Cortical Aerenchyma formation in hypocotyl and adventitious roots of Luffa cylindrica subjected to soil flooding

BACKGROUND AND AIMS: Aerenchyma formation is thought to be one of the important morphological adaptations to hypoxic stress. Although sponge gourd is an annual vegetable upland crop, in response to flooding the hypocotyl and newly formed adventitious roots create aerenchyma that is neither schizogenous nor lysigenous, but is produced by radial elongation of cortical cells. The aim of this study is to characterize the morphological changes in flooded tissues and the pattern of cortical aerenchyma formation, and to analyse the relative amount of aerenchyma formed. METHODS: Plants were harvested at 16 d after the flooding treatment was initiated. The root system was observed, and sections of fresh materials (hypocotyl, tap root and adventitious root) were viewed with a light or fluorescence microscope. Distributions of porosity along adventitious roots were estimated by a pycnometer method. KEY RESULTS: Under flooded conditions, a considerable part of the root system consisted of new adventitious roots which soon emerged and grew quickly over the soil surface. The outer cortical cells of these roots and those of the hypocotyl elongated radially and contributed to the development of large intercellular spaces. The elongated cortical cells of adventitious roots were clearly T-shaped, and occurred regularly in mesh-like lacunate structures. In these positions, slits were formed in the epidermis. In the roots, the enlargement of the gas space system began close to the apex in the cortical cell layers immediately beneath the epidermis. The porosity along these roots was 11-45 %. In non-flooded plants, adventitious roots were not formed and no aerenchyma developed in the hypocotyl or tap root. CONCLUSIONS: Sponge gourd aerenchyma is produced by the unique radial elongation of cells that make the expansigeny. These morphological changes seem to enhance flooding tolerance by promoting tissue gas exchange, and sponge gourd might thereby adapt to flooding stress.     

Additional evidence for separable responses to auxin in soybean hypocotyl

Additional evidence for two separable responses to auxin is presented. The average of 24 control experiments indicated lag times of 12.4 and 35.4 min, and maximum rates of 0.57 and 0.54 mm hr⁻¹, for the first and second response, respectively. The auxin analog 4-azido-2-chlorophenoxyacetic acid increased the lag time of the second response (but not the first), resulting in the temporal separation of the two responses. Plots of elongation rates against time, taken from the literature, allowed the characterization of the two responses in monocotyls and dicotyls. Study of published rate-time elongation curves showed that the maximum rate of the first response is frequently greater than the maximum rate of the second response; however, the maximum rate of the second response has not yet been shown to exceed the maximum rate of the first response.     

Studies on the initiation of adventitious roots on mung bean hypocotyl

During the early stages of root induction in hypocotyls of mungbean cuttings, four new peroxidase isoenzymes are formed inthat part of the cutting that produces roots. These isoenzymesare in addition to and different from the three already presentin the hypocotyl. Forty-eight hr is required for the changeto take place from the original three to seven isoenzymes. Thechange in isoenzyme pattern and root initiation itself are retardedwhen streptomycin is applied during the very early stages ofroot induction. However, during the latter part of the first24-hr period after the cutting is made and when some of thenew isoenzymes have already been formed, the rooting processbecomes insensitive to this antibiotic. Cytologically, the occurrenceof cell division is paralleled by the formation of the fournew peroxidase isoenzymes. The delay in the occurrence of celldivision caused by the presence of streptomycin is paralleledby a similar delay in the formation of the four new isoenzymes. (Received November 27, 1970; )     

Two barley catalase genes respond differentially to light

Cloned catalase probes from barley ( Hordeum vulgare L.) and maize ( Zea mays L.) were used to examine catalase gene expression in greened and etiolated leaves of several barley lines. Etiolated leaves had greater levels of an mRNA detected by barley Cat1 , compared with greened leaves, in all lines. In contrast, a Cat2 -like mRNA (homologous to Cat2 of maize) was induced by light and accumulated to high levels in greened leaves, compared to the negligible levels detected in etiolated leaves. This suggests that barley contains light-inducible and light-repressible catalase genes. In the catalase-deficient barley mutant RPr 79/4, no hybridization signal was detected when RNA from greened or etiolated leaves was probed with maize Cat2 , indicating that this mutant is deficient for the light-induced Cat mRNA. In etiolated seedlings of both RPr 79/4 and its motherline, the level of the Cat1 mRNA increased coordinately with a steady increase in catalase activity. Even though the mutant RPr 79/4 was able to grow to maturity in normal air, it sustained chlorosis and significant head sterility, probably due to the lack of a light-inducible catalase. Although the mutant RPr 79/4 is not completely lacking catalase (EC 1.11.1.6), the loss of the CAT-1 isozyme is evidently harmful. This observation underscores the protective role of catalases in plants.     

Synergistic effects of plant growth retardants and IBA on the formation of adventitious roots in hypocotyl cuttings of mung bean

The synergistic effect of plant growth retardants, such as daminozide, paclobutrazol and triadimefon, and of indole-3-butyric acid (IBA) on the formation of adventitious roots in hypocotyl cuttings of mung bean was studied. The three retardants and IBA all stimulated adventitious root growth, but IBA was the most effective. However, mixtures of the retardants with IBA have proven generally more effective than IBA alone in promoting adventitious root formation. When IBA was applied to the hypocotyls one day after cutting preparation followed by the growth retardant on the second day, there were even more adventitious roots produced than if applied in the reverse order. The effectiveness of the treatments were in the order, IBA followed by growth retardant, IBA + growth retardant together, and IBA alone.Abbreviations IBA indole-3-butyric acid - GA gibberellin     

Effect of 6-benzyladenine on the initiation of adventitious roots on mung bean hypocotyl

Cuttings incubated with 6-benzyladenine showed extensive celldivision and tracheid differentiation in the cortex and rootprimordia did not form. The histological and enzyme data suggestthat the formation of isoperoxidases is associated with theearly differentiation of specialized parenchyma cells involvedin the formation of root primordia. (Received June 13, 1973; )     

Influence of ethylene on adventitious root formation in mung bean hypocotyl cuttings

The relationship between ethylene and adventitious root formation in mung bean hypocotyl cuttings was studied.Ethephon, an ethylene-releasing compound, at 5 x 10 -5 M increased root number and root dry weight on hypo-cotyl cuttings. When ethephon was applied to hypocotyl at different times after excision, there were two effectivetimes for root production i.e. between 06 h and 18-24 h. These two time periods correspond to the induction phase and the late initiation phase of root development, respectively. After excision, three peaks of ethylene productionwere observed. The first peak commencing at 6 h started the sequence of reactions leading root formation, the second peak appearing at 12 h coincided with the beginning of the increase of the IAA level during primordia initiation, and the third peak showing at 48 h played a role in root differentiation and growth. Ethylene stimulated rooting by enhancing the increase in auxins. Thus it appears that the IAA-induced ethylene production may be a factor involved in the stimulation of adventitious root formation.     

Influence of auxin and incubation on the relative level of polyribosomes in excised soybean hypocotyl

The influence of incubation and auxin (2,4-D) on polyribosome level in soybean hypocotyl was studied.     

Biphasic response of cucumber hypocotyl sections to auxin

The pattern of the response to auxin of cucumber hypocotyl sections,its relation to acid-induced growth and the role of the epidermiswere investigated. In longterm incubation, IAA had practicallyno effect on sections without epidermis (peeled sections). Timecourse measurementsshowed that the response to IAA of nonpeeledsections was biphasic; the first phase started right after IAAtreatment and continued for about 1 hr, then after a periodof 1 hr with a greatly decreased growth rate the second phasestarted with a higher rate. Peeled sections lacked thesecondphase; their growth rate in the first phase depended on theacidity of the incubation medium. These results were confirmedby utilizing one side-peeled sections and measuring the curvatureof the sections which represented the difference in the growthrate between the peeled and nonpeeled sides. The following pointswere suggested. The first phase resembles acid-induced growthand the growth capacity is limited by the epidermis; the secondphase is specific to auxin and requires the presence of theepidermis. The first phase was hardly influenced by temperature,while the second one was greatly affected; its Q,₁₀ being morethan 4.0, suggesting that the first phase is of physical nature. (Received December 26, 1975; )     

Involvement of differentially accumulated proteins and endogenous auxin in adventitious root formation in micropropagated shoot cuttings of Cedrela fissilis Vellozo (Meliaceae)

Plant Cell, Tissue and Organ Culture (PCTOC) - Investigation of the biochemical and molecular changes during rooting in woody species can reveal how specific molecules are involved in adventitious...     

Influence of nicotianamine and iron supply on formation and elongation of adventitious roots in hypocotyl cuttings of the tomato mutant 'chloronerva' (Lycopersicon esculentum)

The influence of nicotianamine (NA) on formation and elongation of adventitious roots in hypocotyls of de-rooted NA-less mutant seedlings of Lycopersicon esculentum Mill, was examined in relation to the iron supply [ferric N-N'-ethylenediaminedi-(2-hydroxyphenylacetate) (FEDDHA), ferric ethylenediaminetetracetate (FeEDTA), ferric N-(2-hydroxyethyl)-ethylenediaminetriacetate (FeHEDTA, Fe-citrate and FeCl₃] in the nutrient solution. The initiation of root primordia in hypocotyl cuttings was independent of NA and occurred with about the same frequency in both, mutant and wild-type. In the mutant the development of primordia to adventitious roots was blocked at all iron sources used, except FeEDTA. Addition of NA (5x 10⁻⁶ to 2 × 10⁻⁵ M ) to the rooting medium resulted in a fast growth of adventitious roots in mutant cuttings with all iron sources tested. Rooting of wild-type cuttings was independent from NA application and iron sources. We suppose that NA is involved in the intracellular transport of iron. Its function is possibly linked with chelation of ferrous iron in the cell.     

Cytokinin receptors are required for normal development of auxin-transporting vascular tissues in the hypocotyl but not in adventitious roots

Plants alter the architecture of their root systems to adapt to the environment by modulating post-embryonic (lateral and adventitious) root formation and growth. To understand better the genetic basis of this regulation, we screened ethylmethane sulfonate-mutagenized lines of Arabidopsis thaliana for adventitious rooting mutants. One mutant showed retardation of the primary root growth, no production of lateral roots and enhanced formation of adventitious roots. Mapping and genetic complementation revealed that this mutant named wooden leg-3 (wol-3) was an allele of ARABIDOPSIS HISTIDINE KINASE 4 (AHK4), a locus known to encode a cytokinin receptor. Although the vascular system of the primary root and hypocotyl in the wol-3 mutant was aborted, that of the adventitious roots was normally developed. In the hypocotyl of the wol-3 mutant, auxin signals accumulated around the aborted vascular system. The application of auxin to primary roots induced lateral root formation in the wol-3 mutant. Transport of radiolabeled auxin from the top of the hypocotyl to the primary root was inhibited in wol-3. Although only a single amino acid alteration had occurred in AHK4, the root morphology in the wol-3 mutant was quite similar to that in the ahk2 ahk3 ahk4 triple mutant, which is a loss-of-function mutant of the three cytokinin receptors. This implies that the functional disturbance of AHK4 affects the function of the other receptors. Our results suggest that cytokinin receptors are necessary for the formation of auxin-transporting vascular tissues in the hypocotyl, but not in adventitious roots.     

Ethylene accumulation in waterlogged Rumex plants promotes formation of adventitious roots

Accumulation of the gaseous plant hormone ethylene is very importantfor the induction of several responses of plants to flooding.However, little is known about the role of this gas in the formationof flooding-induced adventitious roots. Formation of adventitiousroots in Rumex species is an adaptation of these plants to floodedsoil conditions. The large air-spaces in these roots enablesdiffusion of gases between shoot and roots. Application of ethylene to non-flooded Rumex plants resultedin the formation of adventitious roots. In R. palustris Sm.shoot elongation and epinasty were also observed. The numberof roots in R. thyrsiflorus Fingerh. was much lower than inR. palustris, which corresponds with the inherent differencein root forming capacity between these two species. Ethyleneconcentrations of 1.5–2µI I_{– 1} induced a maximumnumber of roots in both species. Quantification of ethylene escaping from root systems of Rumexplants that were de-submerged after a 24 h submergence periodshowed that average ethylene concentrations in submerged rootsreached 1.8 and 9.1 µl I_–1 in R. palustris and R.thyrsiflorus, respectively. Inhibition of ethylene productionin R. palustris by L--(2-aminoethoxyvinyl)-glycine (AVG) or-aminobutyric acid (AIB) decreased the number of adventitiousroots induced by flooding, indicating that high ethylene concentrationsmay be a prerequisite for the flooding-induced formation ofadventitious roots in Rumex species. Key words: Adventitious roots, epinasty, ethylene, flooding, Rumex, shoot elongation