Structure elucidation and phytotoxicity of ecdysteroids from Chenopodium album

The leaves of Chenopodium album were infused in H2O/MeOH. The extract treated with cold acetone gave heavy precipitation, which was removed by centrifugation. Solid material was fractionated into acidic and neutral fractions. The acidic material was subjected to different silica-gel column chromatographies, and then it was purified by reversed-phase HPLC to afford four known ecdysteroids and the new 3beta,14alpha-dihydroxy-5beta-pregn-7-ene-2,6,20-trione that were characterized by extensive spectroscopic investigation, especially by 1D- and 2D-NMR. Their effects on germination and growth of Lactuca sativa L. have been studied. The results are reported as percentage differences of germination, root elongation and shoot elongation, from the control at concentrations ranging from 10(-4) to 10(-7) M.     

Effect of light exposure on in vitro germination and germ tube growth of eight species of rust fungi

The effects of light on urediniospore germination and germ tube elongation was studied with eight species of rust fungi that infect ornamental plants or row crops. Exposure of six species of fungi to cool white fluorescent light at 400 or 600 micromol s(-1) m(-2) for 24 h significantly reduced germination with largest decreases typically observed at 600 micromol s(-1) m(-2). Germination and germ tube elongation did not recover during 24 h dark incubation after 18 h exposure to fluorescent light at 600 micromol s(-1) m(-2), indicating the effects were not reversible. Germ tube elongation of all fungi was negatively affected by increased length of exposure to fluorescent light. Increased exposure to fluorescent light differentially affected germination of the fungi with Puccinia hemerocallidis, Phakopsora pachyrhizi, Pucciniastrum vaccinii and Puccinia menthae negatively affected and Puccinia sorghi, Puccinia triticina, Puccinia pelargonii-zonalis and Puccinia iridis relatively unaffected in 10 h incubation. Exposure of Ph. pachyrhizi and P. triticina urediniospores to sunlight rapidly reduced germination and germ tube elongation with no germination observed for Ph. pachyrhizi after 2.5 h. Germ tube elongation but not germination of hydrated urediniospores of Ph. pachyrhizi and P. triticina was significantly reduced compared to dry urediniospores exposed to 10 h fluorescent light followed by 24 h dark incubation. Exposure to fluorescent light (all fungi) or sunlight (two fungi) negatively affected urediniospore germ tube elongation. Differences observed in urediniospore germination between fungi suggest some species have co-evolved with their host for differing light conditions. Our data suggests exposure of urediniospores to strong light could inactivate rust fungi on plant surfaces or in the atmosphere.     

HRS1 acts as a negative regulator of abscisic acid signaling to promote timely germination of Arabidopsis seeds

In this work, we conducted functional analysis of Arabidopsis HRS1 gene in order to provide new insights into the mechanisms governing seed germination. Compared with wild type (WT) control, HRS1 knockout mutant (hrs1-1) exhibited significant germination delays on either normal medium or those supplemented with abscisic acid (ABA) or sodium chloride (NaCl), with the magnitude of the delay being substantially larger on the latter media. The hypersensitivity of hrs1-1 germination to ABA and NaCl required ABI3, ABI4 and ABI5, and was aggravated in the double mutant hrs1-1abi1-2 and triple mutant hrs1-1hab1-1abi1-2, indicating that HRS1 acts as a negative regulator of ABA signaling during seed germination. Consistent with this notion, HRS1 expression was found in the embryo axis, and was regulated both temporally and spatially, during seed germination. Further analysis showed that the delay of hrs1-1 germination under normal conditions was associated with reduction in the elongation of the cells located in the lower hypocotyl (LH) and transition zone (TZ) of embryo axis. Interestingly, the germination rate of hrs1-1 was more severely reduced by the inhibitor of cell elongation, and more significantly decreased by the suppressors of plasmalemma H(+)-ATPase activity, than that of WT control. The plasmalemma H(+)-ATPase activity in the germinating seeds of hrs1-1 was substantially lower than that exhibited by WT control, and fusicoccin, an activator of this pump, corrected the transient germination delay of hrs1-1. Together, our data suggest that HRS1 may be needed for suppressing ABA signaling in germinating embryo axis, which promotes the timely germination of Arabidopsis seeds probably by facilitating the proper function of plasmalemma H(+)-ATPase and the efficient elongation of LH and TZ cells.     

Inhibition of Lettuce Seed Germination and Root Elongation by Derivatives of Auxin and Reversal by Derivatives of Cycocel

Lettuce seed germination or lettuce root elongation after germination in water was inhibited by 5.7 × 10^-6M indoleacetic acid (IAA) and designated other IAA derivatives. These IAA-inhibited growth responses were reversed by 10^-5 to 10^-6M Cycocel, (2-chloroethyl) trimethylammonium chloride, which alone was without effect. Only those Cycocel analogs, which have previously been shown to be active as plant growth retardants, were effective in reversing IAA inhibition of germination or root elongation. These results are consistent with the concept that Cycocel at low concentrations acts as an auxin antagonist. However. Cycocel did not reverse the inhibitory effects from indole-3-propionic acid or indole-3-butyric acid.     

Control of Aspergillus growth and aflatoxin production using antioxidants at different conditions of water activity and pH

AIMS: The effect of butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), trihydroxybutyrophenone (THB) and propyl paraben (PP) (at concentrations of 1, 10 and 20 mmol l(-1)) on germination, growth and aflatoxin B1 production by Aspergillus section Flavi was evaluated. METHODS AND RESULTS: Studies on the percentage of spore germination, elongation rate, growth rate and aflatoxin B1 production were carried out in vitro in relation to water activity (aw) at 0.982, 0.937, 0.809 and 0.747 values. At 0.809 and 0.747aw values none of the isolates was able to germinate. Overall, PP and BHA were the antioxidants most effective at inhibiting germination of both species. In the presence of the lowest concentration of BHA and PP (1 mmol l(-1)) the conidial germination percentage ranged from 2 to 19% after 15 h of incubation at the highest water activity tested. BHA and PP at 10-20 mmol l(-1) completely inhibited conidial germination. The antioxidants more efficient in controlling Aspergillus elongation rate were PP, BHT and BHA. All strains were much more sensitive to all antioxidants tested on the percentage of spore germination and growth rate at 0.937aw. The antioxidants PP and BHA completely inhibited aflatoxin B1 production by all strains when added at 1 mmol l(-1). Decreased aflatoxin B1 levels in comparison with the control, were observed with BHT at 1, 10 and 20 mmol(-1) with the strain T20 at 0.982aw. In contrast, stimulation was observed with the antioxidant THB at 10 and 20 mmol l(-1) at 0.937aw with the strains T20 and T23. The effect of BHA and PP at 1 mmol l(-1) on lag phase and growth rate was maintained in the pH range between 6 and 8. At all pH values the inhibitory effect of BHA was higher than PP. No aflatoxin B1 was detected at all pH values. CONCLUSIONS: The data show that BHA and PP could be considered as effective fungitoxicants for A. flavus and A. parasiticus. SIGNIFICANCE AND IMPACT OF THE STUDY: The information obtained show promise for controlling growth and aflatoxin B1 in stored maize. Futher studies should be carried out to examine the potential for antioxidants, such as BHA and PP to effectively control both growth and aflatoxin production.     

Purification and properties of two chitinolytic enzymes of Serratia plymuthica HRO-C48

The chitinolytic rhizobacterium Serratia plymuthica HRO-C48 was previously selected as a biocontrol agent of phytopathogenic fungi. One endochitinase (E.C. 3.2.1.14), CHIT60, and one N-acetyl-beta-1,4- D-hexosaminidase (E.C. 3.2.1.52), CHIT100, were purified and characterized. The endochitinase CHIT60, with an apparent molecular mass of 60.5 kDa, had a N-terminal amino acid sequence highly similar to that of chitinases A from Serratia liquefaciens and Serratia marcescens. The enzyme activity had its peak at 55 degrees C and pH 5.4, and increased by more than 20% in the presence of 10 mM Ca(2+), Co(2+) or Mn(2+). Activity was inhibited by 80% in the presence of 10 mM Cu(2+). CHIT100 appeared to be a monomeric enzyme with a molecular mass of 95.6 kDa and a pI of 6.8. Optimal activity was obtained at 43 degrees C and pH 6.6, and decreased by more than 90 % in the presence of 10 mM Co(2+) or Cu(2+). CHIT100 (100 microg ml(-1)) inhibited spore germination and germ tube elongation of the phytopathogenic fungus Botrytis cinerea by 28 % and 31.6 %, respectively. With CHIT60 (100 microg ml(-1)), the effect was more pronounced: 78 % inhibition of of germination and 63.9 % inhibition of germ tube elongation.     

myo-Inositol Metabolism in Lilium longiflorum Pollen: Uptake and Incorporation of myo-Inositol-2-H

Germinating Lilium longiflorum pollen absorbs and metabolizes myo-inositol-2-(3)H (MI-2-(3)H) with a pronounced lag when label is supplied from the beginning of germination. If MI-2-(3)H is given after 3 hours of germination, incorporation of labeled metabolic products into pollen tube polysaccharides is constant over a range of 0.56 mm to 2.78 mm MI. When MI-2-(3)H is supplied as a 0.5-hour pulse 3 hours after germination, the proper precursor-product relationship to tube wall polysaccharides is observed. Replacing 10% of the germination media with sigmatic exudate from a compatible cultivar hastens germination and tube elongation. Enhanced MI metabolism accompanies tube growth in this exudate-enriched media.     

不同工艺阶段味精废水对作物种子发芽和根伸长的毒性效应

为了对味精废水的污染进行全面的评价,针对不同工艺阶段排放的味精废水,采用小麦、白菜和西红柿等作物种子,以污水为环境介质,进行了种子发芽和根伸长的污染暴露实验,并结合污染现场,进行了相应的定量分析.结果表明,在高浓度原味精废母液中, 小麦种子的发芽抑制率和根伸长的抑制率与废水的浓度呈显著正相关.味精废水对3种作物种子的发芽和根伸长的毒性强弱顺序为：西红柿＞白菜＞小麦西红柿对味精废水毒性响应最为敏感,可以认为是一种较为理想的生物毒性指示作物.味精生产不同工艺阶段所排放的污水对这3种作物种子的发芽半抑制浓度(IC₅₀)为22.0～32432 mg·L^-1,对根伸长的半抑制浓度(IC₅₀)为17.3～3320 mg·L^-1.     

Effect of cadmium on pollen germination and tube growth in Lilium longiflorum and Nicotiana tabacum

Cadmium had a highly toxic effect on pollen germination and tube growth, which were greatly inhibited as metal concentrations increased. Cadmium concentrations up to 10(-2) M completely stopped pollen germination and pollen showed an increasing tendency to burst within 1 h. At low concentrations, the metal caused a slight stimulation of pollen germination, growth rate and tube elongation at the initial stages of tube development. Comparing the two plants studied, cadmium was more toxic for Nicotiana tabacum than for Lilium longiflorum pollen. Pollen tubes showed a range of strong morphological abnormalities, characterized by uneven or aberrant growth, including apical branching or swelling at the tip of the pollen tube. Cell wall intrusions at or near the tip were evident on the inner side, whereas a loose network formed from fibrillar material was observed on the outer layers. After prolonged cadmium exposure, round (ball-like) aggregates were embedded in a fine fibrillar network. Increased cadmium concentrations (10(-3)-10(-2) M) decreased or completely paralyzed cytoplasmic streaming. No typical cytoplasmic zonation existed, while cell organelles (plastids, lipid droplets) were relocated toward the tip. The vesicular apical zone was drastically reduced, with vesicles dispersed into the subapical region. Mitochondria were distributed throughout the subapical region and among the vesicles of the tube apex. Visible ultrastructural changes in cell organelles were not observed.     

The in vitro effect of gossypol and its interaction with salts on conidial germination and viability of Fusarium oxysporum sp. vasinfectum isolates

AIMS: To assess the effect of different concentrations of gossypol (0, 2, 4, 10 and 20 mg l(-1)) in combination with NaCl and Na(2)SO(4) (20 mS cm(-1)) on the conidial germination and viability of Fusarium oxysporum f.sp. vasinfectum (Fov). METHODS AND RESULTS: A multinomial logistic model was developed to estimate the germination probability of Fov. The inhibitory effect was markedly evident at the two highest concentrations of gossypol; it varied among the isolates tested and with time, and it was attenuated by the presence of sodium salts. The inhibition was temporary as the germination probability increased after 8 h. Fluorescent staining revealed that gossypol either killed the conidia or retarded the elongation of the germ tubes. CONCLUSION: Fov showed the ability to overcome gossypol inhibition over time, and the inhibitory effect is reduced under saline conditions. Differential responses among Fov isolates to the presence of gossypol suggest that gossypol tolerance is genetically determined in the pathogen. SIGNIFICANCE AND IMPACT OF THE STUDY: This study suggests that selecting for high plant gossypol cultivars would have minimal effect on the overall Fov resistance of cotton. A new statistical model was developed to explore the statistical significance of plant-pathogen interactions.     

Uptake and translocation of phytochemical 2-benzoxazolinone (BOA) in radish seeds and seedlings

The molecular aspects of phytochemical interactions between plants, especially the process of phytochemical translocation by the target plant, remain challenging for those studying allelopathy. 2-Benzoxazolinone (BOA) is a natural chemical produced by rye (Secale cereale) and is known to have phytotoxic effects on weed seeds and seedlings. The translocation of BOA into target plants has been poorly investigated. Therefore, the total absorption of [ring U 14C] BOA was estimated by oxidizing whole seedlings of Raphanus sativus cv. for 8 days and quantifying the radioactivity. Non-radiolabelled BOA in seedlings was also estimated by HPLC. BOA applied at 10(-3) M was readily taken up by germinated radish at a rate of 1556 nmol g(-1) FW. At these same concentrations, BOA reduced radish germination by 50% and caused a delay in radicle elongation. Exogenous BOA was responsible for the observed germination inhibition. At a concentration of 10(-5) M, BOA was taken up by germinated seeds (31 nmol g(-1) FW), but this quantity did not affect radish germination. Labelled BOA was not mineralized in the culture medium during seedling growth as no 14CO2 was recovered. Both 10(-3) and 10(-5) M BOA were translocated into radish organs, mainly into roots and cotyledons. These organs were then identified as potential physiological target sites. Cotyledons remained the target sink (44% of the total radioactivity). The kinetics of BOA uptake at 10(-3) and 10(-5) M in radish seedlings was identical: BOA accumulation was proportional to its initial concentration. A comparison between radioactivity and HPLC quantification for 10(-3) M BOA indicated that BOA (along with some metabolites) could effectively be recovered in radish organs using chromatography.     

基于小麦种子发芽和根伸长的麝香酮污染毒性效应

研究了新型污染物麝香酮对小麦种子发芽及幼苗生长的影响.结果表明,土壤麝香酮（10 mg·kg^-1）污染对小麦发芽率抑制作用达到显著水平（P<0.05）,对小麦根长和芽长的抑制作用达极显著水平（P<0.01）,并且芽长、根长与土壤中麝香酮浓度之间存在极显著（P<0.01）的剂量 效应关系.表明小麦根和芽伸长可以指示土壤麝香酮的污染程度.在土壤麝香酮污染条件下,小麦幼苗的根长和芽长之间呈极显著（P<0.01）的正相关性,发芽率和生物量之间呈极显著（P<0.01）的负相关性.有机污染物的毒性除了与它在水中的溶解度有关外,还与其化学性质及对靶标生物的毒性机制有关.     

LONGIFOLIA1 and LONGIFOLIA2, two homologous genes, regulate longitudinal cell elongation in Arabidopsis.

Plants have diversified their leaf morphologies to adapt to diverse ecological niches. The molecular components responsible for regulating leaf morphology, however, have not been fully elucidated. By screening Arabidopsis activation-tagging lines, we identified a dominant mutant, which we designated longifolia1-1D (lng1-1D). lng1-1D plants were characterized by long petioles, narrow but extremely long leaf blades with serrated margins, elongated floral organs, and elongated siliques. The elongated leaves of the mutant were due to increased polar cell elongation rather than increased cell proliferation. Molecular characterization revealed that this phenotype was caused by overexpression of the novel gene LNG1, which was found to have a homolog, LNG2,in Arabidopsis. To further examine the role of the LNG genes, we characterized lng1 and lng2 loss-of-function mutant lines. In contrast to the elongated leaves of lng1-1D plants, the lng1 and lng2 mutants showed slightly decreased leaf length. Furthermore, the lng1-3 lng2-1 double mutant showed further decreased leaf length associated with less longitudinal polar cell elongation. The leaf widths in lng1-3 lng2-1 mutant plants were similar to those in wild type, implying that the role of LNG1 and LNG2 on polar cell elongation is similar to that of ROTUNDIFOLIA3 (ROT3). However, analysis of a lng1-3 lng2-1 rot3-1 triple mutant and of a lng1-1D rot3-1 double mutant indicated that LNG1 and LNG2 promote longitudinal cell elongation independently of ROT3. Taken together, these findings indicate that LNG1 and LNG2 are new components that regulate leaf morphology by positively promoting longitudinal polar cell elongation independently of ROT3 in Arabidopsis.     

In vitro effect of phenolic antioxidants on germination, growth and aflatoxin B accumulation by peanut Aspergillus section Flavi

AIMS: The effectiveness of the food-grade antioxidants butylated hydroxytoluene (BHT), trihydroxybutyrophenone (THB), propyl paraben (PP) and butylated hydroxyanisole (BHA) at 1, 10 and 20 mmol l(-1) concentrations on germination, growth, and aflatoxin B(1) (AFB(1)) production by Aspergillus section Flavi strains was determined. METHODS AND RESULTS: Assays on the lag phase of germination, germination percentage, germ tube elongation rate, lag phase, growth rate and AFB(1) production by three strains of Aspergillus flavus and three of Aspergillus parasiticus were carried out in vitro on peanut extract meal agar conditioned at different water activities (a(w): 0.982, 0.971, 0.955, 0.937). The antioxidants PP and BHA efficiently inhibited the germination of the two species tested at the doses 10 and 20 mmol(-1). The antioxidants PP and BHA at 1 mmol l(-1) and THB at 20 mmol l(-1) reduced the germ tube elongation rate most effectively, regardless of a(w) levels. An increase in the lag time and a reduction in the growth rate of 100% of the strains was observed, this was due to the action of BHT at the doses 10 and 20 mmol(-1) at 0.982, 0.971 and 0.955 a(w), although these treatments stimulated the AFB(1) accumulation in most of the fungi tested. The more effective antioxidants were PP and BHA, which increased the lag phase, reduced the growth rate and AFB(1) production in all of the strains at the four a(w) assayed. At concentrations 10 and 20 mmol l(-1), these antioxidants totally inhibited fungal development. CONCLUSIONS: The study shows that the antioxidants BHA and PP are effective fungal inhibitors to peanut Aspergillus section Flavi in wide range of water activity. SIGNIFICANCE AND IMPACT OF THE STUDY: The results suggest that phenolic antioxidants, BHA and PP, can be effective fungitoxicants on aflatoxigenic strains in peanut at industrial level.     

Expression of maize D-type cyclins: comparison, regulation by phytohormones during seed germination and description of a new D cyclin

While cloning maize D-type cyclins previously reported in databases (described as of D1, D2 and D4 types), a fourth D cyclin was cloned that showed high homology (75%) with the D1 cyclin. Because this D1 cyclin has been recently described as a D5-type cyclin (D5;1), the new cyclin was named D5;2. All maize cyclins have been compared among themselves and among D cyclins from other plant species. All maize D cyclins possess the retinoblastoma protein–binding motif and cyclin boxes but no PEST sequences or destruction box sequences are required for protein degradation. D5 and D2 cyclins also have canonical cyclin-dependent kinase (Cdk)–phosphorylation sites. Every cyclin showed a different expression pattern during seed germination, standing out cyclin D5;2, which seems to be expressed only during the early stages (equivalent to postmitotic interphase), and cyclin D4;1, which progressively accumulates from an almost undetectable level in dry seed embryo axes. Phytohormones like cytokinins and auxins, which accelerate the germination process, change the expression pattern of all cyclins, with cytokinins promoting an increase in expression during the early hours of germination (by 6 h), whereas auxins promote a constant increase in the levels of three out of the four D cyclins (except D5;1). Cyclin D5;1 is the least expressed of all cyclins in all tissues measured (embryo axes, seedlings and plantlets), and all cyclins are expressed in both meristematic and non-meristematic tissues. We discuss their relevance for the germination process and plantlet establishment.     

Identification and characterization of mutants capable of rapid seed germination at 10 degrees C from activation-tagged lines of Arabidopsis thaliana

Five cold temperature germinating (ctg) mutants, completing germination at 10 degrees C faster than wild type, have been recovered from activation-tagged populations of Arabidopsis thaliana. Three (ctg10-D, 41-D, and 144-D) were tagged and segregated 3:1 for BASTA resistance in the F2 when crossed with wild type. None of the tagged ctg mutants was disturbed in sensitivity to abscisic acid or glucose but all were less sensitive to GA4+7 and osmoticum. The other two mutants (ctg156 and ctg225) were recessive, BASTA sensitive, and exhibited a transparent testa (tt) phenotype. They were more sensitive to abscisic acid, paclobutrazol, and GA4+7 than wild type but had similar sensitivity to osmoticum. Dimethylaminocinnamaldehyde staining of seeds from the two tt mutants, compared with stained seeds from the publicly available tt lines 1-10, suggested that ctg156 was a new allele of tt1, while ctg225 was similar to tt7-1. However, reciprocal crosses determined that ctg156 was not allelic to tt1 while ctg225 was a new allele of tt7. When the gene was sequenced from ctg225 it was missing 10 bp in the second exon, resulting in the incorporation of two spurious amino acids (G282E and D283A) followed by a stop. The screen successfully recovered mutants completing germination faster than wild type at 10 degrees C.     

Characterization of a newly discovered Beauveria bassiana isolate to Franklimiella occidentalis Perganda, a non-native invasive species in China

In this study, a new virulent Beauveria bassiana isolate (B. bassiana-CYT5) had been identified as a new member of the species B. bassiana. The B. bassiana-CYT5 isolate was compared with four other virulent B. bassiana isolates and found to be highly infectious and virulent against the Franklimiella occidentalis Perganda. The B. bassiana-CYT5 could approximately 93.08% mortality of F. occidentalis 6 days post inoculation in the concentration of 1×10(8) conidia/mL. The phylogenetic tree based on ITS and partial sequence of elongation factor 1-alpha (EF1-alpha) indicated that B. bassiana-CYT5 isolate was in a cluster of B. bassiana. Furthermore, B. bassiana-CYT5 isolate demonstrated high heat tolerance (60-100% relative germination) between 1-h and 2-h exposure at 37 °C, 38 °C, 39 °C and 40 °C, respectively. So our results suggested that B. bassiana-CYT5 isolate could be a new efficient biocontrol agent against F. occidentalis.     

Profilin promotes barbed-end actin filament assembly without lowering the critical concentration

The mechanism of profilin-promoted actin polymerization has been systematically reinvestigated. Rates of barbed-end elongation onto Spectrin.4.1.Actin seeds were measured by right angle light scattering to avoid confounding effects of pyrenyl-actin, and KINSIM was used to analyze elongation progress curves. Without thymosin-beta4, both actin and Profilin.Actin (P.A) are competent in barbed-end polymerization, and kinetic simulations yielded the same bimolecular rate constant ( approximately 10 x 10(6) M(-1) s(-1)) for actin monomer or Profilin.Actin. When measured in the absence of profilin, actin assembly curves over a 0.7-4 microM thymosin-beta4 concentration range fit a simple monomer sequestering model (1 microM K(D) for Thymosin-beta4.Actin). The corresponding constant for thymosin-beta4.pyrenyl-Actin, however, was significantly higher ( approximately 9-10 microM), suggesting that the fluorophore markedly weakens binding to thymosin-beta4. With solutions of actin (2 microM) and thymosin-beta4 (2 or 4 microM), the barbed-end assembly rate rose with increasing profilin concentration (0.7-2 microM). Actin assembly in presence of thymosin-beta4 and profilin fit a simple thermodynamic energy cycle, thereby disproving an earlier claim (D. Pantaloni and M.-F. Carlier (1993) Cell 75, 1007-1014) that profilin promotes nonequilibrium filament assembly by accelerating hydrolysis of filament-bound ATP. Our findings indicate that profilin serves as a polymerization catalyst that captures actin monomers from Thymosin-beta4.Actin and ushers actin as a Profilin.Actin complex onto growing barbed filament ends.     

Two dominant photomorphogenic mutations of Arabidopsis thaliana identified as suppressor mutations of hy2

By screening suppressor mutants of the hy2 mutation of Arabidopsis thaliana , two dominant photomorphogenic mutants, shy1-1D and shy2-1D , for two genetic loci designated as SHY1 and SHY2 ( s uppressor of hy 2 mutation) have been isolated. Both of these non-allelic, extragenic suppressor mutations of hy2 are located on chromosome 1 of the Arabidopsis genome. Both mutations suppress the elongated hypocotyl phenotype of hy2 by light-independent inhibition of hypocotyl growth as well as by increasing the effectiveness of light inhibition of hypocotyl elongation. The shy1-1D mutation is partially photomorphogenic in darkness with apical hook opening and reduced hypocotyl elongation. The shy2-1D mutant displays highly photomorphogenic characteristics in darkness such as true leaf development, cotyledon expansion, and extremely reduced hypocotyl growth. In regard to hypocotyl elongation, however, the shy2-1D mutation is still light sensitive. Examination of red/far-red light responses shows that the shy1-1D mutation suppresses the hypocotyl elongation of the hy2 mutation effectively in red light but not effectively in far-red light. The shy2-1D suppresses hypocotyl elongation of the hy2 mutation effectively in both red and far-red light. Both mutations can also suppress the early-flowering phenotype of hy2 and have a distinct pleiotropic effect on leaf development such as upward leaf rolling. The data obtained suggest that SHY1 and SHY2 represent a novel class of components involved in the photomorphogenic pathways of Arabidopsis . This is the first report on the identification of dominant mutations in the light signal transduction pathway of plants.