Genetic variation and pathogenicity of anastomosis group 2 isolates of Rhizoctonia solani in Australia

A collection of isolates of Rhizoctonia solani anastomosis group (AG) 2 was examined for genetic diversity and pathogenicity. Anastomosis reactions classified the majority of isolates into the known subgroups of AG 2-1 and AG 2-2 but the classification of several isolates was ambiguous. Morphological characters were consistent with the species, with no discriminating characters existing between subgroups. Vertical PAGE of pectic enzymes enabled the separation of zymogram group (ZG) 5 and 6 within AG 2-1, but not the separation of ZG 4 and 10 within AG 2-2. PCR analysis using inter-simple sequence repeats (ISSR) and the intron-splice junction (ISJ) region supported the separation of ZG 5 and 6, while the AG 2-2 isolates were separated by geographic region. A comparison of distance matrices produced by the zymogram analysis and PCR indicated a strong correlation between the marker types. Pathogenicity studies suggested canola (Brassica napus) cultivars were most severely affected by AG 2-1, while cultivars of two species of medic (Medicago truncatula cv. Caliph and M. littoralis cv. Herald) were susceptible to both AG 2-1 and 2-2. The results indicate that AG 2 is a polyphyletic group in which the classification of subtypes is sometimes difficult. Further investigation of the population structure within Australia is required to determine the extent and origin of the observed diversity.

Intraspecific variation of Rhizoctonia solani AG 3 isolates recovered from potato fields in Central Iran and South Australia

Pectic zymogram, RFLP and PCR analyses were used to characterize Rhizoctonia solani AG 3 isolates collected from diseased potatoes in South Australia. The pectic zymogram data were compared with those obtained for isolates collected from central Iran. Analyses of bands corresponding to pectin esterase and polygalacturonase revealed three zymogram subgroups (ZG) in AG 3. In addition to the previously reported ZG7 (here renamed ZG7-1), two new zymogram subgroups, ZG7-2 and ZG7-3, were identified. Of the 446 isolates tested, 50% of the South Australian and 46% of the Iranian isolates were ZG7-1. The majority of the isolates originating from stem and root cankers were ZG7-1, whereas most of the isolates designated ZG7-2 and ZG7-3 originated from tuber-borne sclerotia. Pathogenicity tests revealed that ZG7-1 generally produced fewer sclerotia and more severe cankers of underground parts of the potato plants than the other two ZGs. Two random DNA clones, one originating from an AG 3 isolate and the other from an AG 4 isolate, were used as probes for RFLP analyses of Australian isolates. The AG 3 probe, previously identified to be specific to this group, detected a high level of genetic diversity, with 11 genotypes identified amongst 50 isolates analysed. The low-copy AG 4 probe resolved three genotypes amongst 24 isolates. For 23 isolates analysed with both markers, the combined data distinguished a total of six genotypes and similarity analysis resolved the isolates into two main groups with 50% homology. PCR, using primers for the plant intron splice junction region (R1), also revealed variation. No obvious relationship among pectic zymogram groups, RFLP and PCR genotypes was observed.

Isolation of fungal cell wall degrading proteins from barley (Hordeum vulgare L.) leaves infected with Rhynchosporium secalis

Proteins with antifungal activity towards Rhynchosporium secalis conidia were isolated from the intercellular washing fluid (IWF) of barley leaves. The active components were purified by high-performance liquid chromatography under conditions that maintained biological activity. Five major barley IWF proteins deleterious to the cell wall of viable R. secalis conidia were isolated and identified by a combination of N-terminal amino acid sequencing, peptide mapping, and determination of mass and isoelectric point. They were a 32-kDa beta-1,3-glucanase (Pr32), a 25-kDa chitinase (Pr25), and three 22-kDa thaumatin-like (TL) proteins (Pr22-1, Pr22-2, and Pr22-3). Pr22-1 and Pr22-2 were similar to the protein R class of TL proteins, whereas Pr22-3 was more similar to the S class. Pr22-3 was shown to digest laminarin, indicating that this TL protein has glucanase activity. In addition, Pr22-3 was more active in the spore bioassay than Pr22-2. Various combinations of the five proteins had a greater effect on R. secalis spores than did the individual proteins. The extraction of proteins with antifungal activity from the IWF of barley leaves indicates their possible role in defense against leaf pathogens. A similar bioassay may be developed for other systems to identify particular isoforms of pathogenicity-related proteins that might have a role in plant disease resistance.

Kinetics of ATP synthesis in pea cotyledon submitochondrial particles

A kinetic study of oxidative phosphorylation by pea submitochondrial particles gave two K(m) values for ADP, one low, the other high. The high value probably reflected a damaged site or a population of leaky mitochondria. Only the high affinity site with a low K(m) for ADP was involved in ATP synthesis. alpha,beta-Methylene ADP was found to be a competitive inhibitor of ATP synthesis. The inorganic phosphate analog, thiophosphate, decreased the apparent K(m) of ADP while the rate of the reaction remained approximately the same. Adenyl imidodiphosphate, a specific inhibitor of ATP hydrolysis activity, had little effect on oxidative phosphorylation. A slight decrease in the K(m) of the high affinity binding site for ADP was noted. Aurovertin was found to be a potent inhibitor of oxidative phosphorylation in pea submitochondrial particles. The K(m) of the high affinity site was increased 10-fold. Also, the inhibition normally exerted by ADP on ATPase activity was severely reduced by aurovertin. In contrast, increasing the concentration of aurovertin only slightly affected the level of inhibition caused by adenyl imidodiphosphate on ATP hydrolysis.

Regulation of ribosomal RNA accumulation by auxin in artichoke tissue

Artichoke (Helianthus tuberosus L.) tuber tissue cultured in the presence of the auxin 2,4-dichlorophenoxyacetic acid accumulates ribosomal RNA at a rate of 0.135 micrograms per hour per explant whereas there is little accumulation in nontreated tissue. The addition of auxin enhanced the transcription of the 2.5 x 10(6) precursor 3.5-fold and increased the rate of processing 1.8-fold. The major effect of auxin, however, was a vast increase in the rate of processing of the 1.39 x 10(6) precursor to the 1.3 x 10(6) mature ribosomal RNA. The incorporation of label into the 0.7 x 10(6) mature ribosomal RNA of treated tissue was in 10-fold excess over the control after a 30-minute pulse and remained so throughout the remainder of the labeling period. This level, however, was not reached for the complementary 1.3 mature RNA until 3 hours of continuous labeling, decreasing from a initial value of 40-fold excess. A complication in the processing of ribosomal RNA is the apparent increase in the stability of the 0.7 x 10(6) mature RNA with auxin treatment.

Purine metabolism and differential inhibition of spore germination in Phytophthora infestans

An analysis of the effect of various purines and pyrimidines on the germination process in three different isolates of the late blight fungus, Phytophthora infestans, revealed increased rates of indirect germination in one isolate by adenine, hypoxanthine, and the riboside of dimethylaminopurine. This observation coupled with the capacity of sporangia of the race affected (1.2.3.4) for the uptake and interconversion of purines, as demonstrated by experiments with labelled purines under in vivo and in vitro conditions, pointed to hypoxanthine as a key intermediate in the purine metabolism directly associated with spore formation and development. This enhanced germination contrasted sharply with the almost complete arrest of indirect germination that occurred when sporangia were incubated with the purine analogue, benzimidazole, or with either of the respiratory inhibitors, sodium azide and 2,4-dinitrophenol. The pattern of differential inhibition exhibited by sporangia versus zoospores upon treatment with actinomycin D, 4-FLUOROURACIL, OR CYCLOHEXIMIDE INDICATED THat continued translation on preformed messenger RNA may be one essential requirement for the formation and release of zoospores, whereas their subsequent germination and development may depend upon renewed transcription as well.