GH3 Auxin-Amido Synthetases Alter the Ratio of Indole-3-Acetic Acid and Phenylacetic Acid in Arabidopsis

Auxin is the first discovered plant hormone and is essential for many aspects of plant growth and development. Indole-3-acetic acid (IAA) is the main auxin and plays pivotal roles in intercellular communication through polar auxin transport. Phenylacetic acid (PAA) is another natural auxin that does not show polar movement. Although a wide range of species have been shown to produce PAA, its biosynthesis, inactivation and physiological significance in plants are largely unknown. In this study, we demonstrate that overexpression of the CYP79A2 gene, which is involved in benzylglucosinolate synthesis, remarkably increased the levels of PAA and enhanced lateral root formation in Arabidopsis. This coincided with a significant reduction in the levels of IAA. The results from auxin metabolite quantification suggest that the PAA-dependent induction of GRETCHEN HAGEN 3 (GH3) genes, which encode auxin-amido synthetases, promote the inactivation of IAA. Similarly, an increase in IAA synthesis, via the indole-3-acetaldoxime pathway, significantly reduced the levels of PAA. The same adjustment of IAA and PAA levels was also observed by applying each auxin to wild-type plants. These results show that GH3 auxin-amido synthetases can alter the ratio of IAA and PAA in plant growth and development.

Paleomagnetic results from three rock units of New Brunswick and their bearing on the Lower Paleozoic tectonics of North America

Thermal, alternating field (AF), and two-stage (AF followed by thermal) experiments performed on one Silurian (Mascarene Group) and two Devonian (St. George and St. Stephen plutons) rock units indicate that considerable polar movement relative to New Brunswick took place in Siluro-Devonian times. The Mascarene Group yields a pole at 122°E, 02°S, the St. Stephen pluton, a pole at 136°E, 43°S, and the St. George pluton, two poles at 085°E, 38°S and 093°E, 29°N respectively. Although the southern poles lie some 70° south of most North American Silurian and Devonian poles, they are in agreement with poles obtained from two recent studies of Newfoundland rock units. It is unknown if this apparent polar displacement is representative of the entire North American craton or of its eastern margin only. A graphic representation of all available North American data is disconcerting and shows that little is known about the behaviour of the Lower Paleozoic field relative to North America and that many more data are needed to reconstruct the paleomagnetic record.The St. George pluton carries two dual polarity remanences which often are not easily discernible, especially since one of the remanences has an intensity much weaker than the other. The usefulness of vector diagrams for the analysis of multicomponent remanences is demonstrated. Examples are shown where the dual polarity of the weaker remanence, which might otherwise escape notice, can be readily detected on enlarged scale vector diagrams.

Nonpolar movement of N6-benzyladenine-14C in coleoptile, stem, petiole, and floral organ sections

Movement of N6-benzyladenine-methylene-14C in Avena sativa coleoptiles, Colens blumei stems and petioles, and Cleome hassleriana stamen filaments, gynophores, and pedicels was studied by suspending sections horizontally between donor and receiver agar cylinders and determining radioactivity in receivers by scintillation counting. No polar movement was found in any of the plant organs. In time-course experiments using oat coleoptiles, the amount of radioactivity in receivers continued to increase for 24 h and the velocity was 1.5-2 mm/h, suggesting movement by passive diffusion. More radioactivity moved through stamen filaments and gynophore sections from mature expanded flowers than through those from young buds, apparently as a result of larger uptake and exit areas in expanded flowers. A significantly greater acropetal and basipetal movement through young pedicels is not due to area differences and probably results from a metabolic difference.

The Great Logan Paleomagnetic Loop — The Polar Wandering Path from Canadian Shield Rocks During the Neohelikian Era

Normally magnetized dikes and reversely magnetized sills of Neohelikian age near the northwest shore of Lake Superior form two distinct paleomagnetic groups with mean pole positions of 179° W, 35° N, and 140° W, 47° N respectively. Thermal and alternating field demagnetization and the study of magnetic properties and opaque minerals indicate that directions of magnetization of these rocks were acquired at the time of their intrusion. Field evidence suggests that the sills are older than the dikes and radiogenic age determinations indicate that the intrusion of both occurred between 1000 and 1100 m.y. ago.These pole positions, together with those for the Franklin intrusions pole at 167° E, 08° N (675 m.y.), the Abitibi dikes, at 134° W, 27° N (1150 m.y.), and the Mackenzie igneous events, at 171° W, 4° N (1200 m.y.) are used to define the Logan Loop, the path that the pole took in Neohelikian time relative to the Canadian Shield. Other poles well defined magnetically, but less well dated, from rocks of this era, fit the curve quite well.Analysis of available data supports the hypothesis that the relative polar movement that gave rise to the Logan Loop was preceded and followed by polar stability vis à vis North America, whereas polar movement may have been quite rapid during the formation of the loop, which coincided with the time of the Grenvillian orogeny. Comparison of the depositional environment of Neohelikian rocks of the Canadian Shield with their probable paleolatitude as indicated by the 5 key points on Logan Loop should provide a test for the validity of this movement.