Transcriptional regulation of gibberellin metabolism genes by auxin signaling in Arabidopsis

Auxin and gibberellins (GAs) overlap in the regulation of multiple aspects of plant development, such as root growth and organ expansion. This coincidence raises questions about whether these two hormones interact to regulate common targets and what type of interaction occurs in each case. Auxins induce GA biosynthesis in a range of plant species. We have undertaken a detailed analysis of the auxin regulation of expression of Arabidopsis (Arabidopsis thaliana) genes encoding GA 20-oxidases and GA 3-oxidases involved in GA biosynthesis, and GA 2-oxidases involved in GA inactivation. Our results show that auxin differentially up-regulates the expression of various genes involved in GA metabolism, in particular several AtGA20ox and AtGA2ox genes. Up-regulation occurred very quickly after auxin application; the response was mimicked by incubations with the protein synthesis inhibitor cycloheximide and was blocked by treatments with the proteasome inhibitor MG132. The effects of auxin treatment reflect endogenous regulation because equivalent changes in gene expression were observed in the auxin overproducer mutant yucca. The results suggest direct regulation of the expression of GA metabolism genes by Aux/IAA and ARF proteins. The physiological relevance of this regulation is supported by the observation that the phenotype of certain gain-of-function Aux/IAA alleles could be alleviated by GA application, which suggests that changes in GA metabolism mediate part of auxin action during development.     

The interaction of light quality and irradiance with gibberellins, cytokinins and auxin in regulating growth of Helianthus annuus hypocotyls

A reduced red to far-red (R/FR) light ratio and low photosynthetically active radiation (PAR) irradiance are both strong signals for inducing etiolation growth of plant stems. Under natural field conditions, plants can be exposed to either a reduced R/FR ratio or lower PAR, or to a combination of both. We used Helianthus annuus L., the sunflower, to study the effect of reduced R/FR ratio, low PAR or their combination on hypocotyl elongation. To accomplish this, we attempted to uncouple light quality from light irradiance as factors controlling hypocotyl elongation. We measured alterations in the levels of endogenous gibberellins (GAs), cytokinins (CKs) and the auxin indole-3-acetic acid (IAA), and the effect of exogenous hormones on hypocotyl growth. As expected, both reduced R/FR ratio and lower PAR can significantly promote sunflower hypocotyl elongation when given separately. However, providing the reduced R/FR ratio at a low PAR resulted in the greatest hypocotyl growth, and this was accompanied by significantly higher levels of endogenous IAA, GA1, GA8, GA20 and of a wide range of CKs. Providing a reduced R/FR ratio under normal PAR also significantly increased growth and again gave significantly higher levels of endogenous IAA, GAs and CKs. However, only under the de-etiolating influence of a normal R/FR ratio did lowering PAR significantly increase levels of GA1, GA8 and GA20. We thus conclude that light quality (e.g. the R/FR ratio) is the most important component of shade for controlling hypocotyl growth and elevated growth hormone content.     

Role of auxin and gibberellin in differentiation of primary Phloem fibers

The hypothesis that auxin and gibberellic acid (GA₃) control the differentiation of primary phloem fibers is confirmed for the stem of Coleus blumei Benth. Indoleacetic acid (IAA) alone sufficed to cause the differentiation of a few primary phloem fibers. In long term experiments auxin induced a considerable number of fibers in mature internodes. GA₃ by itself did not exert any effect on fiber differentiation. Combinatiosn of IAA with GA₃ completely replaced the role of the leaves in primary phloem fiber differentiation qualitatively and quantitatively. Although the combined effect of the two growth hormones diminished considerably with increasing distance from the source of induction, auxin with GA₃ or IAA alone induced fibers in a few internodes below the application site. When various combinations of both hormones were applied, high concentrations of IAA stimulated rapid differentiation of fibers with thick secondary walls, while high levels of GA₃ resulted in long fibers with thin walls. The size of the primary phloem fibers correlated with the dimensions of the differentiating internode, thereby providing evidence that both growth regulators figure in the control of stem extension. High IAA/low GA₃ concentrations have an inhibitory effect on internode elongation, whereas low IAA/high GA₃ concentrations promote maximal stem elongation.     

Genetic Dissection of the Relative Roles of Auxin and Gibberellin in the Regulation of Stem Elongation in Intact Light-Grown Peas

Exogenous gibberellin (GA) and auxin (indoleacetic acid [IAA]) strongly stimulated stem elongation in dwarf GA1-deficient le mutants of light-grown pea (Pisum sativum L.): IAA elicited a sharp increase in growth rate after 20 min followed by a slow decline; the GA response had a longer lag (3 h) and growth increased gradually with time. These responses were additive. The effect of GA was mainly in internodes less than 25% expanded, whereas that of IAA was in the older, elongating internodes. IAA stimulated growth by cell extension; GA stimulated growth by an increase in cell length and cell number. Dwarf lkb GA-response-mutant plants elongated poorly in response to GA (accounted for by an increase in cell number) but were very responsive to IAA. GA produced a substantial elongation in lkb plants only in the presence of IAA. Because lkb plants contain low levels of IAA, growth suppression in dwarf lkb mutants seems to be due to a deficiency in endogenous auxin. GA may enhance the auxin induction of cell elongation but cannot promote elongation in the absence of auxin. The effect of GA may, in part, be mediated by auxin. Auxin and GA control separate processes that together contribute to stem elongation. A deficiency in either leads to a dwarfed phenotype.     

The role of auxin in growth of the plumular hook section of etiolated pea seedling

Externally applied GA greatly promoted elongation of the plumularhook section of the etiolated Alaska pea seedling, but IAA hadno such effect when given either alone or with GA. PCIB inhibitedelongation of the plumular hook section both in the presenceand absence of applied GA. The PCIB effect in the absence ofGA was partially overcome by IAA, but not completely. On theother hand, the PCIB effect in the presence of GA was completelyovercome by IAA. No antagonic response was, however, obtainedbetween GA and PCIB. CCC also retarded elongation of the sectionand this inhibition was completely overcome by GA, but not byIAA. There was little difference in the amount of endogenous auxindetectable in GA treated and untreated sections. These resultssuggest that auxin is necessary for the growth of both GA treatedand untreated plumular hook sections and that auxin and gibberellinact differently on the growth of the section. (Received April 24, 1968; )     

Auxin promotes gibberellin biosynthesis in decapitated tobacco plants

Excision of the apical bud (decapitation) of tobacco (Nicotiana tabacum L.) plants reduced the endogenous levels of indole-3-acetic acid (IAA), gibberellin A20 (GA20), and GA1 (the bioactive GA), in internode tissue below the excision site. Application of IAA to the stump of decapitated plants dramatically increased GA20 content, to a level 3-fold greater than in intact plants. Gibberellin A1 content was also increased by IAA. Decapitation reduced the conversion of [14C]GA19 to [14C]GA20 and of [14C]GA20 to [14C]GA1, and appeared to promote the deactivation pathway [14C]GA20 to [14C]GA29 to [14C]GA29-catabolite. Application of auxin counteracted these effects, but did not restore the conversion of [14C]GA20 to [14C]GA1 to the level found in intact plants. The results indicate that auxin is necessary for normal GA biosynthesis in stems of tobacco.     

Stimulation of Pisum sativum epicotyl elongation by gibberellin and auxin. – Different effects of two hormones on osmoregulation and cell walls

The possible involvement of auxin in the action of gibberellin in stimulating cell elongation was examined by comparing the effects of gibberellic acid (GA) and IAA on the growth, osmoregulation and cell wall properties of the Alaska pea ( Pisum sativum L. cv. Alaska) subhook. Both GA and IAA stimulated cell elongation in the subhook region of derooted cuttings. Cotyledon excision decreased the stimulating effect of GA on the growth of the subhook region, but did not affect that of IAA. As the subhook region elongated, the osmotic potential of the cell sap and the total amount of osmotic solutes increased. Cotyledon excision accelerated the increase in the osmotic potential and suppressed the accumulation of osmotic solutes. In cuttings with cotyledons. GA partly counteracted the increase in the osmotic potential and substantially promoted the accumulation of osmotic solutes. On the other hand, in cuttings without cotyledons. GA did not affect the change in the osmotic potential although it slightly promoted the accumulation of osmotic solutes. IAA accelerated the increase in the osmotic potential, but did not affect the accumulation of osmotic solutes. IAA enhanced the extensibility of the cell wall, while GA did not affect it. These results suggest that at least in the Alaksa pea subhook region. GA does not stimulate cell elongation by affecting the level of auxin.     

Gravistimulation leads to asymmetry of both auxin and gibberellin levels in barley pulvini

The auxin indole-3-acetic acid (IAA) is known to promote the biosynthesis of active gibberellins (GAs) in barley ( Hordeum vulgare ). We therefore investigated the possibility that this interaction might contribute to the gravitropic response of barley leaf sheath pulvini. Barley plants at the inflorescence stage were gravistimulated for varying times, and the pulvini were then separated into upper and lower halves for quantification of IAA and GAs by GC-MS. Consistent with the Cholodny–Went theory, the lower portion contained more IAA than did the upper portion. This difference was detected as early as 2.5 h after the start of gravistimulation, and bending was also observed at this stage. At later time points tested (6 h and 24 h), but not at 2.5 h or 3 h, the higher auxin content of the lower half was associated with a higher level of GA₁, the main bioactive GA in barley. Consistent with that result, the expression of Hv3ox2 , which encodes a key enzyme for the conversion of GA₂₀ to GA₁, was higher in the lower side than in the upper, after 6 h. It is suggested that in gravistimulated leaf sheath pulvini, auxin accumulates in the lower side, leading to a higher level of GA₁, which contributes to the bending response. Further evidence that GAs play a role in the gravitropic response was obtained from GA-related mutants, including the elongated sln1c mutant, in which GA signalling is constitutive. Pulvinar bending in the sln1c mutant was greater than in the wild-type. This result indicates that in the lower side of the gravistimulated pulvinus, the relatively high level of bioactive GA facilitates, but does not mediate, the bending response.     

Fertilization-dependent auxin response in ovules triggers fruit development through the modulation of gibberellin metabolism in Arabidopsis

Fruit development is usually triggered by ovule fertilization, and it requires coordination between seed development and the growth and differentiation of the ovary to host the seeds. Hormones are known to synchronize these two processes, but the role of each hormone, and the mechanism by which they interact, are still unknown. Here we show that auxin and gibberellins (GAs) act in a hierarchical scheme. The synthetic reporter construct DR5:GFP showed that fertilization triggered an increase in auxin response in the ovules, which could be mimicked by blocking polar auxin transport. As the application of GAs did not affect auxin response, the most likely sequence of events after fertilization involves auxin-mediated activation of GA synthesis. We have confirmed this, and have shown that GA biosynthesis upon fertilization is localized specifically in the fertilized ovules. Furthermore, auxin treatment caused changes in the expression of GA biosynthetic genes similar to those triggered by fertilization, and also restricted to the ovules. Finally, GA signaling was activated in ovules and valves, as shown by the rapid downregulation of the fusion protein RGA-GFP after pollination and auxin treatment. Taken together, this evidence suggests a model in which fertilization would trigger an auxin-mediated promotion of GA synthesis specifically in the ovule. The GAs synthesized in the ovules would be then transported to the valves to promote GA signaling and thus coordinate growth of the silique.     

Developmental and genotypic differences in the response of pea stem segments to auxin

The objective of this investigation was to examine the response to exogenous auxin (indole-3-acetic acid; IAA)of stem segments at two developmental stages. The standard auxin response of excised stem segments and intact plants consists of an initial growth response and a prolonged growth response. We found that this biphasic response does not occur in internodes at very early stages. Stem segments of light grown pea of various genotypes were cut when the fourth internode was at 6–13% of full expansion (early-expansion) or at 18–25% of full expansion (mid-expansion). Length measurements of excised segments were made after 48 hours of incubation on buffer with or without auxin. An angular position transducer linked to a computerized data collection system provided high-resolution measurement of growth of stacks of segments incubated in buffer over 20 hours. Early-expansion segments of all genotypes deviated from the standard auxin response, while mid-expansion segments responded in a manner consistent with previous reports. Early-expansion segments of tall, light-grown plants were unique in showing an auxin-induced inhibition of growth. The auxin-induced inhibition correlated with high endogenous auxin content, as determined by HPLC and GC/MS, across genotypes and between early-expansion and mid-expansion segments of tall plants. Measurement of ethylene evolved from stem segments in response to auxin, and treatment of segments with the ethylene action inhibitor, norbornadiene, showed the inhibition to be mediated in part by heightened ethylene sensitivity. Growth of early-expansion segments of dwarf and severe dwarf plants was stimulated by exogenous auxin, but the growth rate increase was delayed compared to that in mid-expansion segments. This is the first time that such a growth response, termed the delayed growth response has been emonstrated. It is concluded that developmental stage and endogenous hormone content affect tissue response to exogenous auxin.     

The interaction of light irradiance with auxin in regulating growth of <Emphasis Type="Italic">Helianthus annuus</Emphasis> 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.