Characteristic early membrane effects induced by tryptophan in pulvinar motor cells

Tryptophan at concentrations higher than 0.1 mM, triggered characteristic early physiological effects such as rapid (within 5 min) dose-dependent membrane hyperpolarization in Mimosa pudica motor cells and modification of the time course of the spontaneous proton efflux monitored in the incubation medium of pulvinar tissues. The rapid modifications of the leaf turgor-mediated movements seen on the primary pulvini of M. pudica following a shock and on Cassia fasciculata leaflets during a transition from light to darkness indicate that tryptophan disturbed the ionic migrations involved in the electrophysiological events and in the osmocontractile reaction of the motor cells. These reactions were specific to tryptophan compared to those induced by serine and 5-hydroxytryptophan. The tryptophan mode of action cannot be linked to a direct modification of the plasma membrane H⁺-ATPase activity as monitored on purified pulvinar plasma membrane vesicles. The tryptophan metabolism-linked products tryptamine and indole also inhibited the motile reactions, activated in a continuous manner the H⁺ secretion of pulvinar tissues and showed properties of a protonophore and an ATPase activity inhibitor on plasma membrane vesicles, respectively. The specific behavior of tryptophan in the reaction studies here is discussed in light of the previously reported action of phytohormones.

Cytotoxic allelochemicals induce ultrastructural modifications in Cassia tora L. and mitotic changes in Allium cepa L.: a weed versus weed allelopathy approach

The stress induced by allelochemicals present in stem aqueous extract (SAE) of Nicotiana plumbaginifolia on alterations in growth, ultrastructure on Cassia tora L., and mitotic changes on Allium cepa L. were inspected. Application of SAE at different concentrations (0.5, 1, 2, and 4%) expressively reduced the growth of C. tora in terms of seedling length and dry biomass. Moreover, the ultrastructural variations induced in the epidermis of Cassia leaf (adaxial and abaxial surface) of 15-day-old saplings were analyzed through scanning electron microscopy (SEM). The variations noticed are rupturing and shrinking of cells along epidermis; damaged margins, extensively curled leaf apex along with the appearance of puff-like structures, grooves, and thread-like structures on the leaf surface. The epidermal cells of samples exposed to treatment no longer appear smooth relative to control, besides showing necrosis as well. Upon exposure to different concentrations of extract, A. cepa root tip cells showed aberrations in chromosome arrangement and disparity in the shape of the interphase and prophase nuclei along various phases of mitotic cycle as compared to control. The mitotic index (MI) showed a concentration-dependent decline in onion root tips exposed to SAE. The aberrations appearing frequently were formation of multinucleated cells, sticky metaphase and anaphase with bridges, sticky telophase, disturbed polarity, etc. The results also show the induction of elongated cells, giant cells, and cells with membrane damage by extract treatment. To our knowledge, this is the first gas chromatography-mass spectrometry (GC-MS) analysis of the methanolic extract of N. plumbaginifolia stem. Overall, 62 compounds were reported, covering 99.61% of the entire constituents, which can be considered responsible for the allelopathic suppression of C. tora. The chief component was 4-tert-butylcalix[4]arene with the highest composition of 19.89%, followed by palmitic acid (12.25%), palmitoleic acid (8.23%), precocene 2 (7.53%), isophytyl acetate (4.01%), and betastigmasterol (3.95%).

In vitro formation of the anthranoid scaffold by cell-free extracts from yeast-extract-treated Cassia bicapsularis cell cultures

The anthranoid skeleton is believed to be formed by octaketide synthase (OKS), a member of the type III polyketide synthase (PKS) superfamily. Recombinant OKSs catalyze stepwise condensation of eight acetyl units to form a linear octaketide intermediate which, however, is incorrectly folded and cyclized to give the shunt products SEK4 and SEK4b. Here we report in vitro formation of the anthranoid scaffold by cell-free extracts from yeast-extract-treated Cassia bicapsularis cell cultures. Unlike field- and in vitro-grown shoots which accumulate anthraquinones, cell cultures mainly contained tetrahydroanthracenes, formation of which was increased 2.5-fold by the addition of yeast extract. The elicitor-stimulated accumulation of tetrahydroanthracenes was preceded by an approx. 35-fold increase in OKS activity. Incubation of cell-free extracts from yeast-extract-treated cell cultures with acetyl-CoA and [2-(14)C]malonyl-CoA led to formation of torosachrysone (tetrahydroanthracene) and emodin anthrone, beside two yet unidentified products. No product formation occurred in the absence of acetyl-CoA as starter substrate. To confirm the identities of the enzymatic products, cell-free extracts were incubated with acetyl-CoA and [U-(13)C(3)]malonyl-CoA and (13)C incorporation was analyzed by ESI-MS/MS. Detection of anthranoid biosynthesis in cell-free extracts indicates in vitro cooperation of OKS with a yet unidentified factor or enzyme for octaketide cyclization.

Protecting cell walls from binding aluminum by organic acids contributes to aluminum resistance

Aluminum-induced secretion of organic acids from the root apex has been demonstrated to be one major Al resistance mechanism in plants. However, whether the organic acid concentration is high enough to detoxify Al in the growth medium is frequently questioned. The genotypes of Al-resistant wheat, Cassia tora L. and buckwheat secrete malate, citrate and oxalate, respectively. In the present study we found that at a 35% inhibition of root elongation, the Al activities in the solution were 10, 20, and 50 muM with the corresponding malate, citrate, and oxalate exudation at the rates of 15, 20 and 21 nmol/cm(2) per 12 h, respectively, for the above three plant species. When exogenous organic acids were added to ameliorate Al toxicity, twofold and eightfold higher oxalate and malate concentrations were required to produce the equal effect by citrate. After the root apical cell walls were isolated and preincubated in 1 mM malate, oxalate or citrate solution overnight, the total amount of Al adsorbed to the cell walls all decreased significantly to a similar level, implying that these organic acids own an equal ability to protect the cell walls from binding Al. These findings suggest that protection of cell walls from binding Al by organic acids may contribute significantly to Al resistance.

The emergence of embryos from hard seeds is related to the structure of the cell walls of the micropylar endosperm, and not to endo-beta-mannanase activity

BACKGROUND AND AIMS

Seeds of carob, Chinese senna, date and fenugreek are hard due to thickened endosperm cell walls containing mannan polymers. How the radicle is able penetrate these thickened walls to complete seed germination is not clearly understood. The objective of this study was to determine if radicle emergence is related to the production of endo-beta-mannanase to weaken the mannan-rich cell walls of the surrounding endosperm region, and/or if the endosperm structure itself is such that it is weaker in the region through which the radicle must penetrate.

METHODS

Activity of endo-beta-mannanase in the endosperm and embryo was measured using a gel assay during and following germination, and the structure of the endosperm in juxtaposition to the radicle, and surrounding the cotyledons was determined using fixation, sectioning and light microscopy.

KEY RESULTS

The activity of endo-beta-mannanase, the major enzyme responsible for galactomannan cell wall weakening increased in activity only after emergence of the radicle from the seed. Thickened cell walls were present in the lateral endosperm in the hard-seeded species studied, but there was little to no thickening in the micropylar endosperm except in date seeds. In this species, a ring of thin cells was visible in the micropylar endosperm and surrounding an operculum which was pushed open by the expanding radicle to complete germination.

CONCLUSIONS

The micropylar endosperm presents a lower physical constraint to the completion of germination than the lateral endosperm, and hence its structure is predisposed to permit radicle protrusion.