Antonio Graniti

Phytotoxins and Plant Pathogenesis

In a broad sense, “phytotoxins” can be considered as microbial metabolites, other than enzymes, that damage or are harmful to plants at very low concentrations (1–3, 23, 25). Many plant pathogenic bacteria and fungi produce phytotoxins both in culture and in their hosts during the infection process. In several cases — especially if they are produced during the early stages of plant disease development — these compounds have a function in pathogenesis and cause part or even all of the symptoms of the disease (4, 5, 20). For microbial products which are not phytotoxic per se but have a role in pathogenesis, the term aggressions has been proposed (1).

Phytotoxins and their involvement in plant diseases. Introduction

Bacterial and fungal plant pathogens are known to produce diffusible toxins in infected plants. These phytotoxins are harmful to plants in very low concentrations, and many reproduce at least some of the symptoms of the relevant bacterial or fungal disease. Several phytotoxins have been shown to be involved in pathogenesis. Recent years have seen substantial progress in our knowledge about the nature, structure and mode of action of phytotoxins, and this is briefly reviewed. Finally, possible applications in fields other than plant pathology are mentioned.

Infection of grapevines by some fungi associated with esca. II. Interaction among Phaeoacremonium chlamydosporum, P. aleophilum and Fomitiporia punctata

Cross-inoculation experiments were designed to study the interaction among the three fungi, Phaeoacremonium chlamydosporum (Pch), P. aleophilum (Pal) and Fomitiporia punctata (Fop) most commonly associated with esca of grapevine. The experiments were carried out in southern Italy on grapevines cv. Italia and Matilde, and the inoculations were performed in January 1999 by infecting trunks (cv. Italia) and branches (cv. Matilde) through wounds. Fop, Pch and Pal were inoculated singly or in groups of two or three in all possible combinations. Pch, Pal or both were inoculated above or below the inoculation site of Fop. From the results obtained in the first eight months of experiments planned for three years, the following conclusions can be drawn. All fungi inoculated singly spread along the woody tissue and caused brown streaking downward and upward along the trunk and branches of inoculated vines. On the basis of internal symptoms, vines cv. Italia were more susceptible than those cv. Matilde; in particular, the brown wood-streaking induced by P. aleophilum was much more severe on cv. Italia. Co-infection with the two tracheiphilous species of Phaeoacremonium caused wood gummosis and discoloration, whereas the lignicolous basidiomycete F. punctata alone caused wood deterioration and decay (white rot). Severity of internal symptoms, assessed as extent and colour of the discoloured wood, varied with the growth and interaction of the inoculated fungi. The wood discoloration caused by F. punctata was not hampered by P. chlamydosporum, but it was always limited by P. aleophilum. A similar interaction was observed in vitro with cultures of the three fungi together, which showed a marked antagonistic effect of P. aleophilum against F. punctata. Although not consistent, foliar symptoms (interveinal and marginal chlorotic areas) developed within six months on ‘Matilde’ vines co-inoculated with either species of Phaeoacremonium, or within three months after a syringe containing a liquid culture of P. chlamydosporum was inserted into current-season ‘Italia’ shoots.

Three-Year Observation of Grapevines Cross-Inoculated with Esca-Associated Fungi

Cross-inoculation experiments were carried out in southern Italy (Apulia) on grapevines cv. Italia and Matilde in order to study interactions among the white rot fungus Fomitiporia punctata (Fop) and two tracheiphilous hyphomycetes, Phaeoacremonium aleophilum (Pal) and Phaeomoniella chlamydospora (Pch), commonly associated with esca and brown wood-streaking of grapevine. In January 1999 the three fungi were inoculated through wounds on the trunks and spurs of cv. Italia, and branches and spurs of cv. Matilde. Pch, Pal and Fop were inoculated singly or in groups of two or three in all possible combinations. From the results so far obtained, the following conclusions can be drawn: 1. all three fungi are pathogenic; 2. inoculation of the above fungi on standing grapevines produces symptoms on trunks, shoots, leaves and berries that are reminiscent of those shown by esca-affected vines in the vineyard. In particular, symptoms in the wood (brown streaking, gummosis or “black goo”, and wood decay or white rot), and on the leaves (various forms of chlorosis, reddening and necrosis, distortion of the lamina) and berries (black measles) were reproduced; 3. severity of internal symptoms, assessed as length of brown streaks and intensity of colour change of the infected wood, varied with the growth and interactions between the inoculated fungi; 4. the wood streaking which developed after inoculation with Pal or Pch on the spurs extended to the supporting shoot or branch, and even to the stem; 5. a non-synergistic, competitive association of the two hyphomycetes, and an inhibition of Fop growth by Pal was observed in planta. A similar marked antagonistic effect of Pal against Fop was previously shown to occur when examining interactions between the three fungi in vitro; 6. the wood discolouration caused by Fop was not hampered by Pch, but it is always limited by Pal; 7. on the basis of internal symptoms, grapevines cv. Italia were more susceptible to the esca fungi than grapevines cv. Matilde.

A new syringopeptin produced by bean strains of Pseudomonas syringae pv. syringae.

Two strains (B728a and Y37) of the phytopathogenic bacterium Pseudomonas syringae pv. syringae isolated from bean (Phaseolus vulgaris) plants were shown to produce in culture both syringomycin, a lipodepsinonapeptide secreted by the majority of the strains of the bacterium, and a new form of syringopeptin, SP(22)Phv. The structure of the latter metabolite was elucidated by the combined use of mass spectrometry (MS), nuclear magnetic resonance (NMR) spectroscopy and chemical procedures. Comparative phytotoxic and antimicrobial assays showed that SP(22)Phv did not differ substantially from the previously characterized syringopeptin 22 (SP(22)) as far as toxicity to plants was concerned, but was less active in inhibiting the growth of the test fungi Rhodotorula pilimanae and Geotrichum candidum and of the Gram-positive bacterium Bacillus megaterium.

Infection of Grapevines by Some Fungi Associated with Esca. I. "Fomitiporia punctata" as a Wood-Rot Inducer

Inoculation experiments with three strains of Fomitiporia punctata on grapevine cv. Sangiovese and on grafted ‘Italia’ rootstocks were carried out in southern Italy in 1992-1993. Inoculations were performed on fresh wounds made on the spurs, branches and trunks of vines showing no symptoms of esca. The fungus developed in the discoloured wood around the inoculation site, and caused white rot within two years. No symptoms were induced on foliage or fruit of the infected vines, nor was there any significant difference in virulence of the strains of F. punctata. After 2 years, re-isolation of F. punctata from the diseased woody tissues was successful, whereas no other species of fungi suspected to act as a “precursor” of wood decay were isolated. In 1999, further experiments were carried out with one strain of F. punctata on standing vines cv. Italia and Matilde free of any sign of wood deterioration. The development of internal symptoms was recorded monthly. The results indicated that the cv. Matilde was less susceptible than the cv. Italia. The first signs of spongy wood decay appeared 6 months after inoculation on both cultivars. F. punctata was re-isolated from the infected vines, whereas no species of Phaeoacremonium or other wood-decaying fungi were isolated from either inoculated or non-inoculated vines. These findings suggest that F. punctata behaves as a primary pathogen, being able to cause wood deterioration and spongy decay both on adult and young grapevines in a relatively short time, without the prior or concurrent action of other fungi.

Occurrence of fusicoccin in plant tissues infected by Fusicoccum amygdali del

Abstract Green peaches were artificially inoculated with Fusicoccum amygdali Del. and the infected tissues extracted with chloroform. To isolate the phytotoxic substance, extracts were purified on chromatographic columns. About 5 mg pure toxin were obtained from 1 kg infected peach pulp. A similar procedure yielded about 10 mg toxin per kg of tissues from naturally infected almond shoots with cankers of F. amygdali. The physical and biological properties of the purified toxin were found to be identical to those of authentic fusicoccin. The results of this work show that fusicoccin is a vivotoxin sensu Dimond & Waggoner.

New phytotoxic butenolides produced bySeiridium cardinale, the pathogen of cypress canker disease

Two new butenolides, seiridin andiso-seiridin, were isolated from culture filtrates ofSeiridium cardinale, the pathogen of cypress canker, a destructive disease ofCupressus and relatedConiferae These metabolites were characterized as 3-methyl-4-(2-hydroxyheptyl)-2(5H)-furanone and its 4-(3-hydroxyheptyl) isomer, respectively. Chlorotic, and necrotic symptoms were produced on leaves of either host or non-host test plants by absorption of 0.3 mg/ml solutions of either compound. These also showed antibacterial activity.

Stomatal response of two almond cultivars to fusicoccin

Abstract The leaves of two cultivars of almond (Prunas amygdalus St.), one (“Filippo Ceo”) resistant and one (“Occhiorosso”) susceptible to the bud canker caused by Fusicoccum amygdali Del., were painted with or allowed to absorb fusicoccin at various concentrations from 2 × 10−5 m to 2 × 10−7 m , or control solutions. At various times after treatment, the stomatal resistance to transpiration by the leaves was measured with a ventilated diffusion porometer. Fusicoccin stimulated stomata of both cultivars to open in both the light and dark. The stomata of the resistant cultivar opened more slowly than those of the susceptible cultivar, but little difference in stomatal transpiration between the resistant and susceptible cultivars was observed after either painting with or absorption of fusicoccin. When absorbed into the transpiration stream, low concentrations of fusicoccin accumulated in the leaf to concentrations sufficient to open stomata. Inoculation of almond branches with F. amygdali induced changes in the diffusive resistance of leaves distal to the site of inoculation similar to those induced by absorption of pure fusicoccin preparations. The results show that the resistant cultivar, “Filippo Ceo”, had little capacity to limit the transfer of the toxin to the leaf or to reduce the opening effect of fusicoccin. It is concluded that the major resistance of the cultivar to the disease probably occurs in limiting the development of infection or containment of the toxin at the site of infection.

A simple technique for obtaining functionally isolated guard cells in epidermal strips of Vicia faba.

SummaryUltrasonic irradiation of Vicia faba L. epidermal strips for 2 min preferentially disrupts the epidermal cells but does not impair guard cell movements. Maximal opening induced by fusicoccin requires that K+ be provided to the guard cells from external sources. A mobile organic anion is not required.