Hormone phytohormones, plant hormones

These variations in behavior indicate that harvesting melons at different stages of maturity causes subsequent biochemical events involved in amino acid accumulation to follow markedly different pathways. Recent work shows that melon fhiit harvested up to ten days before commercial maturity exhibits climacteric behavior with respect to ethylene production showing that at least this aspect of ripening is not completely inhibited by premature separation from the plant(P). However, the amount of ethylene produced is dependent on maturity at harvest and fruit harvested five days prematurely generated only about half of the amount of ethylene produced by fruit harvested two days before maturity. Also the lag time required to initiate ethylene production after harvest depended on maturity and was longer for prematurely harvested fruit. Changes in the content of the phytohormone abscisic acid were also correlated with that of ethylene. However whether the different maturity related metabolic responses observed above result from the action of these or other plant hormones awaits further study. 

There are Arabidopsis thaliana (ampl, [29]) and Physcomitrella patens (ove, [30]) mutants showing an altered cytokinin accumulation, perhaps due to changes in the biosynthetic pathway. In Arabidopsis ampl the product of the ampl gene, AMPl, is suggested to regulate the isopentenyltransferase-like enzyme and maybe also the hydrolysis of cytokinins from their conjugates. The use of plant hormone mutants in phytohormone research is discussed elsewhere in this issue. 

Among the numerous discoveries that we owe to botanists and pharmacognosts are the development of tryptophan metabolites, and especially indolylacetic acid. This compound acts as growth hormone in plants. Para-chlorinated phenoxyacetic acids (MCPA or methoxone 2,4-D or chloroxone) are mimics of indolylacetic acid (bioisostery) and show similar phytohormonal properties at high doses they serve as weeders. Ring-chlorinated phenoxyacetic acids were later introduced in molecules as varied as meclofenoxate (cerebral metabohsm), clofibrate (Upid metabolism) and ethacrynic acid (diuretic). 

Plant hormones (phytohormones) see plant growth substances. 

For many plant hormones, the use of forward genetic approaches has enabled the identification of the direct target proteins (also known as receptors) and other components of their signaling pathway. Along these lines, researchers screened for plant mutants that were insensitive to the corresponding phytohormone and - after analysis of these mutants - were able to identify key components of the corresponding signaling pathway [2]. 

Aside from possible exposure through drinking water sources, dietary exposure to EDCs results from components of diet known as bioflavanoids, phytohormones, or phytoestrogens. These natural plant hormones are less potent than the estrogens found in mammals and in terms of a mass balance approach it has been suggested that the action of such weak or antiestrogens may work to combat the action of elevated environmental estrogen exposure. Some flavones and isoflavones may play an important role in cancer prevention as they are found in plants that are associated with reduced cancer rates. 

Letham, D. S., Goodwin, T. W., Higgins, T. J. V. (eds.) Phytohormones and Related Compounds, a Comprehensive Treatise, Vols. 1 and 2. Elsevier/North Holland, Amsterdam 1978 Sembdner, G., Gross, D., Liebisch, H.-W., Schneider, G. Biosynthesis and metabolism of plant hormones. In Encyclopedia of Plant Physiology, New Series, Vol. 8 Hormonal Regulation of Development I (J. MacMillan, ed.), pp. 281-444. Springer, Berlin-Heidelberg-New York 1980... 

The problem of selectivity of detection has been a major aspect of plant hormone analysis since the early days of bioassay, as shown by the plethora of studies on naturally occurring auxin synergists and auxin antagonists . Two complementary approaches have been taken to improve the selectivity of detection. The first approach has been the application of high resolution techniques of separation science in order to purify the phytohormone away from interfering compounds. The second approach has been the use of selective detectors following chromatographic purification. The quality of an analytical method is dependent on the interplay of these two factors, resolution and detector selectivity. 

Several different approaches to plant hormone analysis have been proposed and the appropriate method to use for each problem is not always obvious. One of the major problems in the field of development of analytical methods for lAA has been the failure to establish reference methods for the evaluation of new techniques. It was consideration of this situation that lead us to produce a multiple labeled, highly enriched, C-IAA and to develop methods for its use in routine analysis [11]. We feel that our current methods, using GC-MS, meet six important criteria 1) they are applicable to a wide variety of plant tissues with only minor modification, 2) they are highly selective in that ions specific to the target molecule are selected, 3) by use of multiple ion pairs they are self validating, 4) both in terms of tissue sample size and levels of detection these GC-MS techniques offer high sensitivity, 5) the techniques are rapid, simple, and easy to teach to those not trained in the problems of phytohormone analysis, and 6) they are absolute methods, thus requiring little complex standardization or calibration. 

Since the pattern of hormone activity in the developing fruits of seeded and naturally parthenocarpic fruit is similar and only one phytohormone treatment is necessary in the chemically induced parthenocarpic fruits, it seems most probable some phytohormones act as inducing agents starting the filling of metabolic sinks , while others like abscisic and jasmonic acids stop protoplasmic growth (Dathe et al. 1981). Thus the role of plant hormones in fruit development remains obscure. 

Among known plant hormones, only auxin exhibits basipetally polar transport. Auxin transport has a striking polarity which mirrors the structural and functional axial polarity of the plant body. Thus, even though other phytohormones are known to cause morphogenic effects, a prominent (and probably dominant) role in the spatial and temporal control of plant morphogenesis must be ascribed to auxin and especially to the still obscure processes comprising the mechanisms of its basipetal transport. 

Phytohormones are substances that, at low concentration, function to coordinate plant growth and development. The compormds that have been considered as plant hormones include indole-3-acetic acid (auxin), cytokinins, gibberelhns (GA), ethylene and abscisic acid (ABA). In addition, brassinosteroids, jasmonic acid (JA) and saUcyhc acid (SA) have been shown to display important growth regulating activities and are also considered to function as phytohormones. 

---