Interaction between HY1 and H2O2 in auxin-induced lateral root formation in Arabidopsis

Haem oxygenase-1 (HO-1) and hydrogen peroxide (H₂O₂) are two key downstream signals of auxin, a well-known phytohormone regulating plant growth and development. However, the inter-relationship between HO-1 and H₂O₂ in auxin-mediated lateral root (LR) formation is poorly understood. Herein, we revealed that exogenous auxin, 1-naphthylacetic acid (NAA), could simultaneously stimulate Arabidopsis HO-1 (HY1) gene expression and H₂O₂ generation. Subsequently, LR formation was induced. NAA-induced HY1 expression is dependent on H₂O₂. This conclusion was supported by analyzing the removal of H₂O₂ with ascorbic acid (AsA) and dimethylthiourea (DMTU), both of which could block NAA-induced HY1 expression and LR formation. H₂O₂-induced LR formation was inhibited by an HO-1 inhibitor zinc protoporphyrin IX (Znpp) in wild-type and severely impaired in HY1 mutant hy1-100. Simultaneously, HY1 is required for NAA-mediated H₂O₂ generation, since Znpp inhibition of HY1 blocked the NAA-induced H₂O₂ production and LR formation. Genetic data demonstrated that hy1-100 was significantly impaired in H₂O₂ production and LR formation in response to NAA, compared with wild-type plants. The addition of carbon monoxide-releasing molecule-2 (CORM-2), the carbon monoxide (CO) donor, induced H₂O₂ production and LR formation, both of which were decreased by DMTU. Moreover, H₂O₂ and CORM-2 mimicked the NAA responses in the regulation of cell cycle genes expression, all of which were blocked by Znpp or DMTU, respectively, confirming that both H₂O₂ and CO were important in the early LR initiation. In summary, our pharmacological, genetic and molecular evidence demonstrated a close inter-relationship between HY1 and H₂O₂ existing in auxin-induced LR formation in Arabidopsis.     

Heme oxygenase is involved in H2O2-induced lateral root formation in apocynin-treated rice

Key message

Apocynin is a natural organic compound structurally related to vanillin. We demonstrated that hydrogen peroxide and heme oxygenase participated in apocynin-induced lateral root formation in rice.

Abstract

Apocynin, also known as acetovanillone, is a natural organic compound structurally related to vanillin. Information concerning the effect of apocynin on plants is limited. In this study, we examined the effect of apocynin on lateral root (LR) formation in rice. Treatment with apocynin induced LR formation and increased H₂O₂ production, but had no effect on nitric oxide production. Diphenyleneiodonium chloride, an inhibitor of H₂O₂ generating NADPH oxidase, was effective in reducing apocynin-induced H₂O₂ production and LR formation. Apocynin treatment also increased superoxide dismutase activity and decreased catalase activity. H₂O₂ application was able to increase the number of LRs. Moreover, H₂O₂ production caused by H₂O₂ and apocynin was localized in the root area corresponding to the LR emergence. Treatment with H₂O₂ and apocynin also increased heme oxygenase (HO) activity and induced OsHO1 mRNA expression. Lateral root formation and HO activity induced by H₂O₂ and apocynin were reduced by Zn protoporphyrin IX (the specific inhibitor of HO). Our data suggest that both H₂O₂ and HO are required for apocynin-induced LR formation in rice.     

Arbuscular mycorrhizas and their role in plant growth, nitrogen interception and soil gas efflux in an organic production system

Background and aims

Roots and mycorrhizas play an important role in not only plant nutrient acquisition, but also ecosystem nutrient cycling.

Methods

A field experiment was undertaken in which the role of arbuscular mycorrhizas (AM) in the growth and nutrient acquisition of tomato plants was investigated. A mycorrhiza defective mutant of tomato (Solanum lycopersicum L.) (named rmc) and its mycorrhizal wild type progenitor (named 76R) were used to control for the formation of AM. The role of roots and AM in soil N cycling was studied by injecting a ¹⁵N-labelled nitrate solution into surface soil at different distances from the 76R and rmc genotypes of tomato, or in plant free soil. The impacts of mycorrhizal and non-mycorrhizal root systems on soil greenhouse gas (CO₂ and ¹⁴⁺¹⁵N₂O and ¹⁵N₂O) emissions, relative to root free soils, were also studied.

Results

The formation of AM significantly enhanced plant growth and nutrient acquisition, including interception of recently applied NO ₃ ^? . Whereas roots caused a small but significant decrease in ¹⁵N₂O emissions from soils at 23?h after labeling, compared to the root-free treatment, arbuscular mycorrhizal fungi (AMF) had little effect on N₂O emissions. In contrast soil CO₂ emissions were higher in plots containing mycorrhizal root systems, where root biomass was also greater.

Conclusions

Taken together, these data indicate that roots and AMF have an important role to play in plant nutrient acquisition and ecosystem N cycling.     

Aluminum induces low phosphate adaptive responses and modulates primary and lateral root growth by differentially affecting auxin signaling in Arabidopsis seedlings

Aims

The aims of this work were to investigate the aluminum (Al) and phosphate (P) interactions in the regulation of root system architecture of Arabidopsis thaliana seedlings and the contribution of auxin signaling in primary and lateral root growth in response to Al toxicity.

Methods

Detailed analyses of root system architecture and cell division were performed in Arabidopsis WT seedlings and in low phosphorus insensitive mutants lpi1-3 and lpr1-1 lpr2-1 in response to Al. Expression studies of P-deficiency regulated phosphate transporter AtPT2 were also conducted. The role of auxin as a mediator of root morphogenetic changes by Al was evaluated by using the auxin-signaling mutants tir1, tir1 afb2 afb3, and arf7 arf19.

Results

Al inhibited primary root growth by affecting cell cycle progression and causing differentiation of cells in the root meristem. These effects were reduced in low phosphorus insensitive lpi1-3 and low phosphate resistant lpr1-1 lpr2-1 Arabidopsis mutants. Al also activated the expression of the low phosphate-induced P transporter AtPT2 in roots. Lateral root formation by Al decreased in tir1 afb2 afb3 while arf7 arf19 mutants were highly resistant to Al in both primary root inhibition and lateral root induction.

Conclusions

Our results suggest that lateral root formation in response to Al toxicity and P deficiency may involve common signaling mechanisms, while a pathway involving ARF7 and ARF19 is important for primary root growth inhibition by Al.     

Hydrogen sulfide alleviates aluminum toxicity in barley seedlings

Aims

Aluminum (Al) toxicity is one of the major factors that limit plant growth. Low concentration of hydrogen sulfide (H₂S) has been proven to function in physiological responses to various stresses. The objective of this study is to investigate the possible role of H₂S in Al toxicity in barley (Hordeum vulgare L) seedlings.

Methods

Barley seedlings pre-treated with sodium hydrosulfide (NaHS), a H₂S donor, and subsequently exposed to Al treatment were studied for their effects on root elongation, Al accumulation in seedlings, Al-induced citrate secretion and oxidative stress, and plasma membrane (PM) H⁺-ATPase expression.

Results

Our results showed that H₂S had significant rescue effects on Al-induced inhibition of root elongation which was correlated well with the decrease of Al accumulation in seedlings. Meanwhile, Al-induced citrate secretion was also significantly enhanced by NaHS pretreatment. Al-induced oxidative stress as indicated by lipid peroxidation and reactive oxygen species burst was alleviated by H₂S through the activation of the antioxidant system. Moreover, Al-induced reduction in PM H⁺-ATPase expression was reversed by exogenous NaHS.

Conclusions

Altogether, our results suggest H₂S plays an ameliorative role in protecting plants against Al toxicity by inducing the activities of antioxidant enzymes, increasing citrate secretion and citrate transporter gene expression, and enhancing the expression of PM H⁺-ATPase.     

Influence of carbohydrate source on xanthone content in root cultures of Gentiana dinarica Beck

The effects of different types and concentrations of sugars on root growth and xanthone production in root culture of Gentiana dinarica were investigated. The results showed that sucrose, glucose and fructose all supported root growth, and sucrose was superior in terms of growth index, dry mass and fresh/dry mass ratio then fructose or glucose at the same concentrations. However, considering equimolar concentration of sugars, their contribution to the root growth was similar. The HPLC analysis of roots indicated the presence of xanthone compounds, and the contents of norswertianin-1-O-primeveroside (1), norswertianin-1-O-glucoside (2), gentioside (3) and norswertianin (4) were evaluated. In all samples, norswertianin-1-O-primeveroside (1) was present in highest concentration, followed by norswertianin-1-O-glucoside (2), whereas gentioside (3) and norswertianin (4) were present in lower amounts. The production of xanthones was affected by both type and concentration of sugar. In general, roots growing in media supplemented with sucrose contained higher levels of xanthones. The amounts of xanthone primeveroses (1) and (3) increased with the increase of concentrations of all types of sugars, whereas higher sugar concentrations resulted in reduction of the contents of norswertianin-1-O-glucoside (2) and aglycone norswertianin (4). The roots were also evaluated regarding the content of total phenolics and higher accumulation of total phenolic compounds was observed in roots grown in fructose-containing medium. Antioxidant activity was determined by 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging assay, and high correlation between total phenolic content and antiradical activity was observed (r = ?0.83).     

Hydrogen-rich water regulates cucumber adventitious root development in a heme oxygenase-1/carbon monoxide-dependent manner

Hydrogen gas (H₂) is an endogenous gaseous molecule in plants. Although its reputation is as a “biologically inert gas”, recent results suggested that H₂ has therapeutic antioxidant properties in animals and plays fundamental roles in plant responses to environmental stresses. However, whether H₂ regulates root morphological patterns is largely unknown. In this report, hydrogen-rich water (HRW) was used to characterize H₂ physiological roles and possible signaling transduction pathways in the promotion of adventitious root (AR) formation in cucumber explants. Our results showed that a 50% concentration of HRW was able to mimic the effect of hemin, an inducer of a carbon monoxide (CO) synthetic enzyme, and heme oxygenase-1 (HO-1), in restoring AR formation in comparison with the inhibition effect conferred by auxin-depletion treatment alone. It was further shown that the inducible effect of HRW could be further blocked by the co-treatment with N-1-naphthylphtalamic acid (NPA; an auxin transport inhibitor). The HRW-induced response, at least partially, was HO-1-dependent. This conclusion was supported by the fact that the exposure of cucumber explants to HRW up-regulates cucumber HO-1 gene expression and its protein levels. HRW-mediated induction of representative target genes related to auxin signaling and AR formation, such as CsDNAJ-1, CsCDPK1/5, CsCDC6, CsAUX22B-like, and CsAUX22D-like, and thereafter AR formation (particularly in the AR length) was differentially sensitive to the HO-1 inhibitor zinc protoporphyrin IX (ZnPP). Above blocking actions were clearly reversed by CO, further confirming that the above response was HO-1/CO-specific. However, the addition of a well-known antioxidant, ascorbic acid (AsA), failed to influence AR formation triggered by HRW, thus ruling out the involvement of redox homeostasis in this process. Together, these results indicated that HRW-induced adventitious rooting is, at least partially, correlated with the HO-1/CO-mediated responses. We also suggested that exogenous HRW treatment on plants might be a good option to induce root organogenesis.     

Density functional studies on photophysical properties and chemical reactivities of the triarylboranes: effect of the constraint of planarity

The geometric and electronic structures, absorption spectra, transporting properties, chemical reactivity indices and electrostatic potentials of the planar three-coordinate organoboron compounds 1-2 and twisted reference compound Mes ₃ B, have been investigated by employing density functional theory (DFT) and conceptual DFT methods to shed light on the planarity effects on the photophysical properties and the chemical reactivity. The results show that the planar compounds 1-2 exhibit significantly lower HOMO level than Mes ₃ B, owing to the stronger electronic induction effect of boron centers. This feature conspicuously induces a blue shifted absorption for 1, although 1 seemingly possesses more extended conjugation framework than Mes ₃ B. Importantly, the reactivity strength of the boron atoms in 1-2 is much lower than that in Mes ₃ B, despite the fact that the tri-coordinate boron centers of 1-2 are completely naked. The interesting and abnormal phenomenon is caused by the strong p-π electronic interactions, that is, the empty p-orbital of boron center is partly filled by π-electron of the neighbor carbon atoms in 1-2, which are confirmed by the analysis of Laplacian of the electron density and natural bond orbitals. Furthermore, the negative electrostatic potentials of the boron centers in 1-2 also interpret that they are not the most preferred sites for incoming nucleophiles. Moreover, it is also found that the planar compounds 1-2 can act as promising electron transporting materials since the internal reorganization energies for electron are really small.

Interaction between NO 3 − and abscisic acid in the regulation of lateral root elongation in Zea mays L.

Lateral root (LR) elongation rate of 7–8-day maize seedlings depends on the availability of NO ₃ ^? , NO ₂ ^? , and abscisic acid (ABA) in an environment. Four-hour exposure to 0.01–1.5 mM NO ₂ ^? increases the relative LR elongation rate; in the case of NO ₂ ^? , the stimulation occurs only at an NO ₂ ^? concentration equal to 0.01 mM. Exogenous ABA (10^?6 M) inhibits the LR elongation process. In the case of a combined influence of NO ₃ ^? and ABA or NO ₂ ^? and ABA, the character of the response elongation reaction is different. The NO role in the regulation of LR elongation is discussed.     

Nitric oxide contributes to copper tolerance by influencing ROS metabolism in Arabidopsis

Key message

Nitric oxide improves copper tolerance via modulation of superoxide and hydrogen peroxide levels. This reflects the necessity of a well-coordinated interplay between NO and ROS during stress tolerance.

Abstract

Copper (Cu) excess causes toxicity and one probable consequence of this is the disturbance of cell redox state maintenance, inter alia, by reactive oxygen- (ROS) and nitrogen species (RNS). The objective of this paper was to examine the role of nitric oxide (NO) in Cu stress tolerance and its relationship with ROS in Arabidopsis. In agar-grown seedlings, concentration-dependent Cu accumulation was observed. The 5 μM Cu resulted in reduced cell viability in the NO overproducing nox1 and gsnor1-3 root tips compared to the wild-type (WT). In contrast, 25 and 50 μM Cu caused higher viability in these mutants, while in the NO-lacking nia1nia2 lower viability was detected than in the WT. The exogenous NO donor enhanced cell viability and scavenging endogenous NO decreased it in Cu-exposed WT seedlings. Besides, SNP in nia1nia2 roots led to the improvement of viability. The ascorbic acid-deficient mutants (vtc2-1, vtc2-3) possessing slightly elevated ROS levels proved to be Cu sensitive, while miox4 showing decreased ROS production was more tolerant to Cu than the WT. In nox1 and gsnor1-3, Cu did not induce superoxide formation, and H₂O₂ accumulation occurred only in the case of NO deficiency. Based on these, under mild stress NO intensifies cell injury, while in the case of severe Cu excess it contributes to better viability. ROS were found to be responsible for aggravation of Cu-induced damage. NO alleviates acute Cu stress via modulation of O ₂ ^·? and H₂O₂ levels reflecting the necessity of a well-coordinated interplay between NO and ROS during stress tolerance.