Effect of auxin on mesocotyl elongation of dark-grown maize under different seeding depths

In order to elucidate the physiological mechanism of maize mesocotyl elongation induced by auxin under different seeding depths, seeds of five maize inbred lines, including 3681-4 line tolerant to deep seeding, were treated with IAA and triiodobenzoic acid (TIBA) under seeding depths of 20 or 2 cm. Under deep seeding conditions, maize mesocotyls grew by 1.5–2.0 times faster than under shallow seeding conditions. IAA (10⁻⁶ to 10⁻⁴ M) applied to roots stimulated mesocotyl elongation only of 3681-4 line and only under deep seeding conditions. TIBA (10⁻⁵ and 10⁻⁴ M) applied to roots inhibited mesocotyl elongation in all lines, but only 3681-4 was sensitive to 10⁻⁶ M TIBA. IAA promoted only cell elongation, and TIBA inhibited both cell elongation and cell division. After IAA and TIBA treatments, endogenous IAA content changed in parallel with the mesocotyl growth rate under different seeding depths. Furthermore, ABP1 gene expression changed in parallel with the mesocotyl growth rate under deep seeding conditions. Therefore, deep seeding tolerance of 3681-4 line was achieved due to auxin-regulated rapid mesocotyl elongation.     

A role for polar auxin transport in rhizogenesis

Internodal shoot sections of the easy-to-root Forsythia×intermedia cv. Lynwood, and the difficult-to-root Syringa vulgaris cv. Madame Lemoine were used in vitro to investigate the role of polar auxin transport (PAT) in rhizogenesis. Syringa internodes required the distal application of indole-3-acetic acid (IAA), indole-3-butyric acid (IBA) or naphthaleneacetic acid to induce rooting, while 2,4-dichlorophenoxyacetic acid was ineffective. In contrast, Forsythia internodes rooted equally well when IBA was applied at either end of the internode. Using [³H]IAA showed transport of exogenous auxin was basipetal, and that despite similar transport velocities, the intensity of auxin transport in Syringa was greater than in Forsythia. Basipetal transport of exogenous auxin was blocked using the PAT inhibitors 2,3,5-triiodobenzoic acid (TIBA) and naringenin (Nar); where Forsythia proved more sensitive to TIBA, but less so to Nar, in comparison with Syringa. In both species, percentage rooting and the number of roots formed were greater in 5-mm-long internodes than in shorter internodes. The results demonstrate the importance of PAT for root initiation in Syringa, whereas Forsythia tissue appears to be more sensitive to the direct application of auxin.     

Gibberellins and auxin regulate soybean hypocotyl elongation under low light and high-temperature interaction

Soybean is an important oilseed crop grown globally. However, two examples of environmental stresses that drastically regulate soybean growth are low light and high-temperature. Emerging evidence suggests a possible interconnection between these two environmental stimuli. Low light and high-temperature as individual factors have been reported to regulate plant hypocotyl elongation. However, their interactive signal effect on soybean growth and development remains largely unclear. Here, we report that gibberellins (GAs) and auxin are required for soybean hypocotyl elongation under low light and high-temperature interaction. Our analysis indicated that low light and high-temperature interaction enhanced the regulation of soybean hypocotyl elongation and that the endogenous GA₃, GA₇, indole-3-acetic acid (IAA), and indole-3-pyruvate (IPA) contents significantly increased. Again, analysis of the effect of exogenous phytohormones and biosynthesis inhibitors treatments showed that exogenous GA, IAA, and paclobutrazol (PAC), 2, 3, 5,-triiodobenzoic acid (TIBA) treatments significantly regulated soybean seedlings growth under low light and high-temperature interaction. Further qRT-PCR analysis showed that the expression level of GA biosynthesis pathway genes (GmGA3ox1, GmGA3ox2 and GmGA3) and auxin biosynthesis pathway genes (GmYUCCA3, GmYUCCA5 and GmYUCCA7) significantly increased under (i) low light and high-temperature interaction and (ii) exogenous GA and IAA treatments. Altogether, these observations support the hypothesis that gibberellins and auxin regulate soybean hypocotyl elongation under low light and high-temperature stress interaction.     

Hydrogen Peroxide-Mediated Growth of the Root System Occurs via Auxin Signaling Modification and Variations in the Expression of Cell-Cycle Genes in Rice Seedlings Exposed to Cadmium Stress

The link between root growth, H₂O₂, auxin signaling, and the cell cycle in cadmium (Cd)‐stressed rice (Oryza sativa L. cv. Zhonghua No. 11) was analyzed in this study. Exposure to Cd induced a significant accumulation of Cd, but caused a decrease in zinc (Zn) content which resulted from the decreased expression of OsHMA9 and OsZIP. Analysis using a Cd‐specific probe showed that Cd was mainly localized in the meristematic zone and vascular tissues. Formation and elongation of the root system were significantly promoted by 3‐amino‐1,2,4‐triazole (AT), but were markedly inhibited by N,N’‐dimethylthiourea (DMTU) under Cd stress. The effect of H₂O₂ on Cd‐stressed root growth was further confirmed by examining a gain‐of‐function rice mutant (carrying catalase1 and glutathione‐S‐transferase) in the presence or absence of diphenylene iodonium. DR5‐GUS staining revealed close associations between H₂O₂ and the concentration and distribution of auxin. H₂O₂ affected the expression of key genes, including OsYUCCA, OsPIN, OsARF, and OsIAA, in the auxin signaling pathway in Cd‐treated plants. These results suggest that H₂O₂ functions upstream of the auxin signaling pathway. Furthermore, H₂O₂ modified the expression of cell‐cycle genes in Cd‐treated roots. The effects of H₂O₂ on root system growth are therefore linked to auxin signal modification and to variations in the expression of cell‐cycle genes in Cd‐stressed rice. A working model for the effects of H₂O₂ on Cd‐stressed root system growth is thus proposed and discussed in this paper.     

Exogenous application of salicylic acid alleviates cadmium toxicity and reduces hydrogen peroxide accumulation in root apoplasts of Phaseolus aureus and Vicia sativa

We examined ameliorative effects of salicylic acid (SA) on two cadmium (Cd)-stressed legume crops with different Cd tolerances, viz. Phaseolus aureus (Cd sensitive) and Vicia sativa (Cd tolerant). Cd at 50 μM significantly increased the production of hydrogen peroxide (H₂O₂) and superoxide anion (O₂^·−) in root apoplasts of P. aureus and V. sativa. When comparing the two species, we determined that Cd-induced production of H₂O₂ and O₂^·− was more pronounced in P. aureus root apoplasts than in V. sativa root apoplasts. V. sativa had higher activities of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) than P. aureus in root symplasts and apoplasts. Seed-soaking pretreatment with 100 μM SA decreased Cd-induced production of H₂O₂ and O₂^·− in apoplasts of both species, and increased activities of symplastic and apoplastic SOD, symplastic APX, and apoplastic CAT under Cd stress. Hence, SA-induced Cd tolerances in P. aureus and V. sativa are likely associated with increases in symplastic and apoplastic antioxidant enzyme activities.     

Targeting of auxin carriers to the plasma membrane: differential effects of brefeldin A on the traffic of auxin uptake and efflux carriers

Monensin and brefeldin A (BFA), inhibitors of Golgi-mediated protein secretion, rapidly perturb the transport catalytic activity of specific plasma membrane-associated efflux carriers for indole-3-acetic acid (IAA) and inhibit polar transport of IAA. To determine if these responses result solely from perturbation of the efflux carrier or whether specific auxin uptake carrier function is also affected, the influence of BFA on the cellular transport of a range of auxins with contrasting affinities for specific auxin uptake and efflux carriers was investigated in zucchini (Cucurbita pepo L.) hypocotyl tissue. In-flight addition of BFA (3 · 10⁻⁵ mol · dm⁻³) caused a rapid (lag < 10 min) and substantial (fourfold) increase in the rate of [1-¹⁴C]IAA net uptake by zucchini hypocotyl tissue. In the presence of the specific auxin efflux carrier inhibitor N-1-naphthylphthalamic acid (NPA; 3 · 10⁻⁶ mol · dm⁻³), BFA slightly reduced the rate of [1-¹⁴C]IAA net uptake. Stimulation of [1-¹⁴C]IAA net uptake by BFA was concentration-dependent. In the absence of BFA, net uptake of [1-¹⁴C]IAA exhibited the characteristic biphasic response to increasing concentrations of competing cold IAA but in the presence of BFA, [1-¹⁴C]IAA uptake decreased smoothly with increase in concentration of competing unlabelled IAA, indicating a loss of auxin efflux carrier activity but retention of functional uptake carriers. The half-time for mediated efflux of [1-¹⁴C]IAA from preloaded zucchini tissue was substantially increased by BFA (t_1/2 = 51 min, controls; 107 min, BFA-treated). Treatment with BFA and/or NPA did not significantly affect the net uptake by, or efflux from, zucchini tissue of [1-¹⁴C]2,4-dichlorophenoxyacetic acid ([1-¹⁴C]2,4-D), a substrate for the auxin uptake carrier but not the auxin efflux carrier. Uptake of [1-¹⁴C]2,4-D declined smoothly with increasing concentrations of competing unlabelled IAA whether or not BFA was included in the uptake medium, confirming the failure of BFA to perturb auxin uptake carrier function. Transport of 1-[4-³H]naphthaleneacetic acid (1-NAA) exhibited little response to BFA or NPA, confirming that it is only a weakly transported substrate for the efflux carrier in zucchini cells. Received: 12 November 1997 / Accepted: 27 January 1998     

Tiba inhibition of <Emphasis Type="Italic">in vitro</Emphasis> organogenesis in excised tobacco leaf explants

Summary Triiodobenzoic acid (TIBA), an anti-auxin, was found to inhibit both shoot and root formation in cultured excised leaf explants of tobacco (Nicotiana tabacum L.). The shoot formation (SF) medium used required only exogenous cytokinin (N⁶-benzyladenine) and the root formation (RF) medium required both auxin (indole-3-butyric acid) and cytokinin (kinetin). By transferring the explants from SF or RF media to SF or RF media with TIBA (4.0×10⁻⁵ M), respectively or vice versa, at different times in culture, it was found that TIBA inhibition was at the time of meristemoid formation and after determination of organogenesis. This indicates that TIBA interfered with endogenous auxin involvement in organized cell division.     

MAPKs as a cross point in H2O2 and auxin signaling under combined cadmium and zinc stress in rice roots

Previously, we have reported the role of MAPKs (mitogen-activated protein kinases) under cadmium stress. This work continue to explore the relationship between MAPKs, H₂O₂, auxin signaling, and OsHMA and OsZIP gene expression in rice (Oryza sativa L.) roots under combined cadmium (Cd) and zinc (Zn) stress. Compared with Cd, Cd+Zn reduced Cd levels but increased Zn accumulation in the roots. Three OsMAPK genes were negatively regulated, while two OsHMA and two OsZIP genes were positively regulated by MAPK pathways under Cd+Zn stress. Transgenic rice expressing DR5-GUS exhibited enhanced GUS activity in H₂O₂-, PD (MAPKK inhibitor PD98059)-, or (Cd+Zn)-treated roots, which also exhibited increased H₂O₂ concentrations, whereas GUS staining decreased in roots in response to Cd+Zn+PD, DMTU (N,N′-dimethylthiourea, a H₂O₂ scavenger), or Cd+Zn+DMTU treatment, with reduced H₂O₂ levels. GUS levels were consistent with H₂O₂ levels, suggesting that MAPK pathway-mediated auxin redistribution occurs via H₂O₂, and H₂O₂ functions downstream of MAPK but upstream of auxin signaling pathways. Furthermore, MAPK pathways serve specific functions in regulating the expression of some key genes of auxin signaling (OsYUCCA, OsPIN, OsARF, and OsIAA) under Cd+Zn stress. Overall, MAPK cascades function in the integration of metal transport, H₂O₂ generation, and auxin signaling in rice seedlings grown under Cd+Zn stress.     

Function of nitric oxide and superoxide anion in the adventitious root development and antioxidant defence in <Emphasis Type="Italic">Panax ginseng</Emphasis>

The involvement of NO in O₂ ^·− generation, rootlet development and antioxidant defence were investigated in the adventitious root cultures of mountain ginseng. Treatments of NO producers (SNP, sodium nitroprusside; SNAP, S-nitroso-N-acetylpenicillamine; and sodium nitrite with ascorbic acid), and NO scavenger (PTIO, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl3-oxide) revealed that NO is involved in the induction of new rootlets. Severe decline in number of new rootlets compared to the control under PTIO treatment indicates that NO acts downstream of auxin action in the process. NO producers (SNP, SNAP and sodium nitrite with ascorbic acid) activated NADPH oxidase activity, resulting in greater O₂ ^·− generation and higher number of new rootlets in the adventitious root explants. Moreover, treatment of diphenyliodonium chloride, a NADPH oxidase inhibitor, individually or along with SNP, inhibited root growth, NADPH oxidase activity and O₂ ^·− anion generation. NO supply also enhanced the activities of antioxidant enzymes that are likely to be responsible for reducing H₂O₂ levels and lipid peroxidation as well as modulation of ascorbate and non-protein thiol concentrations in the adventitious roots. Our results suggest that NO-induced generation of O₂ ^·− by activating NADPH oxidase activity is related to adventitious root formation in mountain ginseng.     

Induction mechanism of adventitious root from leaf explants of <Emphasis Type="Italic">Morinda citrifolia</Emphasis> as affected by auxin and light quality

An efficient protocol for adventitious root induction from leaf explants of Morinda citrifolia treated with different concentrations of indole-3-butyric acid (IBA) and α-naphthaleneacetic acid (NAA) was established in relation to physiological process changes during adventitious root induction under different light sources (fluorescent, red, blue, red + blue, and far-red). Among the different concentrations of IBA and NAA, 1.0 mg l⁻¹ IBA was proven as the best auxin source for adventitious root induction under fluorescent light. Higher concentrations of IBA and NAA trigger callus formation in both light and dark conditions. Maximum numbers of adventitious roots were induced under red light (26) followed by blue light (22) and the lowest under far-red light (6). In contrast, numerous callus formations were induced by red + blue followed by red and blue, while the highest root length (1.66 cm) with negligible callusing was observed under fluorescent light. Catalase and guaicacol peroxidase activities were highest under red light followed by fluorescent light and the lowest under red + blue light, but superoxide dismutase activity was not significantly influenced by different light sources. Ascorbate peroxidase played an important role in detoxification of the harmful effects of hydrogen peroxide (H₂O₂). Under fluorescent light, significantly lower accumulation of H₂O₂ was observed. Accumulation of H₂O₂ in the induced root under different light showed a positive correlation with peroxidation of lipids and was observed higher under far-red followed by red + blue and blue light.     

NADPH oxidase as the source of ROS produced under waterlogging in roots of mung bean

The objective of this study was to examine the role of NADPH oxidase on superoxide radical production under waterlogging in mung bean (Vigna radiata) cvs. T 44 (tolerant) and Pusa Baisakhi (PB) (susceptible), and wild species Vigna luteola. Two days of waterlogging caused decline in superoxide radical (O₂ ^·−) contents in all the genotypes, however, further waterlogging up to 8 d caused significant increase in O₂ ^·− contents. In control and revived plants O₂ ^·− contents were higher in PB, while under waterlogging stress T 44 and V. luteola showed greater increases in the O₂ ^·− contents. During waterlogging the increase in O₂ ^·− content was found to be due to the diphenylene iodonium chloridesensitive NADPH oxidase (NOX). This was further confirmed by the waterlogging induced increase in NOX activity, which was higher in tolerant genotypes T 44 and V. luteola compared with PB. Gene expression studies showed enhanced expression of NOX in the roots of waterlogged V. luteola and T 44, while little expression was observed in control or treated plants of PB. PCR band products were cloned and sequenced, and partial cDNAs of NOX was obtained. Results suggest that increase in O₂ ^·− content during waterlogging could be due to the induction of membrane linked NOX.     

Rapid micropropagation of mature wild cherry

Explants taken from the mature vigorous tree of wild cherry (Prunus avium L.) were assayed for their organogenic capacity under various phytohormonal treatments. The highest rate of adventitious shoot multiplication was recorded at a combination of 0.5 mg dm⁻³ 6-benzylaminopurine (BAP) and 0.05 mg dm⁻³ thidiazuron (6.83 shoots per explant). No differences in multiplication rates were found among media supplemented with BAP, BAP + α-naphthaleneacetic acid (NAA) or BAP + indole-3-butyric acid (IBA). Shoot elongation was significantly affected by the concentration of BAP, regardless of auxin addition to medium. Up to 73 % of microshoots rooted after using 0.3 mg dm⁻³ IBA, otherwise the adventitious rooting occurred at reasonable frequencies in all auxin treatments. Regenerated plantlets were successfully hardened ex vitro and continued to grow after the transfer to soil. No morphological aberrations were observed in the regenerates.     

生长素与乙烯在沙田柚上胚轴不定芽再生中的作用

结果表明,6 BA只能诱导较低频率的不定芽再生,IBA的加入可以促进不定芽的再生。从再生率与单位外植体再生芽数综合考虑,以高浓度IBA(1.5mg·L 1)与低浓度6 BA(0.5mg·L 1)配合的效果最佳,加入生长素极性运输的调节剂TIBA与Flavone可进一步提高不定芽的再生频率。乙烯抑制上胚轴不定芽的再生,而乙烯生理作用的拮抗剂Ag+与生物合成抑制剂AOA、Co+可以显著提高不定芽再生频率。水杨酸(SA)也抑制不定芽的再生。     

Plant regeneration from shoot apical meristems of Melia azedarach L. (Meliaceae)

A protocol was developed for plant regeneration of Melia azedarach L. by in vitro culture of apical meristem (0.5 mm in length). The influence of six clones was investigated. The culture procedure comprised two sequential steps: 1) Induction of shoots by in vitro culture of axillary buds from adult trees (10–15 years old) by culture on Murashige and Skoog (1962) medium (MS) supplemented with 0.5 mg·dm⁻³ BAP (6-benzylaminopurine), 0.1 mg·dm⁻³ IBA (indolebutyric acid), and 0.1 mg·dm⁻³ GA₃ (gibberellic acid). The Multiplication of the regenerated shoots was achieved in MS + 0.5 mg·dm⁻³ BAP + 0.1 mg·dm⁻³ GA₃. 2) In vitro culture of the apical meristems from the regenerated shoots in MS medium (0.7 %) supplemented with various combinations of BAP and IBA. Maximum shoot proliferation was obtained on MS medium supplemented with 0.5 mg·dm⁻³ BAP and 0.1 mg·dm⁻³ IBA. Regenerated shoots were rooted on MS + 3.5 mg·dm⁻³ IBA (4 days) followed by subculture on MS lacking growth regulators (30 days). Complete plants were transferred to soil.     

Cadmium modulates NADPH oxidase activity and expression in sunflower leaves

The production of reactive oxygen species (ROS) and the ways by which ROS are generated are very important facts related to heavy metal toxicity in plants. In this work, superoxide anion (O₂ ^·−) generation diminished in cadmium treated sunflower (Helianthus annuus L.) leaf discs, and this reduction was time and Cd-concentration dependent. In line with these findings, we observed that NADPH-dependent oxidase activity was significantly inhibited by 0.1 and 0.5 mM Cd²⁺ treatments and the expression of the NADPH oxidase putative gene related to O₂ ^·− synthesis in sunflower leaves was 83 % inhibited by 0.1 mM CdCl₂ and almost completely depleted by 0.5 mM CdCl₂.     

Location of transported auxin in etiolated maize shoots using 5-azidoindole-3-acetic Acid

A study was undertaken using the photoaffinity labeling agent, tritiated 5-azidoindole-3-acetic acid ([³H],5-N₃IAA), to identify cells in the etiolated maize (Zea mays L.) shoot which transport auxin. Transport of [³H],5-N₃IAA was shown to be polar, inhibited by 2,3,5-triiodobenzoic acid (TIBA) and essentially freely mobile. There was no detectable radiodecomposition of [³H],5-N₃IAA within tissue kept in darkness for 4 hours. Shoot tissue which had taken up [³H],5-N₃IAA was irradiated with ultraviolet light to covalently fix the photoaffinity labeling agent within cells that contained it at the time of photolysis. Subsequent microautoradiography showed that all cells contained radioactivity; however, the amount of radioactivity varied among different cell types. Epidermal cells contained the most radioactivity per area, approximately twofold more than other cells. Parenchyma cells in the mature stelar region contained the next largest amount and cortical cells, sieve tube cells, tracheary cells, and all cells in the leaf base contained the least amount of the radioactive label. Two observations suggest that the auxin within the epidermal cells is transported in a polar manner: (a) the amount of auxin in the epidermal cells is greatly reduced in the presence of TIBA, and (b) auxin accumulates on the apical side of a wound in the epidermis and is absent on the basal side. While these results indicate that auxin in the epidermis is polarly transported, this tissue cannot be the only pathway since the epidermis is only a small fraction of the shoot volume. The greater than twofold difference between the concentration of auxin in the epidermal and subtending cells demonstrates that physiological differences in the concentration of auxin can occur between adjacent cells.