Involvement of Macromolecule Biosynthesis in Auxin and Fusicoccin Enhancement of β-Glucan Synthase Activity in Pea

Abstract

In pea stem segments whose cuticle has been made permeable by abrading it, actinomycin D (ActD) and 80S ribosomal protein synthesis inhibitors such as cycloheximide (CHI) inhibit enhancement by indole 3-acetic acid (IAA) of the activity of the cell wall biosynthetic enzyme, glucan synthase I (GS). This supersedes earlier, negative results with inhibitors, obtained with segments having an intact cuticle, which prevents adequate inhibitor uptake. Since these inhibitors also block IAAstimulated H' extrusion, which according to earlier results is involved in the GS response, the significance of these inhibitions would be ambiguous without additional evidence. ActD does not inhibit fusicoccin (FC) enhancement of GS activity, which indicates existence of a post-trnscriptional control mechanism for GS, but does not preclude involvement of transcription in the GS response to IAA. Although protein synthesis inhibitors such as CHI do not block FC-stimulated H' extrusion, they do inhibit FC enhancement of GS activity, indicating an involvement of protein synthesis in the GS response to FC, and presumably also to LAA. However, protein synthesis inhibitors (but not ActD) by themselves paradoxically elevate GS activity, less strongly than IAA does but resembling the UIA enhancement in several characteristics. These results suggest that IAA may enhance GS activity at least in part by inhibiting the synthesis or action of a labile repressor of the transcription of, or a labile destabilizer of, mRNA for GS or some polypeptide that enhances GS activity. However, resemblances between the IAA and FC effects on GS suggest that UIA also has a posttanscriptional GS-enhancing action like that of FC. Lipid biosynthesis may be involved in this aspect of the response since both IAA and FC enhancements of GS activity are inhibited by cerulenin.IAA causes a rapid increase in GS2 activity, in pea third internode tissue segments whose GS activity has declined after isolation from the plant (16). This response, which probably contributes to the auxin stimulation of cell wall synthesis (1) extrusion plays a role in this enhancement (18). To study this I used pea stem segments abraded with emergy particle suspension to make their cuticle H+-permeable, so that IAA-and FCstimulated H+ extrusion could be measured (18). With such tissue I obtained effects of protein and RNA synthesis inhibitors on the GS activity response to auxin that differed importantly from the results I had previously obtained with unabraded tissue. This prompted further investigation of apects of metabolism involved in the GS response to auxin and FC, the outcome of which is presented here. MATERIALS AND METHODSEtiolated pea seedlings, Pisum sativum cv Alaska, were grown, and third internode segments 8 mm long were cut from them, as described previously (1), except that unless stated otherwise the third internode was always abraded with emergy powder paste as described (18) and the internode was rinsed by vigorous agitation in two changes oftap water prior to cutting t...

Hormonal and Gravitropic Specificity in the Regulation of Growth and Cell Wall Synthesis in Pulvini and Internodes from Shoots of Avena sativa L. (Oat)

“…The stimulation of pulvinar cell wall synthesis by FC is similar to the findings of Brummell and Hall (1983) in elongating, auxin-responsive pea stem segments, in which the effect is apparently mediated at least in part by increased activity of glucan synthase (Ray, 1985(Ray, ,1987. In the present work, FC stimulated incorporation predominately into the matrix (TFA soluble) component of the wall, whereas IAA and gravistimulus promoted incorporation proportionally into both fractions (Figs.…”

Further characterization of auxin-regulated mRNAs in hypocotyl sections of mung bean [Vigna radiata (L.) Wilczek]: Sequence homology to genes for fatty-acid desaturases and atypical late-embryogenesis-abundant protein, and the mode of expression of the mRNAs

“…The auxininduced increase in the m R N A for fatty-acid desaturase that I observe might be related to membrane flow. It is also of interest that cerulenin, an inhibitor of the biosynthesis of fatty acids, inhibits the auxin-induced elonga-tion of sections of oat coleoptiles or etiolated pea epicotyls (Cleland 1982;Theologis et al 1985;Ray 1987;Theologis 1987). Metabolism of fatty acids in general, as well as their desaturation, may be an important element of the molecular machinery of elongation (Theologis 1987;Hager et al 1991).…”

“…Therefore, an examination was made of the kinetics of induction and the distribution in seedlings of the mRNAs, as well as of the effects of a phytotoxin, fusicoccin (FC), by Northern blot analysis. Fusicoccin induces elongation of plant stems through a mechanism different from that of auxin (Cleland 1982;Ray 1987;Theologis 1987;Hager et al 1991). Comparisons of the effects of auxin and FC on gene expression may provide important insights into the molecular machinery of the action of auxin.…”