Implication of Pectic Components in Cell Surface Interactions between Tomato Root Cells and Fusarium oxysporum f. sp. radicis-lycopersici: A Cytochemical Study by Means of a Lectin with Polygalacturonic Acid-Binding Specificity

Aplysia gonad lectin, a polygalacturonic acid-binding lectin isolated from the sea mollusc Aplysia depilans, was complexed to colloidal gold and used for localizing polygalacturonic-acid-containing molecules in tomato root tissues infected with Fusarium oxysporum f. sp. radicis-lycopersici (FORL). Colonization of host tissues by FORL was associated with striking wall modifications including disruption and even loss of middle lamellae. According to the labeling pattern observed in host wall areas adjacent to fungal penetration channels, it is likely that FORL pectolytic enzymes act through localized wall degradation. The release of polygalacturonic acid-rich wall fragments and the accumulation of polygalacturonic acid-containing molecules in some altered phloem cells were frequently observed and considered to be specific host reactions to fungal attack. The heavy deposition of such molecules at strategic sites such as wall oppositions and intercellular spaces provides support to their implication in the plant defense system. The possible interrelation between polygalacturonic acid-containing molecules and other polymers such as lignin and phenolic compounds remains to be investigated further. The role of these molecules in host-pathogen interactions is discussed in relation to plant defense.     

Polyamines, hydroxycinnamoylputrescines, and root formation in leaf explants of tobacco cultivated in vitro: effects of the suicide inhibitors of putrescine synthesis

In vitro formation of roots is obtained directly, without intermediate growth of callus, from foliar explants of a tobacco (Nicotiana tabacum) plant cultured on Murashige and Skoog medium containing IAA. Auxin-induced root formation was accompanied by significant changes in hydroxycinnamoylputrescine levels. Increasing levels were found in leaf explants during the first 14 days in culture; this was followed by a sharp decline after 20 days. Early changes in putrescine conjugates were detected in leaf explants before the visible appearance of roots. An early and transitory accumulation of hydroxycinnamoylputrescines was observed in the roots. Free polyamines (putrescine, spermidine, and spermine) in leaf explants and roots were always at a low level and only small changes in their concentrations were observed, α-dl-difluoromethylarginine and α-dl-difluoromethylornithine, specific, irreversible inhibitors of arginine decarboxylase and ornithine decarboxylase, respectively, inhibited putrescine accumulation and root initiation and reduced the fresh and dry weights of leaf explants. These effects were reversed by free putrescine or hydroxycinnamoylputrescines. The results reported here suggest that hydroxycinnamoylputrescines are associated with root formation. The relationship among free polyamines, hydroxycinnamoylputrescines, cell division, and root formation is discussed.     

Synergistic inhibition of Escherichia coli aspartate transcarbamylase by CTP and UTP: binding studies using continuous-flow dialysis.

The allosteric control of Escherichia coli aspartate transcarbamylase (ATCase) involves feedback inhibition by both CTP and UTP, although it is only in the presence of CTP that UTP appears to inhibit the activity of the enzyme. In order to better understand the parts played by both pyrimidine nucleotides in this synergistic inhibition, binding studies were performed by continuous-flow dialysis and ultracentrifugation methods. The results obtained show that UTP binds to ATCase in the absence of CTP. Nevertheless, this binding does not induce any inhibition unless CTP is present. The mutual influence of CTP and UTP on their respective binding constants suggests that they bind to the same regulatory sites. However, the results obtained cannot be satisfactorily explained by a simple competition between the nucleotides, and it is shown that reciprocal affinity enhancements play a fundamental role. CTP enhances the affinity of UTP for the regulatory sites 80-fold, and conversely, UTP enhances the affinity of CTP 5-fold. Interestingly, the isolated regulatory subunits bind the two pyrimidine nucleotides following the same pattern as the entire enzyme. These observations indicate that the synergistic inhibition mechanism relies entirely on interactions between the two adjacent allosteric sites which belong to the same regulatory dimer.     

The tryptophan residues of aspartate transcarbamylase: site-directed mutagenesis and time-resolved fluorescence spectroscopy.

Aspartate transcarbamylase (EC 2.1.3.2) contains two tryptophan residues in position 209 and 284 of the catalytic chains (c) and no such chromophore in the regulatory chains (r). Thus, as a dodecamer [(c3)2(r2)3] the native enzyme molecule contains 12 tryptophan residues. The present study of the regulatory conformational changes in this enzyme is based on the fluorescence properties of these intrinsic probes. Site-directed mutagenesis was used in order to differentiate the respective contributions of the two tryptophans to the fluorescence properties of the enzyme and to identify the mobility of their environment in the course of the different regulatory processes. Each of these tryptophan residues gives two independent fluorescence decays, suggesting that the catalytic subunit exists in two slightly different conformational states. The binding of the substrate analog N-phosphonacetyl-L-aspartate promotes the same fluorescence signal whether or not the catalytic subunits are associated with the regulatory subunits, suggesting that the substrate-induced conformational change of the catalytic subunit is the essential trigger for the quaternary structure transition involved in cooperativity. The binding of the substrate analog affects mostly the environment of tryptophan 284, while the binding of the activator ATP affects mostly the environment of tryptophan 209, confirming that this activator acts through a mechanism different from that involved in homotropic cooperativity.     

A tomato cDNA inducible by salt stress and abscisic acid: nucleotide sequence and expression pattern

We have characterized a new tomato cDNA, TAS14, inducible by salt stress and abscisic acid (ABA). Its nucleotide sequence predicts an open reading frame coding for a highly hydrophilic and glycine-rich (23.8%) protein of 130 amino acids. Southern blot analysis of tomato DNA suggests that there is one TAS14 structural gene per haploid genome. TAS14 mRNA accumulates in tomato seedlings upon treatment with NaCl, ABA or mannitol. It is also induced in roots, stems and leaves of hydroponically grown tomato plants treated with NaCl or ABA. TAS14 mRNA is not induced by other stress conditions such as cold and wounding. The sequence of the predicted TAS14 protein shows four structural domains similar to the rice RAB21, cotton LEA D11 and barley and maize dehydrin genes.     

A new species of Lernanthropus De Blainville, 1822 (Copepoda: Lernanthropidae) parasitic on Menticirrhus ophicephalus (Jenyns) (Teleostei: Sciaenidae) from the Peruvian coast

Lernanthropus huamani n. sp. (Copepoda: Lernanthropidae), a parasite of the Peruvian sciaenid fish Menticirrhus ophicephalus (Jenyns), is described and illustrated. The new species differs from all other species of Lernanthropus by a combination of characters, including the dorsal plate, legs and other appendages. L. guacoldae Villalba & Fernández, 1984 is considered a synonym of L. pacificus Oliva & Duran, 1982.     

Influence of the Molecular Structure of Steroids on Their Ability to Interrupt Gap Junctional Communication

17β-estradiol propionate was found to reduce the gap junctional communication in a concentration range similar to that of testosterone propionate, in primary cultures of rat Sertoli cells and cardiac myocytes. Uncoupling was reversible on washing out and occurred without concomitant rise in the intracellular calcium concentration. Esterification was a prerequisite for the activity of extracellularly applied steroid compounds (for example, testosterone was ineffective even at external concentrations up to 100 μm, whereas its intracellular application at 1 μm totally interrupted intercellular communication), but their uncoupling efficiency did not depend on the nature of the ester chain nor on its position on the steroid nucleus. The derivatives of two other androgen hormones (derivatives of the androstane nucleus) were also efficient as junctional uncouplers. Among five steroid molecules belonging to the pregnane family, only one (pregnanediol diacetate) interrupted the junctional communication. Neither cholic acid nor cholesteryl acetate or ouabain showed this effect. Altogether, no correlation with the presence or position of double bonds nor with the trans- or cis-fusion of the A and B rings could be recognized. These results suggest that this reversible, nondeleterious uncoupling effect of steroids is independent of the shape of the molecules and is more probably related to their size and liposolubility, that condition their insertion into the lipid bilayer. Their incorporation into the membrane could disturb the activity of the membrane proteins by a physical mechanism. Received: 10 April 1995/Revised: 27 October 1995