition of root growth in both coi1-2 and wild type (WT). Root length of 7-day-old seedlings grown in MS medium containing 0, 5 and 10 μM MeJA without (−) or with (+) 0.5 μM Brz was expressed as a percentage of root length in MS without (−) or with (+) 0.5 µM Brz. Error bars represent SE (n > 30).It has been demonstrated that JA connects with other plant hormones including auxin, ethylene, abscisic acid, salicylic acid and gibberellin to form complex regulatory networks modulating plant developmental and stress responses.^11–15 We found that BR negatively regulates JA inhibition of root growth, suggesting that a cross talk between JA and BR exists in planta, which extends our understandings on the JA signal transduction.COI1 is a JA receptor¹⁶ and DWF4 catalyzes the rate-limiting step in BR-biosynthesis pathway.⁸ We found that JA inhibits DWF4 expression, this inhibition was dependent on COI1,⁷ indicating that DWF4 is downregulated by JA and is located downstream of COI1 in the JA signaling pathway.Since the effects of psc1, Brz, and exogenous BR on JA inhibition of root growth were mild, and the DWF4 expression was partially repressed by JA (Ren et al. 2009, Fig. 1), we suggest that BR negatively finely regulates JA inhibition of root growth, and propose a model for these regulations. As shown in Figure 2A, JA signal passes COI1 repressing substrates, such as JAZs,^17,18 i.e., JA activates degradation of substrates via SCF^COI1-26S proteasome,^16–18 whereas substrates positively regulate root growth through other regulators. JA also partially inhibits DWF4 expression through COI1, reducing BR that is required for root growth.^7,9 Mutation in COI1 interrupts JA signaling for failing in degradation of substrates and repression of DWF4 as well, resulting in JA-insensitivity (Fig. 2B). However, mutation in DWF4 or treatment with Brz causes a reduction in BR, which affects root growth, leading to JA-hypersensitivity in wild-type COI1 background (Fig. 2C and E) and partial restoration of JA sensitivity in coi1-2 background (Fig. 2D and F). Whereas, an application of exogenous BR could eliminate the effect of BR reduction resulted from repression of DWF4 by JA on root growth, attenuating JA sensitivity in wild type (Fig. 2G). Because the inhibition of DWF4 expression by JA is dependent on COI1, the coi1 mutant treated with exogenous BR do not show alteration in JA sensitivity (Fig. 2H).Open in a separate windowFigure 2A model for that BR negatively finely regulates JA inhibition of root growth in Arabidopsis. (A–D) Treatment with JA in wild type (A), coi1-2 (B), psc1 (C) and psc1coi1 (D). (E and F) Treatments with JA and Brz in wild type (E) and coi1-2 (F). (G and H) Treatments with JA and exogenous BR in wild type (G) and coi1-2 (H). Arrows indicate positive regulation or enhancement, whereas blunted lines indicate repression or negative regulation. Crosses indicate interruption or impairment. The letter “S” indicates substrates of SCF^COI1. Thicker arrows and blunted lines represent the central JA signaling pathway regulating JA inhibition of root growth. Broken arrows represent JA signaling pathway in which other regulators are involved. The intensity of gray boxes represents the degree of JA inhibition on root growth.