B. J. Deverall

Histochemical and chemical evidence for lignin accumulation during the expression of resistance to leaf rust fungi in wheat

Abstract Primary leaves of wheat ( Triticum aestivum ) were examined after inoculation with avirulent and virulent strains of the leaf rust fungus ( Puccinia recondita f. sp. tritici ) at stages when previous work had shown fungal and host cells to be affected by the expression of the Lr20 or Lr28 alleles for resistance. Histochemical stains for lignin were first shown to react in characteristic ways with cell walls in stomata, vascular tissue and mechanical fibres in sections of uninfected leaves. Toluidine blue and two different chlorine-sulphite reagents reacted positively for lignin with walls of hypersensitive mesophyll cells in the Lr28 -bearing leaves, but only toluidine blue in the Lr20 -bearing leaves. Phloroglucinol did not react positively with these cells. Aniline blue stained guard cells in the infected Lr28 -bearing leaves and hypersensitive cells strongly in these leaves but less so in Lr20 -bearing leaves, suggesting the presence of callose. Sudan IV failed to indicate suberin in any situation. Lignin was also extracted and measured as a complex with thioglycolic acid (LTGA). LTGA derived from uninfected and infected leaves had a characteristic UV absorption spectrum with a distinctive peak at 280 nm. Lignin was present in high concentration 48 h after inoculation of Lr20 -bearing leaves with an avirulent strain, when widespread hypersensitivity had occurred around fungal colonies. Lignin was also detected at moderate concentrations 24 h after inoculation of Lr28 -bearing leaves with an avirulent strain, when one or a few cells had undergone hypersensitivity in the presence of haustoria. Enzymatic digestion of these leaves yielded fractions containing hypersensitive cells and the lignin content in these fractions was significantly higher than in comparable fractions from uninfected leaves. Some evidence was obtained that infection also increased the lignin content of stomatal guard cells. The results suggested that lignification, in cells undergoing a hypersensitive response, may play a rote in the highly specific cultivar resistance expressed by wheat to the leaf rust fungus.

Changes in phenolic acid levels in wheat leaves expressing resistance to Puccinia recondita f. sp. tritici

Abstract Levels of individual phenolic acids were examined in primary leaves of wheat ( Triticum aestivum ) after inoculation with avirulent and virulent strains of the leaf rust fungus ( Puccinia recondita f. sp. tritici ) at stages when previous work had shown fungal and host cells to be affected by expression of the Lr 20 or Lr 28 alleles for resistance. The predominant phenolic acid, ferulic acid, as well as p -coumaric and syringic acids were detected in primary leaves in both unbound and bound forms. They were not detected in germinating urediniospores of either rust strain. Levels of unbound phenolic acids changed little in response to infection. In Lr28 -bearing leaves inoculated with an avirulent strain, increased concentrations of bound phenolic acids and three other unidentified compounds were observed about 4 h after many single or small groups of cells had undergone hypersensitive collapse. In an Lr20 -bearing cultivar, levels of bound phenolic acids fell in leaves inoculated with either a virulent or avirulent rust strain. Coniferyl aldehyde and coniferyl alcohol were not detected in healthy or inoculated leaves of either wheat cultivar. Attempts to affect expression of resistance by application of inhibitors of phenylalanine ammonia-lyase were not successful and both wheat cultivars remained resistant to avirulent rust strains. The bound phenolic acids which accumulate in cells undergoing a hypersensitive response may play a role in resistance of Lr28 -bearing wheat to the leaf rust fungus.

Postharvest control of green mould on oranges by a strain of Pseudomonas glathei and enhancement of its biocontrol by heat treatment.

Abstract A bacterial isolate related to Pseudomonas glathei did not produce antibiotics, but gave moderate biocontrol of green mould caused by Penicillium digitatum on postharvest oranges at ambient conditions (25 °C). The biocontrol ability of this isolate was improved greatly when the inoculated fruits were incubated at 30 °C for 24 h before being moved to 25 °C. Heat treatment at 30 °C stimulated multiplication of the protective bacteria, but retarded the spore germination of the pathogen. This allowed the bacteria to become established at wound sites on fruit before germination of spores and infection by the pathogen occurred.

Effect of 2,6-dichloroisonicotinic acid or benzothiadiazole on Alternaria leaf spot, bacterial blight and Verticillium wilt in cotton under field conditions

In field experiments, cotton (Gossypium hirsutum/G. barbadense) plants were treated with foliar applications of the synthetic activators 2, 6-dichloroisonicotinic acid (INA) or benzo [1, 2, 3] thiadiazole-7-carbothioic acid S-methyl ester (BTH) and the severity of natural infection with Alternaria macrospora, Xanthomonas campestris pv. malvacearum and Verticillium dahliae was assessed. Percentage leaf area infected with A. macrospora was significantly (P=0.05) lower in plants following one application of INA or BTH in the 95/96 and 97/98 seasons, respectively, compared with untreated plants. There was also significantly (P=0.05) less defoliation in the treated plants. Plants treated with one application of BTH had significantly (P=0.05) lower leaf areas infected with X. campestris pv. malvacearum and decreased defoliation compared with untreated plants in the 97/98 season. The severity of foliar symptoms of Verticillium wilt was significantly (P=0.05) decreased in plants treated with one application of INA or multiple applications of BTH compared with untreated plants in four experiments conducted over four field seasons. The reduced susceptibility of the cotton plants to Alternaria leaf spot, bacterial blight and Verticillium wilt is attributed to systemic acquired resistance following application of INA or BTH.

A benzothiadiazole applied to foliage reduces development and egg deposition by Meloidogyne spp. in glasshouse-grown grapevine roots

Foliar application of a benzothiadiazole (BTH), as Bion WG50 (50 mg active ingredient/mL), to glasshouse-grown Cabernet Sauvignon grapevines caused a significant reduction in egg deposition by root-knot nematodes in 10-week-old plants. No change in the number of nematodes in roots of treated grapevines was observed 3 days after inoculation, but fewer mature nematodes were recorded after a further 18 days. BTH was not toxic to the juvenile nematodes in vitro. The activity of β-1,3-glucanase increased in the leaves of BTH-treated grapevines at 7 and 28 days, and increased slightly and transiently in the roots at 5 days after foliar application. The results are interpreted as indicating the activation of systemic resistance to nematode development in the roots.

Pre-harvest application of 2, 6-dichloroisonicotinic acid, -aminobutyric acid or benzothiadiazole to control post-harvest storage diseases of melons by inducing systemic acquired resistance (SAR)

Summary Field-grown rockmelon plants were treated with -aminobutyric acid (BABA), 2,6-dichloroisonicotinic acid (INA), benzothiadiazole (BTH) or water during fruit development and evaluated for increased resistance against plant diseases or post-harvest pathogens. One experiment was conducted at Camden, NSW, Australia (INA, BABA or water). Two experiments were at Griffith, NSW, Australia (INA, BTH or water). Growing plants and harvested fruits were assessed for disease symptoms from natural infections and assayed for the accumulation of chitinase and peroxidase, two major pathogenesis-related (PR) proteins induced as a result of systemic acquired resistance (SAR). Harvested fruit from both BTH- or INA-treated plants showed a significant (P < 0.05) reduction in the severity and incidence of post-harvest storage diseases mainly caused by Fusarium, Alternaria and Rhizopus. However, there were no differences in disease severity and the incidence of rots between using four fortnightly foliar sprays of INA or BTH during flowering and fruit development, or a single spray of BTH, 2 weeks before harvest. Each approach showed an equivalent reduction in storage diseases. Plants treated with BABA showed less resistance against powdery mildew in the field and storage rots from natural inoculum, and lower increases in chitinase and peroxidase activities than those treated with INA. In all trials, an additional post-harvest dip with guazatine [0.05% (w/v)] gave a substantial reduction in melon storage rots. Pre-harvest application of INA or BTH reduced the occurrence of powdery mildew and downy mildew on the leaves. Over the three field-experiments, INA had a small phytotoxic effect causing lesion-like symptoms on leaves and affected plant growth, but not yield, at Camden, when applied during flowering and 2 weeks after flowering. However, INA did not produce any phytotoxic effects in the two experiments at Griffith when applied serially, four times to plants after flowering.

Induced Resistance in Legumes

Induced resistance is taken to mean heightened resistance in a plant towards pathogens as a result of a previous treatment with a pathogen, an attenuated pathogen or a chemical that is not itself a pesticide. This review chapter is concerned with induced resistance in leguminous plants. It particularly seeks and evaluates evidence for induced resistance to fungal and bacterial pathogens, but makes occasional reference to induced resistance towards viruses and pests where this is related to its main theme. For more extensive coverage of induced resistance to viruses, see Fraser (1985) and Ponz and Bruening (1986). For coverage of induced resistance to pests, see Tallamy and Raupp (1991).

2,6-dichloro-isonicotinic acid (INA) induces resistance in green beans to the rust pathogen, Uromyces appendiculatus, under field conditions

Field trials during late summer to early spring in two consecutive years demonstrated that applying 2,6-dichloroisonicotinic acid (INA) to bean seedlings (cv. Redlands Pioneer) when they were 16 to 20 days old protected plants against rust disease caused by subsequent inoculation with Uromyces appendiculatus. Protected plants had 2–10 times fewer uredinia than control plants, and protection was sustained throughout most of the growing season (is. for at least 5 weeks). Treatment with 2,6-dichloro-isonicotinic acid also curtailed the spread of the disease to upper uninoculated leaves, which had 4–5 times fewer uredinia than the same leaves of control plants. There was no additional resistance gained by applying a second or ‘booster’ WA treatment.

Isolation of haustoria from wheat leaves infected by the leaf rust fungus

A procedure was developed to isolate haustoria from wheat leaves infected by Puccinia recondita . Homogenization of heavily infected leaves in an isolation medium was followed by a period of sedimentation. The resulting suspension was subjected to mild centrifugation and the pellet was separated further by centrifugation on a density gradient. One layer was selected and washed beforc examination by optical microscopy. Two types of fungal structure were distinguished by their shape, size and cytoplasmic activity and with a nuclear stain and lectin probes. One type was identified as hyphae and the other as haustoria. The reasons for identification are discussed in relation to literature on isolated haustorial complexes from powdery mildew fungi and on electron microscopic examination of sections of rust haustoria in wheat leaves.

Genetic tests of the basis of wheat cultivar selectivity in symptom elicitation by preparations from rust pathogens

Abstract Fluids centrifuged from water-infiltrated leaves of a susceptible cultivar infected by strains of the leaf rust fungus ( Puccinia recondita f.sp. tritici ) had previously been shown to be cultivar-selective in causing symptoms when injected into test leaves. The responsiveness of cultivars is indicated herein to depend not upon known Lr genes but upon a factor located on chromosome 5A. Similar symptom elicitation was brought about by fluids obtained from a resistant cultivar infected by the same strains of the leaf rust fungus and from a susceptible cultivar infected by other strains of this fungus or of the stem rust fungus ( P. graminis f.sp. tritici ). Symptom elicitation was also caused by brief and prolonged water washes of uredospores, germination fluids and sonicates of germ-tubes of the leaf rust fungus. The results are discussed in relation to other work implicating selective elicitors in the leaf rust-wheat system and in the interaction between Cladosporium fulvum and tomato.