Ethylene production by cress roots and excised cress root segments and its inhibition by 3,5-diiodo-4-hydroxybenzoic acid

Possible role of light in the maintenance of EIN3/EIL1 stability in Arabidopsis seedlings

To examine the mechanism of EIN3-mediated gene expression by ethylene, the expression patterns of ethylene-inducible genes by ethylene were monitored in Col-0 and ethylene signaling mutants. In Col-0, the inducibility of ACC oxidase by ethylene in light-grown seedlings was much higher than in dark-grown seedlings. While the expression of ACC oxidase was highly increased by ethylene not only in Col-0 but in ein3-1 under light treatment, this pattern was completely abrogated in etiolated ein3-1 seedlings, suggesting the expression of EIN3-mediated ACC oxidase genes could be affected by light. To check if the level of EIN3 and EIL1 was regulated by light, cell-free degradation assays were performed. This resulted in the rapid degradation of these proteins within 1h after adding dark-grown cell extracts and this degradation was retarded by light-grown extracts. Here, we propose that light may act as a negative regulator in the destabilization of EIN3/EIL1.

Differential ethylene sensitivity of epidermal cells is involved in the establishment of cell pattern in the Arabidopsis root

Root hairs of Arabidopsis roots develop on trichoblasts located over the anticlinal (radial) walls of underlying cortical cells. Non-hair cells, on the other hand, develop on atrichoblasts overlying the periclinal (tangential) walls of cortical cells. Dark-grown wild-type seedlings, which produce little ethylene, are largely root hairless. Exogenous treatment of dark-grown plants with either ethylene or 1-aminocyclopropane-1-carboxylic acid (ACC) restores the development of root hairs in cells overlying the anticlinal cortical cell walls, indicating that cells in this position are more sensitive to ethylene than atrichoblasts. We used mutations in genes that overproduce ethylene (eto1, eto2, eto3 and eto4) to illustrate the positive regulatory role of ethylene. The preferential development of root hairs on epidermal cells overlying the cortical anticlinal cell walls in these mutants also illustrates that trichoblasts are more sensitive to ethylene than atrichoblasts. CTR1 is a negative regulator of the ethylene response and might, therefore, be a candidate regulator of differential sensitivity. CTR1 mRNA is expressed in all cell types in the root, suggesting that its transcriptional pattern alone cannot account for the differential sensitivity of epidermal cells to ethylene. Cellular mapping of wild-type and mutant roots supports previous findings indicating that ethylene acts after, and perhaps independently, of TTG during the establishment of cell fate in the root epidermis.

Investigations into the possible regulation of negative gravitropic curvature in intact Avena sativa plants and in isolated stem segments by ethylene and gibberellins

Using Avena sativa L. cv. Victory oat seedlings and excised p-1 stem segments (including the p-1 and p-2 internodes) the effect of exogenously supplied ethylene and the removal of ethylene on internodal extension and gravitropic bending was assessed. Similarly, the ability of the excised system to respond to gravistimulation was assessed in the presence of inhibitors of ethylene action (AgNO3) and ethylene synthesis (3,5-diiodo-4-hydroxybenzoic acid and benzyl isothiocyanate; BITC). The production of ethylene from both intact and excised systems was also measured from 0 to 48 h after gravistimulation, relative to vertical controls. Although gravitropic curvature is initiated, and indeed enters the most rapid phase of upward bending during the first 6 h, there is no difference in ethylene production between vertical and geostimulated plants during this period. The ethylene production of gravistimulated plants rises sharply to a maximum at 24 h, then decreases steeply to almost the control level by 48 h, at which time the rate of upward curvature is diminishing. Neither the addition nor removal of ethylene, nor the addition of inhibitors affecting ethylene-action (AgNO3) or synthesis (DIHB) influence gravitropic bending or internodal extension in excised segments. Although the ethylene synthesis inhibitor BITC slowed down the rate of upward bending, this effect could not be reversed by addition of ethylene. We conclude that the burst in ethylene production that develops in leaf-sheath bases (pulvini) after they have started to curve upwards is not primary to the induction of curvature. We further suggest that ethylene has no major effect or role in the induction of upward bending after gravistimulation. The metabolism of high specific activity gibberellin A1 ([3H]-GA1) in the excised system was assessed during 1, 2 and 4 h of gravistimulation. Changes in endogenous GAs and GA metabolism have been shown previously to be correlated (at the later stages) with gravistimulated bending in intact Avena shoots. The excised segments "leaked" free [3H]-GAs and [3H]-GA glucosyl conjugate-like substances into the bathing medium, and this was a confounding factor. Nevertheless, gravistimulated stem segments, and especially the bottom half of the segment, were significantly less leaky then vertical segments. Thus, just 1 h after gravistimulation, bottom segment halves retained 22% more precursor [3H]-GA1, 36% more free [3H]-GA-like metabolites, and 48% more [3H]-GA glucosyl conjugate-like metabolites than vertical segments. In contrast, the 1 h gravistimulated top halves retained slightly less (1-4%) precursor [3H]-GA1 and free [3H]-GA metabolites, but 21% more [3H]-GA glucosyl conjugate-like radioactivity than vertical segments.

The effects of 3,5-diiodo-4-hydroxybenzoic acid on the oxidation of IAA and auxin-induced ethylene production by cress root segments

In previous research here, 3,5-diiodo-4-hydroxybenzoic acid (DIHB) was shown to promote the elongation of roots of cress (Lepidium sativum) seedlings growing in light, and to inhibit the auxin-induced production of ethylene in this tissue. Although DIHB is a cofactor for the oxidation of indole-3-acetic acid (IAA) by horse-radish peroxidase, it inhibits the decarboxylation of [1-(14)C]IAA by segments excised from cress roots. The inhibition by DIHB of ethylene production by this tissue does not, therefore, arise from a reduction of IAA levels. These findings are discussed in relation to the effects of DIHB on cress root growth.

The effect of certain phenolic acids on the growth and ethylene production of cress seedling roots

Previous studies showed that 3,5-diiodo-4-hydroxybenzoic acid (DIHB) promotes the root growth of cress seedlings (Lepidium sativum) in the light and also inhibits the production of ethylene by this tissue. Studies with structurally related phenolic acids suggest that there is an inverse correlation between these two effects.

The effect of ethylene on root meristems of Pisum sativum and Zea mays

Ethylene at a concentration of 100 μl l(-1) causes a slight increase in the duration of the mitotic cycle in the primary root meristems of both Pisum sativum L. and Zea mays L. This is due to a lengthening of the G 1 phase; other phases of the cycle are unaffected. Autoradiography and microdensitometry show that the rate of (3)H-thymidine incorporation into nuclei of Pisum is maximal when about half the DNA has been replicated, and that ethylene has no effect upon this rate. Ethylene causes a reduction of the number of dividing cells in the root meristem, particularly in Pisum.

Promotion of cress root elongation in white light by 3,5-diiodo-4-hydroxybenzoic acid

The effect of low concentrations of 3,5-diiodo-4-hydroxybenzoic acid (DIHB) in promoting the elongation of light-exposed cress (Lepidium sativum L.) roots has been further examined. Aeration of the DIHB solution in which the roots were grown largely removed the growth promotion. The addition of ethylene or the ethylene precursor methionine to the solution caused marked inhibition of root elongation and this effect was relieved by DIHB.