Linda J. Walton

Interaction of red to far red light ratio and ethylene in regulating stem elongation of Helianthus annuus

Sunflower (Helianthus annuus L.) stems showed increased elongation under two types of vegetative shade: canopy shade (low red to far red [R/FR] ratio) and neighbouring proximity shade (FR enrichment). Hypocotyls also elongated more under narrow-band FR light than under narrow-band R light. Ethylene levels were determined in actively elongating 7-day-old hypocotyls and 17-day-old internodes under three R/FR ratios. Ethylene levels were lower in both sunflower hypocotyls and internodes when the R/FR ratio was reduced. Both FR enrichment of normal R/FR ratio and narrow-band FR light with very low light irradiance resulted in reduction in ethylene levels in 7-day-old hypocotyls. Further, in application experiments, sunflower stems grown under low R/FR ratio were more sensitive to ethephon and less sensitive to aminoethoxyvinylglycine (AVG) than stems grown under high R/FR ratio. Low R/FR ratio appears to initiate reduction in ethylene levels in sunflower seedlings, allowing maximum stem elongation. These results, and findings of other authors, suggest that various plant species may have developed different ways of regulating stem elongation and ethylene levels in response to low R/FR ratio.

Interactions of temperature and light quality on phytohormone-mediated elongation of Helianthus annuus hypocotyls

Two important environmental signals, shade light, where the red/far-red (R/FR) light ratio is reduced, and elevated temperatures can each promote shoot growth. We examined their interactions using hypocotyl elongation of young sunflower (Helianthus annuus) seedlings, and we did this in the context of a possible hormonal mechanism for the growth increases that were induced by each environmental signal. Seedlings were subjected to combinations of six different temperatures (10, 15, 20, 25, 30 and 35°C) and four R/FR ratios (normal at 1.2 and reduced at 0.9, 0.6 and 0.3). Hypocotyl length was significantly increased by each of elevated temperature and FR enrichment. The magnitude of elongation induced by FR enrichment (low R/FR ratios) was dependent on temperature, with maximal effects of FR enrichment being seen at 20°C. Hypocotyl tissue concentrations of four endogenous gibberellins (GAs) and abscisic acid (ABA) were measured using the stable isotope dilution method. Hypocotyl ethylene evolution was also assessed. Thus, hypocotyl growth in both normal and shade light is highly dependent on temperature, with the most significant increases in FR-induced growth occurring at 20 and 25°C. A causal involvement of endogenous hormones, especially the GAs, in the growth that is induced by elevated temperatures, as well as in FR-induced growth, is strongly implied, with temperature being the stronger signal.

Light regulation of endogenous salicylic acid levels in hypocotyls of Helianthus annuus seedlings

Plants growing in canopy shade typically exhibit increased stem elongation and reduced leaf growth. This is as a result of direct interactions between plant photoreceptors sensing the change (reduction) in the ratio of red to far-red (R/FR) light and photosynthetically active radiation (PAR) and plant hormones, and regulating these morphological traits. The effect of the varying light conditions found in shade on endogenous salicylic acid (SA) content was tested, and the possible role of SA in shade avoidance by sunflower (Helianthus annuus L.) hypocotyls was examined. A logarithmic increase in PAR irradiance levels increased endogenous SA levels roughly 10-fold. Separation of individual light wavelengths (R, FR, and blue) constituting the PAR irradiance of sunlight, established that only FR light had significant and positive effects on endogenous SA levels. Further, a low R/FR ratio significantly increased the endogenous SA content in hypocotyls compared with normal and high R/FR ratios. Uncoupling the e...

Ethylene involvement in silique and seed development of canola, Brassica napus L.

A wide range of plant hormones, including gibberellins (GAs) and auxins are known to be involved in regulating seed and fruit growth and development. Changes in ethylene biosynthesis are also associated with seed and fruit development, but ethylenes role in these processes is poorly understood, as is its possible interaction with the other plant hormones. A major complication of investigating ethylene-induced regulation of developmental processes is ethylenes biphasic mode of action. To investigate ethylenes actions and interactions we used a 1-amino-cyclopropane-1-carboxylic acid (ACC) deaminase transgenic canola line. This line evolves significantly less ethylene from its siliques and seeds, relative to plants from a wild type (WT) background. Plants of the transgenic line also had smaller siliques which were associated with reductions in both seed size and seed number. Application of ethephon, a compound that produces ethylene, to plants of the transgenic line restored the WT phenotype for both siliques and seeds. Application of the same dose of ethephon to WT plants diminished both silique and seed development, showing ethylenes biphasic effect and effectively producing the ACC deaminase transgenic phenotype. There were significant decreases in endogenous concentrations of GA(1) and GA(4) and also of indole-3-acetic acid (IAA), between WT seeds and seedless siliques and seeds and siliques from the transgenic line plants. These differences were emphasized during early stages (10-20 days after pollination) of seed and silique development. The above results strongly suggest that ethylene interacts with other endogenous plant hormones in regulating silique and seed development and growth in WT lines of canola.

Shade light interaction with salicylic acid in regulating growth of sun (alpine) and shade (prairie) ecotypes of Stellaria longipes

The possible involvement of salicylic acid (SA) in a typical growth response of plants to shade light was investigated using the model system Stellaria longipes L. Goldie. The prairie (shade) ecotype of S. longipes is from foothills grassland habitat where it grows under shrubs or among taller grasses. The plants of this ecotype responded, as expected, with increased growth under lower red to far-red (R/FR) ratio and reduced photosynthetically active radiation (PAR). By contrast, the alpine (sun) ecotype is from an open sunny habitat, where canopy shade is a non-factor. The plants of this ecotype failed to respond with increased growth under a lower R/FR ratio, but had increased growth under a reduced PAR level. To examine the possible role of SA in shade light-mediated growth, the two main components of shade light, R/FR ratio and PAR, were uncoupled, and a series of experiments were performed by measuring the endogenous SA content and the plant response to exogenous SA concentrations. Contrary to the alpine plants, the prairie plants had increased endogenous SA content and higher shoot biomass accumulation under a low R/FR ratio treatment compared with normal or high R/FR ratios. Both alpine and prairie plants responded to a low PAR treatment with a decrease in endogenous SA content and an increase in shoot biomass accumulation, but the magnitude of this response was higher in prairie plants. Based on the results of this study, we conclude that shoot SA content is differentially regulated by both R/FR ratio and PAR signals, and SA may contribute, in ecotype specific manner, to growth changes in plants subjected to changing light environments.

The interaction of light irradiance with auxin in regulating growth of Helianthus annuus shoots

Plants growing under canopy shade or in near-neighboring proximity of taller vegetation are the receivers of shade light conditions. The effect of light irradiance (photosynthetically active radiation [PAR]), one of the main components of shade light, on the growth of various tissues of sunflower seedlings and the possible role of auxin were investigated. Gradual reductions in PAR irradiance level from near-normal to low and very low result in significant and gradual increases in sunflower hypocotyl growth and endogenous auxin content. Similar reductions in PAR level resulted in significant and gradual decreases in sunflower cotyledon and leaf growth, and endogenous auxin content. Exogenously applied auxin increased hypocotyl elongation under near-normal PAR, where IAA levels are below optimum, but decreased elongation under very low PAR, where IAA levels are already at optimum. These results suggests that auxin acts as positive growth regulator of sunflower hypocotyls subjected to low light irradiance stress. This is further supported by the transfer experiments where seedlings transferred, for example, from near-normal PAR to very low PAR showed increased elongation associated with increased IAA levels. Therefore, it is reasonable to conclude that light irradiance-mediated changes in hypocotyl elongation of young sunflower seedlings are regulated by endogenous auxin levels.

Narrow-band light regulation of ethylene and gibberellin levels in hydroponically-grown Helianthus annuus hypocotyls and roots

Both hypocotyl and root growth of sunflower (Helianthus annuus) were examined in response to a range of narrow-band width light treatments. Changes in two growth-regulating hormones, ethylene and gibberellins (GAs) were followed in an attempt to better understand the interaction of light and hormonal signaling in the growth of these two important plant organs. Hydroponically-grown 6-day-old sunflower seedlings had significantly elongated hypocotyls and primary roots when grown under far-red (FR) light produced by light emitting diodes (LEDs), compared to narrow-band red (R) and blue (B) light. However, hypocotyl and primary root lengths of seedlings given FR light were still shorter than was seen for dark-grown seedlings. Light treatment in general (compared to dark) increased lateral root formation and FR light induced massive lateral root formation, relative to treatment with R or B light. Levels of ethylene evolution (roots and hypocotyls) and concentrations of endogenous GAs (hypocotyls) were assessed from both 6-day-old sunflower plants either grown in the dark, or treated with FR, R or B light. Both R and B light had similar effects on hypocotyl and root growth as well as on ethylene and on hypocotyl GA levels. Dark treatment resulted in the highest ethylene levels, whereas FR treatment significantly reduced ethylene evolution for both hypocotyls and roots. R- and B-light treatments elevated ethylene evolution relative to FR light. Endogenous GA53 and GA19 levels in hypocotyls were significantly higher and GA44, GA20 and GA1 levels significantly lower, for dark and FR light treatments compared to R and B light-treatments. The patterns seen for changes in GA concentrations indicate FR-, R- and B-light-mediated effects [differences] in the metabolism of the early C20 GAs, GA53 → GA44 → GA19. Surprisingly, GA20, GA1 and GA8 levels in hypocotyls were very much reduced by treatment of the plants with FR light, relative to B and R-light treatments, e.g. the increased hypocotyl elongation induced by FR light was correlated with reduced levels of all three of the downstream C19 GAs. The best explanation, albeit speculative, is that a more rapid metabolism, i.e. GA20 → GA1 → GA8 → GA8 conjugates occurs under FR light. Although this study provided no evidence that elevated ethylene evolution by roots or hypocotyls of sunflower is controlling growth via endogenous GA biosynthesis, there are differences between soil-grown and hydroponically-grown sunflower seedlings with regard to trends seen for hypocotyl GA concentrations and both root and hypocotyl ethylene evolution in response to narrow band width R and FR light signaling.