Physiological Plant Pathology最新文献

Paleness, chlorosis and necrosis were caused in uninfected primary leaves of three Lr2O-bearing wheat cultivars after infiltration with fluids from intercellular spaces of leaves of Triticum aestivum cv. Chinese Spring infected by mycelia of leaf rust (Puccinia recondita Esp. tritici) either avirulent or virulent with respect to the Lr20 allele. No symptoms were caused in cv. Chinese Spring or two other cultivars, all of which lack Lr genes, or in cv. Brevit which bears the Lr2c allele. Symptoms were caused, however, in the Lr14a-bearing cv. Spica. Symptoms were not caused in a Chinese Spring/Axminster line in which chromosome 7A carrying the Lr20 allele had been substituted from the cv. Axminster. No symptoms were caused in any cultivar by fluids from uninfected leaves.

The eliciting agents were partially thermostable macromolecules which were active after many-fold dilution and likely to have been derived from either walls of intercellular hyphae or host cells at the infection court.

A first step for establishing the molecular basis of disease resistance in plants may be to identify genes whose expression closely follows the events occurring during infection. Nine pea clones were available from a prior screening of a cDNA library, developed from a single population of pea mRNA induced 8 h following inoculation with Fusarium solani f. sp. phaseoli. Transcription and accumulation of RNA homologous to these clones were compared over a 48 h infection course following the treatment of pea pods with either compatible or incompatible forms of F. solani or chitosan, a minor component of the fungal cell wall which is known to induce disease resistance. The incompatible reaction of the pea tissue is clearly distinguished from the compatible reaction on the basis of gene expression. Three of the cloned genes, designated Group I clones, show a large induction of their RNAs in disease resisting tissue which temporally correlates with the resistance observed cytologically. In the compatible reaction activation of Group I genes is commonly weaker or delayed, and is suppressed 12–24 h after inoculation, compared with the incompatible reaction. Genes homologous with Group II clones showed only a partial fit to the activity expected for resistance genes. We propose that selecting cloned genes whose time course of induction matches the response of plant issue to infection is a useful preliminary screening technique towards obtaining gene candidates for cross-species transformation experiments.

The progress of infection by Colletotrichum lindemuthianum was examined in susceptible and resistant French bean hypocotyls producing spreading lesions or single hypersensitive cells, respectively.

In susceptible tissue, intracellular infection vesicles formed in epidermal cells, which remained alive. Intracellular primary hyphae developed from the vesicles and colonized further host cells. A matrix layer separated the hyphal wall from the invaginated host plasmalemma. After a period of biotrophy lasting less than 24 h, the cytoplasm of infected cells gradually degenerated. This was associated with loss of the ability of cells to plasmolyse and to exclude tannic acid, here used as a permeability tracer with plant tissue for the first time. Loss of the ability of the tonoplast to contract and for neutral red to accumulate in the vacuole occurred later, and was considered to indicate cell death. In cultivars containing the pigment malvidin-3,5-diglucoside, loss of colour coincided with tonoplast rupture. During the development of the primary mycelium, the sequence of a brief biotrophy phase followed by gradual degeneration and death was repeated as each host cell became infected. Thus, despite the absence of tissue browning, only recently colonized cells at the edge of the infection were alive. As lesions appeared, narrower secondary hyphae grew within host cell walls. Death of host protoplasts and wall dissolution then occurred in advance of secondary hyphae.

In resistant tissue, infection vesicles were not formed, and the fungus was restricted in single hypersensitive epidermal cells. Most hyphae appeared dead, but some had normal ultrastructure.

These findings are discussed in relation to race specificity and the importance of biotrophy to successful pathogenesis by C. lindemuthianum.

Growth of Erysiphe graminis hordei and Erysiphe pisi on the same cells of barley coleoptiles was observed in detail by light microscopy to determine significant factors conditioning host cells toward susceptibility. When E. pisi attempted penetration more than 60 min earlier than E. graminis on the same coleoptile cell, E. pisi never succeeded in penetration (0% penetration efficiency) and the penetration efficiency of E. graminis was lowered from 75·0 to 28·6%, suggesting that the resistance to E. graminis invasion might be enhanced under this condition. When both fungi attempted penetration of the coleoptile cell almost simultaneously (within 30 min of each other), the penetration efficiency of E. pisi increased to 11·8%. Moreover, the penetration efficiency of E. graminis was 55·8%. When E. graminis attempted penetration 60 min or more earlier than E. pisi, the mean penetration efficiency of E. pisi was 29·2% and that of E. graminis to 75·0%. These observations suggest that the coleoptile cells are conditioned toward susceptibility by prior-penetration of E. graminis or even by its post-penetration if it penetrates after E. pisi E. pisi developed haustoria only in coleoptile cells where E. graminis formed haustoria. The induced susceptibility and enhanced resistance states in coleoptile cells had not transferred to the adjacent cells within 3 h.

Extracts from leaves of Nicotiana tabacum cv. Samsun NN which have developed a hypersensitive response to infection by tobacco mosaic virus (TMV), contain at least 10 pathogenesis-related (PR) proteins which are absent from or present in very small amounts in uninfected leaves. When [¹⁴C] amino acids were injected into leaves which were still attached to the plants and which had been inoculated with TMV 3 days earlier, a significant radioactivity became associated with all PR-proteins that were resolvable from other host proteins on non-denaturing gels. The incorporation of labelled amino acids into the individual polypeptides was investigated by a procedure involving two successive electrophoretic migrations, first under non-denaturing and then under denaturing conditions. This procedure, when applied to those PR-proteins whose composition is known, PR-1a, PR-1b, PR-1c, PR-2 and PR-N showed that they all accumulated significant radiolabel within 3 h of feeding the leaves with the [¹⁴C] amino acids. Significant radioactivity was also associated with PR-proteins in inoculated leaves within a few hours of feeding the [¹⁴C] amino acids to detached leaves through the petiole, but this method was much less efficient than the injection procedure. Specific radioactivities of the PR-proteins were compared with those of other host proteins and changes were followed during further incubation with unlabelled amino acids in order to investigate the possibility that the PR-proteins are stable end-products from proteolytic cleavage of constitutive proteins. The results indicate that de novo synthesis rather than proteolytic cleavage is responsible for the production and accumulation of PR-proteins in hypersensitively reacting leaves of tobacco.

Hypocotyls of soybean (cultivar Altona) are resistant to race 4 of Phytophthora megasperma f.sp. glycinea at 25 °C but susceptible to the same race at 32°C. Lower concentrations of the phytoalexin, glyceollin, accumulate in the temperature-induced susceptible response at 32°C than in the resistant response at 25°C. The possibility that this is due to decreased production or activity of elicitors of glyceollin was examined. Following published procedures, elicitors were obtained from culture filtrates and cell walls of P. megasperma f.sp. glycinea race 4, and for comparison from similar preparations from the compatible P. megasperma f.sp. glycinea race 6. Cotyledon bioassays indicated that preparations obtained from cultures of P. megasperma f.sp. glycinea race 4 grown either at 25 or 32 °C were both active. Preparations obtained from P. megasperma f.sp. glycinea race 6 were active also. The cell-wall elicitor preparations obtained from cultures grown at 25 or 32 °C were active in bioassays performed both at 32 and 25 °C. The abiotic elicitor AgNO₃ had similar activity at both temperatures. It is concluded that temperature-induced susceptibility to P. megasperma f.sp. glycinea race 4 is not due to inability to produce elicitors in culture, to inactivity at 32 °C of the elicitors examined or to inability of the host to produce glyceollin at the higher temperature.

Filtrates from 5-week-old cultures of race 4 of Phytophthora infestans contained an elicitor of the hypersensitive reaction in tuber discs of the potato cultivar Kennebec which caused both browning of discs and accumulation of the sesquiterpenoid phytoalexins rishitin and lubimin. Elicitor activity was associated with a fraction of molecular weight of more than 10 000; the most active fractions obtained by preparative isoelectric focussing contained bands which stained for both protein and carbohydrate on analytical gels.

Neither polygalacturonase present in the active fractions, nor any of the other polysaccharide-depolymerizing enzymes present in the crude culture filtrate, was involved in elicitation, since treatments which inactivated polygalacturonase (heat and pronase) did not reduce elicitor activity. Furthermore, these experiments indicated that the protein moiety of the elicitor was not important for its eliciting activity; the importance of the carbohydrate moiety was indicated by the loss of activity after periodate oxidation.

The culture filtrate elicitor did not contain any fatty acids and did not interact synergistically with arachidonic acid.

The antifungal activity of the five structurally related isoflavonoids phaseollin, phaseollinisoflavan, 7-o-methylphaseollinisoflavan, 2′-o-methylphaseollinisoflavan, and 7,2′-di-o-methylphaseollinisoflavan was tested in vitro using the bean pathogens Colletotrichum lindemuthianum, Fusarium oxysporum f. sp. phaseoli, Rhizoctonia solani and Thanatephorus cucumeris, as well as pathogens which do not attack Phaseolus beans such as Aphanomyces euteiches, Phytophthora megasperma f.sp. glycinea, Pythium paroecandrum and Fusarium solani f.sp. pisi. Colony growth on solid medium and growth of sporelings in liquid medium were used to measure the effect of the isoflavonoids. The fungi differed in their sensitivity to these compounds. With the exception of C. lindemuthianum, however, the order of effectiveness of the five isoflavonoids was virtually the same in all target organisms. There appeared to be no clear relationship between lipophilicity and anti-fungal activity of these compounds. The substituent at position 7 of the isoflavans may chiefly affect biological activity. The basis for antifungal activity of this series of isoflavonoids is discussed.

Relationships between the ability to produce indole-3-acetic acid (IAA) and cytokinins in culture and pathogenicity were studied in different strains of Pseudomonas syringae pv. savastanoi from olive and oleander and in their α-methyltryptophan resistant mutants. Wild type olive and oleander strains of pv. savastanoi produced both IAA and cytokinins in culture but the amount produced varied between strains. The α-methyltryptophan resistant mutants produced little or no IAA but about the same amount of cytokinins as their parent strains.

All wild type strains induced knots on olive whereas only the wild type strains from oleander were pathogenic on oleander. The size of the knots on olive and the time necessary for their development varied between strains. The olive strain mutants and the mutant from the oleander strain PBa219 did not usually induce symptoms on either olive or oleander, but mutants derived from oleander strains ITM519 and NCPPB640 induced atypical knots on olive. Mutants from the oleander strains induced necrotic symptoms only on oleander. The results indicate that the amount of IAA produced determines the length of the incubation period, whereas the amount of cytokinin produced determines the size of the knot.

All the wild type olive strains and their mutants exhibited the same plasmid profile whereas the mutants derived from the oleander strains lacked one plasmid. This suggests that the oleander strains do not carry the genetic determinants for cytokinin production on the plasmid which encodes for IAA production.

One-week-old Phaseolus vulgaris cv. Topnotch Golden Wax plants were inoculated with 0, 1000, 5000 or 10 000 freshly hatched Meloidogyne incognita larvae per plant, and maintained under controlled conditions (21 °C, 14 h day at 400 μE m⁻²s⁻¹; 16 °C, 10 h night cycle). At 3 weeks after inoculation, leaf area, dry weight, number of flowers, the total carbon, hydrogen, nitrogen, calcium, copper, iron, manganese, potassium and zinc content of shoots and roots, leaf chlorophyll content, and dark respiration and photosynthetic rates were measured. Respiration rate, percentage shoot nitrogen content, and calcium, copper and iron content (per unit weight and on a shoot: root ratio basis), were significantly increased with increasing inoculum level. Other measured parameters were significantly decreased. Calcium, copper and iron in the shoot and potassium in the root increased per unit weight, while copper and zinc in the roots decreased significantly as a result of nematode infection. However, the overall total content of the nutrient elements per plant was significantly decreased by nematode infection. Differences in the physiology and nutrient content of P. vulgaris plants, as they relate to altered growth and loss of yield of nematode-infected plants, are discussed.