Krishnapillai Sivasithamparam

Antifungal antibiotics produced by Trichoderma spp

In recent years plant pathologists and commercial companies have shown considerable interest in the application of biological control agents. Trichoderma spp have received particular attention as agents for the biological control of fungai pathogens of plants and appear to be likeiy candidates for successful exploitation in the future. Despite extcnsivc research over the last 50 years on the capability of Trichoderma spp to reduce the incidence of disease caused by soil-borne plant pathogens, the mcchanisms by which disease control is achicvcd are not clearly understood. The mechanisms suggcstcd to bc involved in biocontrol by thcsc fungi arc antibiosis, lysis, competition. mycoparasitism and promotion of plant growth (Hcnis, 1984; Papavizas, 1985; Chet, 1987; Baker, 1988; Lynch, 1990) It seems rcasonablc to assume that successful antagonism may rely on a combination of these modes of action. In this rcvicw we document the ability of Tri~hodermu spp to produce antibiotics and discuss the nature and possible ecological relevance of the antibiotics produced by these fungi. Four reviews on Trichoderma have appeared in recent years (Evcleigh, 1985; Papavizas, 1985; Taylor, 1986; Chct, 1987) but none have specifically considered these topics.

Cloning and characterisation of a family of disease resistance gene analogs from wheat and barley

Abstract The most common class of plant disease resistance (R) genes cloned so far belong to the NBS-LRR group which contain nucleotide-binding sites (NBS) and a leucine-rich repeat (LRR). Specific primer sequences derived from a previously isolated NBS-LRR sequence at the Cre3 locus, which confers resistance to cereal cyst nematode (CCN) in wheat (Triticum aestivum L.) were used in isolating a family of resistance gene analogs (RGA) through a polymerase chain reaction (PCR) cloning approach. The cloning, analysis and genetic mapping of a family of RGAs from wheat (cv ‘Chinese Spring’) and barley (Hordeum vulgare L. cvs ‘Chebec’ and ‘Harrington’) are presented. The wheat and barley RGAs contain other conserved motifs present in known R genes from other plants and share between 55–99% amino acid sequence identity to the NBS-LRR sequence at the Cre3 locus. Phylogenetic analysis of the RGAs with other cloned R genes and RGAs from various plant species indicate that they belong to a superfamily of NBS-containing genes. Two of the barley derived RGAs were mapped onto loci on chromosomes 2H (2), 5H (7) and 7H (1) using barley doubled haploid (DH) mapping populations. Some of these loci identified are associated with regions carrying resistance to CCN and corn leaf aphid.

Potential impact of climate change on plant diseases of economic significance to Australia

Burning of fossil fuel, large scale clearing of forests and other human activities have changed global climate. Atmospheric concentration of radiatively active CO2, methane, nitrous oxide and chlorofluorocarbons has increased to cause global warming. In Australia temperature is projected to rise between 1 and 3°C by 2100. This review is the result of a recent workshop on the potential impact of climate change on plant diseases of economic significance to Australia. It gives an overview of projected changes in Australian climate and the current state of knowledge on the effect of climate change on plant diseases. Based on an assessment of important diseases of wheat and other cereals, sugarcane, deciduous fruits, grapevine, vegetables and forestry species, climate change in Australia may reduce, increase or have no effect on some diseases. Impacts will be felt in altered geographical distribution and crop loss due to changes in the physiology of host-pathogen interaction. Changes will occur in the type, amount and relative importance of pathogens and diseases. Host resistance may be overcome more rapidly due to accelerated pathogen evolution from increased fecundity at high CO2, and/or enhanced UV-B radiation. However, uncertainties about climate change predictions and the paucity of knowledge limit our ability to predict potential impacts on plant diseases. Both experimental and modelling approaches are available for impact assessment research. As the development and implementation of mitigation strategies take a long time, more research is urgently needed and we hope this review will stimulate interest.

Major secondary metabolites produced by two commercial Trichoderma strains active against different phytopathogens

Aims:  Trichoderma harzianum strains T22 and T39 are two micro‐organisms used as active agents in a variety of commercial biopesticides and biofertilizers and widely applied amongst field and greenhouse crops. The production, isolation, biological and chemical characterization of the main secondary metabolites produced by these strains are investigated.

Promotion of plant growth by an auxin-producing isolate of the yeast Williopsis saturnus endophytic in maize (Zea mays L.) roots

A plant-growth-promoting isolate of the yeast Williopsis saturnus endophytic in maize roots was found to be capable of producing indole-3-acetic acid (IAA) and indole-3-pyruvic acid (IPYA) in vitro in a chemically defined medium. It was selected from among 24 endophytic yeasts isolated from surface-disinfested maize roots and evaluated for their potential to produce IAA and to promote maize growth under gnotobiotic and glasshouse conditions. The addition of l-tryptophan (L-TRP), as a precursor for auxins, to the medium inoculated with W. saturnus enhanced the production of IAA and IPYA severalfold compared to an L-TRP-non-amended medium. The introduction of W. saturnus to maize seedlings by the pruned-root dip method significantly (P<0.05) enhanced the growth of maize plants grown under gnotobiotic and glasshouse conditions in a soil amended with or without L-TRP. This was evident from the increases in the dry weights and lengths of roots and shoots and also in the significant (P<0.05) increases in the levels of in planta IAA and IPYA compared with control plants grown in L-TRP-amended or non-amended soil. The plant growth promotion by W. saturnus was most pronounced in the presence of L-TRP as soil amendment compared to seedlings inoculated with W. saturnus and grown in soil not amended with L-TRP. In the glasshouse test, W. saturnus was recovered from inside the root at all samplings, up to 8 weeks after inoculation, indicating that the roots of healthy maize may be a habitat for the endophytic yeast. An endophytic isolate of Rhodotorula glutinis that was incapable of producing detectable levels of IAA or IPYA in vitro failed to increase the endogenous levels of IAA and IPYA and failed to promote plant growth compared to W. saturnus, although colonization of maize root tissues by R. glutinis was similar to that of W. saturnus. Both endophytic yeasts, W. saturnus and R. glutinis, were incapable of producing in vitro detectable levels of gibberellic acid, isopentenyl adenine, isopentenyl adenoside or zeatin in their culture filtrates. This study is the first published report to demonstrate the potential of an endophytic yeast to promote plant growth. This is also the first report of the production of auxins by yeasts endophytic in plant roots.

Plant growth promotion and biological control of Pythium aphanidermatum, a pathogen of cucumber, by endophytic actinomycetes

Aims:  To evaluate the potential of Actinoplanes campanulatus, Micromonospora chalcea and Streptomyces spiralis endophytic in cucumber roots, to promote plant growth and to protect seedlings and mature plants of cucumber from diseases caused by Pythium aphanidermatum, under greenhouse conditions.

Analysis of Leptosphaeria maculans Race Structure in a Worldwide Collection of Isolates

ABSTRACT Leptosphaeria maculans, the causal agent of stem canker of oilseed rape, develops gene-for-gene interactions with its hosts. To date, eight L. maculans avirulence (Avr) genes, AvrLm1 to AvrLm8, have been genetically characterized. An additional Avr gene, AvrLm9, that interacts with the resistance gene Rlm9, was genetically characterized here following in vitro crosses of the pathogen. A worldwide collection of 63 isolates, including the International Blackleg of Crucifers Network collection, was genotyped at these nine Avr loci. In a first step, isolates were classified into pathogenicity groups (PGs) using two published differential sets. This analysis revealed geographical disparities as regards the proportion of each PG. Genotyping of isolates at all Avr loci confirmed the disparities between continents, in terms of Avr allele frequencies, particularly for AvrLm2, AvrLm3, AvrLm7, AvrLm8, and AvrLm9, or in terms of race structure, diversity, and complexity. Twenty-six distinct races were identified in the collection. A larger number of races (n = 18) was found in Australia than in Europe (n = 8). Mean number of virulence alleles per isolate was also higher in Australia (5.11 virulence alleles) than in Europe (4.33) and Canada (3.46). Due to the diversity of populations of L. maculans evidenced here at the race level, a new, open terminology is proposed for L. maculans race designation, indicating all Avr loci for which the isolate is avirulent.

Development of in situ and ex situ seed baiting techniques to detect mycorrhizal fungi from terrestrial orchid habitats

An innovative ex situ fungal baiting method using soil collected from field sites which allows the simultaneous detection of mycorrhizal fungi for multiple terrestrial orchids is presented. This method demonstrated that coarse organic matter (> 2 mm) in the litter and topsoil was the most important reservoir of inoculum of these fungi. A new in situ seed baiting method using multi-chambered packets to simultaneously assess germination for different orchid species within soil is also introduced. These in situ and ex situ methods are compared using seed of orchids in the genera Monadenia, Microtis, Caladenia, Pterostylis and Diuris, using urban Banksia woodland sites with high or low weed cover. Both these seed baiting methods detected compatible fungi for these orchids, but common orchids germinated more frequently than those which were uncommon at the field sites. Germination rates were not significantly affected by weed cover even though adult orchids were rare in areas with high weed cover. The two new seed baiting methods vary in efficiency and applicability depending on the situation where they are used. However, the ex situ method allowed the time-course of germination to be observed, resulting in the production of more protocorms and facilitation of the isolation of mycorrhizal fungi. These techniques provide valuable new tools for detection of compatible mycorrhizal fungi to assist orchid research and conservation.

Breakdown of a Brassica rapa subsp. sylvestris Single Dominant Blackleg Resistance Gene in B. napus Rapeseed by Leptosphaeria maculans Field Isolates in Australia

Blackleg, caused by Leptosphaeria maculans, is a major disease of oilseed rape (Brassica napus) grown in Canada, Europe, and Australia. Cv. Surpass 400 was released in Australia in 2000 as the most resistant cultivar to L. maculans. It carries a single dominant resistance gene from B. rapa subsp. sylvestris. This cultivar usually shows a hypersensitive response to L. maculans characterized by small, dark brown lesions that are necrotic, localized, and without pycnidia on cotyledons, leaves, and stems. However, in 2001 on a Western Australian experimental farm, a small proportion of the lesions on the lower stem and crown region of cv. Surpass 400 were typical of those observed in susceptible cultivars, which were brown, necrotic lesions with a darker margin, but they contained fewer pycnidia. Forty seedlings of cv. Surpass 400 and susceptible cv. Westar were inoculated with pycnidiospore suspensions (106/ml) of each of 18 isolates taken from lesions on cv. Surpass 400. All 18 isolates caused collapse of cotyledons of susceptible cv. Westar. Four of these isolates caused large cotyledon lesions with some pycnidia on cv. Surpass 400. Three of these four isolates were subsequently inoculated onto 60 seedlings per isolate, at each of the four cotyledon lobes of each seedling of the two cultivars. Inoculated plants were assessed for disease severity on cotyledons and transplanted to the field 14 days after inoculation. The cotyledons of inoculated cv. Surpass 400 showed characteristic large, necrotic lesions with pycnidia, while the cotyledons of cv. Westar had collapsed and contained a mass of pycnidia. Blackleg disease severity in the crown region of the stem was assessed at 2 weeks before harvest. Fifty-four percent of the cv. Surpass 400 transplanted inoculated plants subsequently developed susceptible symptoms of crown cankers on stems. These symptoms were deep, girdling, brown lesions on the plant crowns with some pycnidia. One hundred percent of cv. Westar plants were infected and dead at this stage. This confirmed the ability of these field isolates to overcome the single dominant resistance gene present in cv. Surpass 400. To our knowledge, this is the first report of breakdown of a single dominant B. rapa subsp. sylvestris gene based resistance to blackleg in oilseed rape in the field.

Growth promotion of bean (Phaseolus vulgaris L.) by a polyamine-producing isolate of Streptomyces griseoluteus

Of seventy-five actinomycetes isolated from a bean rhizosphere in the United Arab Emirates, an isolate of Streptomyces griseoluteus (WT) was found to be capable of producing relatively high levels of putrescine on decarboxylase agar medium and to produce putrescine, spermidine and spermine in liquid decarboxylase medium. In the glasshouse, the application of the WT strain to soil amended with arginine (as a precursor for putrescine) significantly (P < 0.05) promoted the growth of bean plants and increased the fresh and dry weights and lengths of roots and shoots, compared with control plants. Infestation of soil with the WT strain resulted in a significant (P < 0.05) increase in the levels of putrescine, spermidine and spermine, certain endogenous plant growth regulators (PGRs) (indole-acetic acid, and gibberellic acid), chlorophylls (a, b) and carotenoids with a concomitant reduction in the level of abscisic acid in bean plants, compared with control plants. A polyamine non-producing mutant strain (PNPM) obtained from the wild-type isolate (WT), however, failed to promote plant growth. There were no significant (P > 0.05) differences between the levels of polyamines, endogenous PGRs, chlorophylls (a, b), and carotenoids between plants that were not exposed to either of the strain (control) and those grown in soil with the PNPM strain. Both WT and PNPM strains were incapable of producing in vitro detectable levels of PGRs, indole-acetic acid, indole-pyruvic acid, gibberellic acid, isopentenyl adenine and zeatin in the culture filtrates. This study is the first to demonstrate the potential of a polyamine-producing actinomycete to promote plant growth. In addition, it is also the first published report of the production of polyamines by streptomycete actinomycetes.