Aga Schulze

Photo-regulation of the ratio of ester to free indole-3-acetic acid.

Abstract A light exposure, sufficient to cause a 30% reduction in growth rate of seedlings of Zea mays, causes a decrease of 40% in the concentration of free indole-3-acetic acid in the seedling and an increase in the content of esterified indole-3-acetic acid. We conclude that one mechanism for regulation of plant growth is alteration of the ratio of free to conjugated hormone by environmental stimuli.

Estimation of indole-3-acetic acid in gametophytes of the moss, Physcomitrella patens

By means of gas chromatography-selected ion monitoring-mass spectrometry using an isotope-dilution assay with 4,5,6,7-tetradeutero-indole-3-acetic acid as the internal standard, indole-3-acetic acid has been estimated to be present in aseptically cultured gametophytes of wild-type Physcomitrella patens (Hedw.) B.S.G. at a level of 0.075 μg g−1 dry weight or 2.1 ng g−1 fresh weight.

Double-standard isotope dilution assay: I. Quantitative assay of indole-3-acetic acid

Abstract Isotope dilution analysis for the quantitation of labile compounds has been limited in applicability by the amount of sample necessary to determine specific activity. A method is described for the analysis of radiolabeled compounds which allows the direct determination of specific activity by gas chromatography. It requires the availability of the radiolabeled internal standard, as is customarily used in an isotope dilution assay, and also requires a chemically related radiolabeled compound to serve as a second internal standard. It is this second internal standard, added in known amounts, that permits quantitation of the gas chromatography. The method is illustrated by assaying indole-3-acetic acid in plant extracts using [ 14 C]indole-3-acetic acid as the internal standard and adding [ 14 C]indole-3-butyric acid as the second internal standard for quantitation of the gas chromatographic procedures. Used with a nitrogen-specific thermionic detector the method is selective and is sensitive at the nanogram level. The synthesis of [2- ring - 14 C]indole-3-butyric acid is also described.

Possible effects of organelle charge and density on cell metabolism

To respond to gravity a biological system must: First, perceive the stimulus; and, second transduce the stimulus into an appropriate response. This laboratory has studied a system of perception and transduction involving the gravity-induced asymmetric distribution of a plant growth hormone. From these studies we have developed a working theory which states as its postulates that: a) The perception of the gravitational stimulus involved a perturbation of the plants bio-electric field; and b) that the transduction of the stimulus involved voltage-gating of hormone movement from the plants vascular tissue into the hormone responsive growing tissue. These studies may provide the simplest system for studying [correction of studing] the mechanism whereby the gravity signal is translated into a biological response.

Genetics, chemistry, and biochemical physiology in the study of hormonal homeostasis

More than one hundred years ago, on October 11, 1878, the then 26 year old Professor of Chemistry, Jacobus Henricus van ‘t Hoff, gave his inaugural lecture in the Auditorium Maximum of the newly endowed State University of Amsterdam. His lecture was entitled, “Imagination in Science” (Springer, 1967). We may surmise that van ‘t Hoff’s imagination was not coincident with that of his peers. A colleague described van ‘t Hoff’s work as follows: “This natural philosophy, which had been put aside by exact science, is at present being dragged out by pseudo scientists from the junk-room which harbors such failings of the human mind, as is dressed up in modem fashion and rouged freshly like a whore whom one tries to smuggle into good society where she does not belong.” In a similar vein, in 1871, a few years before van ‘t Hoff’s lecture, Theophil Ciesielski published his Ph.D. thesis demonstrating that something was secreted by the root tip which controlled the growth of the elongation zone-discovering in this manner, the plant growth hormone we now call auxin (Ciesielski, 1872). Ciesielski, like van ‘t Hoff, was disagreed with, in this case by the all powerful Sachs, and Ciesielski’s work received little recognition (Heslop-Harrison, 1980). Were it not for the efforts of the prestigious Charles Darwin, even the notion of a plant hormone might not have survived.

The mechanism by which an asymmetric distribution of plant growth hormone is attained.

Zea mays (sweet corn) seedlings attain an asymmetric distribution of the growth hormone indole-3-acetic acid (IAA) within 3 minutes following a gravity stimulus. Both free and esterified IAA (that is total IAA) accumulate to a greater extent in the lower half of the mesocotyl cortex of a horizontally placed seedling than in the upper half. Thus, changes in the ratio of free IAA to ester IAA cannot account for the asymmetric distribution. Our studies demonstrate there is no de novo synthesis of IAA in young seedlings. We conclude that asymmetric IAA distribution is attained by a gravity-induced, potential-regulated gating of the movement of IAA from kernel to shoot and from stele to cortex. As a working theory, which we call the Potential Gating Theory, we propose that perturbation of the plants bioelectric field, induced by gravity, causes opening and closing of transport channels in the plasmodesmata connecting the vascular stele to the surrounding cortical tissues. This results in asymmetric growth hormone distribution which results in the asymmetric growth characteristics of the gravitropic response.

Gravitational effects on plant growth hormone concentration

Numerous studies, particularly those of H. Dolk in the 1930s, established by means of bio-assay, that more growth hormone diffused from the lower, than from the upper side of a gravity-stimulated plant shoot. Now, using an isotope dilution assay, with 4,5,6,7 tetradeutero indole-3-acetic acid as internal standard, and selected ion monitoring-gas chromatography-mass spectrometry as the method of determination, we have confirmed Dolks finding and established that the asymmetrically distributed hormone is, in fact, indole-3-acetic acid (IAA). This is the first physico-chemical demonstration that there is more free IAA on the lower sides of a geo-stimulated plant shoot. We have also shown that free IAA occurs primarily in the conductive vascular tissues of the shoot, whereas IAA esters predominate in the growing cortical cells. Now, using an especially sensitive gas chromatographic isotope dilution assay we have found that the hormone asymmetry also occurs in the non-vascular tissue. Currently, efforts are directed to developing isotope dilution assays, with picogram sensitivity, to determine how this asymmetry of IAA distribution is attained so as to better understand how the plant perceives the geo-stimulus.