Growth distribution during first positive phototropic curvature of maize coleoptiles*

Abastract Measurements of growth increments on the shaded and the irradiated sides of phototropically stimulated maize (Zea mays L.) coleoptiles, obtained over the entire fluence range of the first positive curvature, indicate that the curvature is induced by growth stimulation on the shaded side and compensating inhibition on the irradiated side (length increments on the coleoptile flanks were determined 100 min after 30 s phototropic induction with blue light). At high fluences of blue light, overall stimulation of growth takes place, but this tendency is largely eliminated when only the tip of the coleoptile is irradiated. Time courses for growth increments obtained for the maximum first positive response show that the growth stimulation on the shaded side and the growth inhibition on the irradiated side commence almost simultaneously 20-30 min after the phototropic induction. The growth on the irradiated side almost ceases, but the growth rate on the shaded side is doubled, relative to the control rate. The onset of differential growth migrates basipetally from the tip at a velocity similar to that for polar auxin transport. The first positive phototropic response of the coleoptile is concluded to be the consequence of lateral redistribution of growth, which is not necessarily accompanied by changes in the net growth. The results are consonant with the Cholodny-Went theory of tropisms, in which lateral redistribution of auxin is considered to be the cause of tropic responses.     

Buder revisited: cell and organ polarity during phototropism

The induction of a radial polarity by environmental stimuli was studied at the cellular and organ levels, with phototropism chosen as a model. The light gradient acting on the whole coleoptile was opposed to the light direction acting upon individual cells in the classical Buder experiment, irradiating from the inside out. Alternatively, the stimulus was administered to the coleoptile tip with a microbeam-irradiation device. Tropistic curvature was assayed as a marker for the response of the whole organ, whereas cell elongation and the orientation of cortical microtubules were taken as markers for the responses of individual cells. Upon tip irradiation, signals much faster than basipetal auxin transport migrate towards the base. The data are discussed in terms of an organ polarity that is the primary result of the asymmetric light signal and affects, in a second step, an endogenous radial polarity of epidermal cells.     

Effects of red light on the fluence–response relationship for pulse-induced phototropism of maize coleoptiles

Dark-adapted coleoptiles of maize (Zea mays L.) were treated with red light (3min at 10.5 μmol m^?2S^?1) and were Stimulated, after a dark interval, with a pulse of unilateral blue light to induce phototropism. Phototropic fluence-response curves were obtained in this way for different dark intervals. It was confirmed that the bell-shaped fluence-response curve for the first pulse-induced positive phototropism (FPIPP) shifts to higher fluences following the red-light treatment, the maximal shift being achieved at a dark interval of 2h. We found, however, that the two arms of the Fluence-response curve do not shift synchronously. The shift of the descending arm to higher fluences began at 15 min. The ascending arm showed a slight shift to lower fluences before a greater shift to higher flucnces. the change of the shift direction occurring at 30–40min. Accordingly, the fluence-response curve obtained for a 30 min dark interval was comparatively wide. Although dark-adapted coleoptiles showed only fPIPP, another bell-shaped fluence-response curve, representing the second pulse-induced positive phototropism (sPIPP), appeared gradually after the red-light treatment. These changes of the phototropic fluence– respnse curve following exposure to red light are likely to have adaptive values because they favour phototropism under brighter light.     

Phytochrome is required for the occurrence of time-dependent phototropism in maize coleoptiles

Time-dependent phototropism (TDP), sometimes called second positive curvature, occurs when the duration of phototropic stimulation with blue light (B) exceeds a few minutes. TDP was characterized in maize (Zea mays L.) coleoptiles raised under continuous red light (R). Subsequently, coleoptiles adapted to darkness were used to investigate the effect of R on TDP. It was found that TDP, which is induced in R-grown coleoptiles, does not occur in dark-adapted coleoptiles and that dark-adapted coleoptiles begin to show TDP after treatment with R. The TDP responsiveness became maximal 1-2 h after treatment with a R pulse and decreased during the next few hours. At least 10 min was required after a short pulse of R before the coleoptile began to respond to B for the induction of TDP. The effect of R in establishing the TDP responsiveness was totally suppressed by a pulse of far-red light given immediately after an inductive pulse of R. It is concluded that the mechanism of TDP requires for its establishment a R signal perceived by phytochrome. The TDP of R-grown and R-pretreated coleoptiles showed relationships to stimulation times and fluence rates that are similar to those reported for oat coleoptiles, except that TDP of maize showed a sharp increase in its magnitude within a narrow range of stimulation times as short as 5-10 min.     

Mechanism of detecting light direction in first positive phototropism in Zea mays L.

Abstract. The carotenoid content of corn seedlings was reduced by 80–90% with the herbicide SAN 9789 or by using carotenoidless mutants. This caused a decrease in 'first positive' phototropism by about 50% without affecting geotropism. This reduction in phototropism is attributed to the decreased light gradient across the albino shoot. Decreased screening should increase the response if a focusing mechanism is used to measure the light gradient, but should decrease the response if a screening mechanism is used. Thus, these data support the hypothesis that screening establishes the light gradient used to measure light direction in 'first positive' phototropism.     

White light‐induced sugar distribution controls growth and osmotic properties in the coleoptile and the first leaf in Zea mays seedlings

The correlation of white light‐induced changes in osmotic concentration in the coleoptile and the first leaf and the growth rate of these organs in maize seedlings, was examined in relation to sugar distribution and invertase activity. One hour irradiation with white light decreased the osmotic concentration in basal zones of the coleoptile and increased it in the apical zones of the first leaf. The change in the osmotic concentration was positively correlated with the growth rate of both organs. The amount of total osmotic solutes in each zone was highly correlated with that of soluble sugars. Light decreased the activity of wall‐bound invertase in the coleoptile, but increased it in the first leaf. A high correlation existed between the content of soluble sugars and invertase activity in both organs. During 1 h incubation in the light, ca 2 µmol of soluble sugars per seedling was lost from the coleoptile and gained in the first leaf. Light promoted sugar exudation from the excised coleoptile, but the amount of soluble sugar exuded represented 5% of sugar loss from the coleoptile in intact seedlings.
These results indicate that in maize seedlings white light controls the growth rate of the coleoptile and the first leaf through the osmotic concentration. Light may have an osmoregulatory function in the control of sugar distribution between the coleoptile and the first leaf by regulating the activity of wall‐bound invertase.     

Effects of UV-C radiation on extension growth, H+-extrusion and transmembrane electric potential in maize coleoptile segments

The effect of 253.7 nm ultraviolet radiation on elongation growth, medium acidification and changes in electric potential difference between vacuole and external medium in cells of maize ( Zea mays L.) coleoptile segments was investigated. It was found that irradiation with 390, 1170, 3900 and 5 850 J m⁻² UV-C (ultraviolet radiation 253.7 nm) inhibited elongation growth, whereas at 195 J m⁻² stimulation of growth was observed. The administration of IAA (10⁻⁵ M ) to the incubation medium of coleoptile segments partially abolished the inhibitory effect of UV-C. The pH of the incubation medium, measured simultaneously with growth, showed that the exposure of the segments to UV-C caused inhibition of H⁺-extrusion (or stimulation of H⁺ uptake). The presence of IAA (10⁻⁵ M ) in the incubation medium promoted (except after 5850 J m⁻² irradiation) H⁺-extrusion to a level comparable with that produced by IAA in non-irradiated segments. In UV-C irradiated segments the potential difference underwent significant alterations. Irradiation of coleoptile segments with 390 J m⁻² caused a transient depolarization, which was fully reversible within 30 min, while at higher doses depolarization was irreversible. The hyperpolarization of the membrane potential (MP) in cells of maize coleoptile induced by IAA was completely nullified by subsequent irradiation with UV-C. It is suggested that UV-C inhibited IAA-induced growth by a mechanism independent of cell wall acidification.     

Estimation of growth parameters in salt-stressed maize: comparison of the pressure-block and applied-tension techniques

A custom-built pressure block was used to estimate the effective turgor (turgor pressure minus the yield threshold) and the cell wall extensibility of the growing zone of the third leaves of 8-d-old maize (Zea mays L.) seedlings. In response to cell wall loosening, pressure in the chamber increased rapidly and reached a maximum after approximately 60 min. Plants treated with 80 mol m^?3 NaCl for 4 h were compared to control plants. Pressure-block analysis revealed that salinity reduced effective turgor, but had no effect on cell wall extensibility. These results are qualitatively and quantitatively similar to those obtained with an applied-tension technique used previously in our laboratory. This study indicates that the pressure-block and applied-tension techniques, which use very different methodologies, estimate similar growth parameters.     

Changes in cellular osmotic potential and mechanical properties of cell walls during light-induced inhibition of cell elongation in maize coleoptiles

The growth rate of maize ( Zea mays L. cv. Cross Bantam T51) coleoptiles in the dark was highest at the basal zone and decreased towards the tip. Growth was strongly inhibited by white fluorescent light (5 W m⁻²), especially in the basal zone of coleoptiles. Light irradiation caused an increase in the values of stress-relaxation parameters, the minimum stress-relaxation time and the relaxation rate and a decrease in the extensibility (strain/stress) of the cell walls at all zones. In addition, during growth, the accumulation of osmotic solutes was strongly inhibited by white light irradiation, resulting in an increased osmotic potential. The influences of white light on the mechanical properties of the cell wall and the osmotic potential of the tissue sap were most prominent in the basal zone. Significant correlations were observed between the increment of coleoptile length and the mechanical properties of the cell walls or the osmotic potential of the tissue sap and osmotic solutes content. Furthermore, light inhibited the outward bending of split coleoptile segments. These facts suggest that white light inhibits elongation of maize coleoptiles by modifying both the mechanical properties of the cell walls and cellular osmotic potential, which control the rate of water uptake.     

Size-dependent growth and the development of inequality in maize,sunflower and soybean

Links were investigated between allometry of plant growth and dynamics of size structure of well-fertilized, irrigated crops of soybean (Glycine max L.), sunflower (Helianthus annuus L.) and maize (Zea mays L.) grown at standard plant-population densities (D), as in commercial crops (D = 30, 6 and 8.5 plants m-2, respectively), and at high densities (2D). Patterns of size-dependent growth of shoot and seed mass accumulation were distinctly different among species. In soybean and sunflower, non-linear relationships between size and subsequent growth led to strong hierarchical populations in terms of both shoot and seed biomass. Curvilinear (soybean) and sigmoid (sunflower) size-dependent growth determined strongly asymmetrical (soybean) and bimodal (sunflower) frequency distributions of shoot biomass indicating predominantly size asymmetrical competition among individuals. In comparison, a lower plant-to-plant variation coupled with a typical linear allometry of growth to plant size indicated symmetrical two-sided plant interference in maize. Despite the weak development of hierarchies in shoot biomass, a strong inequality in reproductive output developed in crowded populations of maize indicating an apparent breakage of reproductive allometry.     

The decrease in growth of phosphorus-deficient maize leaves is related to a lower cell production

The spatial distribution of leaf elongation and adaxial epidermal cell production in leaf 6 of maize (Zea mays L. cv. Cecilia) plants grown in a growth chamber under two contrasting availabilities of P in the soil was investigated. Lower displacement velocities from 32.5 mm from leaf base and a shorter growth zone were found in low P (LP) leaves compared with control leaves. P deficiency significantly diminished maximum relative elemental growth rate and shifted its location closer to the leaf base. Cells were significantly longer in LP than in control leaves for all positions from the leaf base except at the end of the growth zone. For both treatments it took a similar time for a cell situated at the leaf base to reach the limit of the growth zone. The average length of the cell division zone was decreased by 21% in LP leaves. Significant differences were found in cell production and cell division rates from 12.5 mm from the leaf base although maximum values were similar between P treatments. A shorter zone of cell division with lower cell production rates along most of its length was the regulatory event that decreased cell production, and ultimately leaf elongation rates, in P‐deficient maize plants.     

Involvement of auxin pathways in modulating root architecture during beneficial plant–microorganism interactions

A wide variety of microorganisms known to produce auxin and auxin precursors form beneficial relationships with plants and alter host root development. Moreover, other signals produced by microorganisms affect auxin pathways in host plants. However, the precise role of auxin and auxin‐signalling pathways in modulating plant–microbe interactions is unknown. Dissecting out the auxin synthesis, transport and signalling pathways resulting in the characteristic molecular, physiological and developmental response in plants will further illuminate upon how these intriguing inter‐species interactions of environmental, ecological and economic significance occur. The present review seeks to survey and summarize the scattered evidence in support of known host root modifications brought about by beneficial microorganisms and implicate the role of auxin synthesis, transport and signal transduction in modulating beneficial effects in plants. Finally, through a synthesis of the current body of work, we present outstanding challenges and potential future research directions on studies related to auxin signalling in plant–microbe interactions.     

An endogenous growth inhibitor, 3-hydroxy-β-ionone. III. Its longitudinal gradients in the first internode of Phaseolus vulgaris

Dark-grown, 10-day-old bean seedlings (Phaseolus vulgaris L. cv. Morocco) were transferred to continuous light, and the resulting changes in growth rate and concentration of 3-hydroxy-β-ionone, an endogenous growth inhibitor, were monitored. The growth rate of the first internodes of the seedlings decreased rapidly and leveled off 20 h after onset of light. This plateau value was about 25% of the growth rate of the non-irradiated control. The concentration of 3-hydroxy-β-ionone in the internodes of the irradiated seedlings increased rapidly and reached a plateau value after 16 h. This increased level of the inhibitor was about 5 times the level in the non-irradiated control. The changes in the levels of the inhibitor in the internodes reflected the light-induced growth inhibition of the internodes. The longitudinal distribution of the growth inhibitor along the first internodes of the seedlings was closely correlated with the light-induced growth inhibition of the corresponding region of the internodes. The present results suggest that the endogenous growth inhibitor 3-hydroxy-β-ionone may play an important role in the inhibition by light of internode growth of bean seedlings.