Ilan Chet

Trichoderma species - opportunistic, avirulent plant symbionts.

Trichoderma spp. are free-living fungi that are common in soil and root ecosystems. Recent discoveries show that they are opportunistic, avirulent plant symbionts, as well as being parasites of other fungi. At least some strains establish robust and long-lasting colonizations of root surfaces and penetrate into the epidermis and a few cells below this level. They produce or release a variety of compounds that induce localized or systemic resistance responses, and this explains their lack of pathogenicity to plants. These root–microorganism associations cause substantial changes to the plant proteome and metabolism. Plants are protected from numerous classes of plant pathogen by responses that are similar to systemic acquired resistance and rhizobacteria-induced systemic resistance. Root colonization by Trichoderma spp. also frequently enhances root growth and development, crop productivity, resistance to abiotic stresses and the uptake and use of nutrients.

Phytoremediation: a novel strategy for the removal of toxic metals from the environment using plants.

Toxic metal pollution of waters and soils is a major environmental problem, and most conventional remediation approaches do not provide acceptable solutions. The use of specially selected and engineered metal-accumulating plants for environmental clean-up is an emerging technology called phytoremediation. Three subsets of this technology are applicable to toxic metal remediation: (1) Phytoextraction—the use of metal-accumulating plants to remove toxic metals from soil; (2) Rhizoflltration—the use of plant roots to remove toxic metals from polluted waters; and (3) Phytostabilization—the use of plants to eliminate the bioavailability of toxic metals hi soils. Biological mechanisms of toxic metal uptake, translocation and resistance as well as strategies for improving phytoremediation are also discussed.

Effect of Trichoderma harzianum on microelement concentrations and increased growth of cucumber plants

The potential of the biocontrol agent Trichoderma harzianum strain T-203 to induce a growth response in cucumber plants was studied in soil and under axenic hydroponic growth conditions. When soil was amended with T. harzianum propagules, a 30% increase in seedling emergence was observed up to 8 days after sowing. On day 28, these plants exhibited a 95 and 75% increase in root area and cumulative root length, respectively, and a significant increase in dry weight (80%), shoot length (45%) and leaf area (80%). Similarly, an increase of 90 and 30% in P and Fe concentration respectively, was observed in T. harzianum inoculated plants. To better characterize the effect of T. harzianum during the early stages of root colonization, experiments were carried out in a gnotobiotic hydroponic system. An increased growth response was apparent as early as 5 days post-inoculation with T. harzianum, resulting in an increase of 25 and 40% in the dry weight of roots and shoots, respectively. Similarly a significant increase in the concentration of Cu, P, Fe, Zn, Mn and Na was observed in inoculated roots. In the shoots of these plants, the concentration of Zn, P and Mn increased by 25, 30 and 70%, respectively. Using the axenic hydroponic system, we showed that the improvement of plant nutritional level may be directly related to a general beneficial growth effect of the root system following T. harzianum inoculation. This phenomenon was evident from 5 days post-inoculation throughout the rest of the growth period, resulting in biomass accumulation in both roots and shoots.

The molecular biology of chitin digestion.

Chitinases catalyze the hydrolysis of chitin, an unbranched polymer of beta-1,4-N-acetylglucosamine. In recent years, soil-borne microorganisms that produce chitinases are considered as potential biocontrol agents against fungi and nematodes which causes diseases of agricultural crops. Chitinases also play an important physiological and ecological role in ecosystems as recyclers of chitin, by generating carbon and nitrogen sources. Many chitinases of varied organisms have been isolated and their corresponding genes cloned.

Involvement of Jasmonic Acid/Ethylene Signaling Pathway in the Systemic Resistance Induced in Cucumber by Trichoderma asperellum T203.

ABSTRACT Trichoderma spp. are effective biocontrol agents for a number of soilborne plant pathogens, and some are also known for their ability to enhance plant growth. It was recently suggested that Trichoderma also affects induced systemic resistance (ISR) mechanism in plants. Analysis of signal molecules involved in defense mechanisms and application of specific inhibitors indicated the involvement of jasmonic acid and ethylene in the protective effect conferred by Trichoderma spp. against the leaf pathogen Pseudomonas syringae pv. lachrymans. Moreover, examination of local and systemic gene expression by real-time reverse transcription-polymerase chain reaction analysis revealed that T. asperellum (T203) modulates the expression of genes involved in the jasmonate/ethylene signaling pathways of ISR (Lox1, Pal1, ETR1, and CTR1) in cucumber plants. We further showed that a subsequent challenge of Trichoderma-preinoculated plants with the leaf pathogen P. syringae pv. lachrymans resulted in higher systemic expression of the pathogenesisrelated genes encoding for chitinase 1, beta-1,3-glucanase, and peroxidase relative to noninoculated, challenged plants. This indicates that Trichoderma induced a potentiated state in the plant enabling it to be more resistant to subsequent pathogen infection.

Plant-beneficial effects of Trichoderma and of its genes.

Trichoderma (teleomorph Hypocrea) is a fungal genus found in many ecosystems. Trichoderma spp. can reduce the severity of plant diseases by inhibiting plant pathogens in the soil through their highly potent antagonistic and mycoparasitic activity. Moreover, as revealed by research in recent decades, some Trichoderma strains can interact directly with roots, increasing plant growth potential, resistance to disease and tolerance to abiotic stresses. This mini-review summarizes the main findings concerning the Trichoderma-plant interaction, the molecular dialogue between the two organisms, and the dramatic changes induced by the beneficial fungus in the plant. Efforts to enhance plant resistance and tolerance to a broad range of stresses by expressing Trichoderma genes in the plant genome are also addressed.

Biological control of fungal pathogens

Biological control of soil-borne plant pathogens is a potential alternative to the use of chemical pesticides, which have already been proved to be harmful to the environment. Several strains of the fungusTrichoderma have been isolated and found to be effective biocontrol agents of various soil-borne plant pathogenic fungi under greenhouse and field conditions. Different application approaches have been used including integration ofTrichoderma with reduced doses of chemical agents. Biochemical and molecular biology studies carried out to explore the mechanisms involved in biological control revealed thatTrichoderma is a rather specific mycoparasite. Lectins were found to be involved in the recognition betweenTrichoderma and its host fungi, whereas chitinase is involved in the degradation of the host cell wall. Genetic engineering techniques were employed in order to increase the effectiveness, stability, and biocontrol capacity ofTrichoderma spp. as well as other biocontrol agents, such asPseudomonass spp. andRhizobium.

Biological control of soilborne plant pathogens by a β-1,3 glucanase-producing Pseudomonas cepacia

Abstract A β-1,3 glucanase-producing bacterium identified as Pseudomonas cepacia was isolated on a synthetic medium with laminarin as sole carbon source. In biocontrol experiments carried out under greenhouse conditions, this bacterium decreased the incidence of diseases caused by Rhizoctonia solani, Sclerotium rolfsii and Pythium ultimwn by 85, 48 and 71%, respectively. A determination of lytic enzymes revealed no chitinolytic activity. However, an active and stable β-1,3 glucanase was detected. The optimal temperature and pH values for its activity were 60°C and 5.0, respectively. The induction of this β-1,3 glucanase by different fungal cell walls as sole carbon source in synthetic medium was correlated with the biocontrol of the respective fungi by Pseudomonas cepacia. The damage caused to R. solani hyphae was observed under light and electron microscopes. The role of the β-1,3 glucanase produced by Pseudomonas cepacia in the biological control of soilborne plant pathogens is discussed.

Concomitant Induction of Systemic Resistance to Pseudomonas syringae pv. lachrymans in Cucumber by Trichoderma asperellum (T-203) and Accumulation of Phytoalexins

ABSTRACT Most studies on the reduction of disease incidence in soil treated with Trichoderma asperellum have focused on microbial interactions rather than on plant responses. This study presents conclusive evidence for the induction of a systemic response against angular leaf spot of cucumber (Pseudomonas syringae pv. lachrymans) following application of T. asperellum to the root system. To ascertain that T. asperellum was the only microorganism present in the root milieu, plants were grown in an aseptic hydroponic growth system. Disease symptoms were reduced by as much as 80%, corresponding to a reduction of 2 orders of magnitude in bacterial cell densities in leaves of plants pretreated with T. asperellum. As revealed by electron microscopy, bacterial cell proliferation in these plants was halted. The protection afforded by the biocontrol agent was associated with the accumulation of mRNA of two defense genes: the phenylpropanoid pathway gene encoding phenylalanine ammonia lyase (PAL) and the lipoxygenase pathway gene encoding hydroxyperoxide lyase (HPL). This was further supported by the accumulation of secondary metabolites of a phenolic nature that showed an increase of up to sixfold in inhibition capacity of bacterial growth in vitro. The bulk of the antimicrobial activity was found in the acid-hydrolyzed extract containing the phenolics in their aglycone form. High-performance liquid chromatography analysis of phenolic compounds showed a marked change in their profile in the challenged, preelicited plants relative to that in challenged controls. The results suggest that similar to beneficial rhizobacteria, T. asperellum may activate separate metabolic pathways in cucumber that are involved in plant signaling and biosynthesis, eventually leading to the systemic accumulation of phytoalexins.

Biological Control of the Root-Knot Nematode Meloidogyne javanica by Trichoderma harzianum

ABSTRACT The fungal biocontrol agent, Trichoderma harzianum, was evaluated for its potential to control the root-knot nematode Meloidogyne javanica. In greenhouse experiments, root galling was reduced and top fresh weight increased in nematode-infected tomatoes following soil pretreatment with Trichoderma peat-bran preparations. The use of a proteinase Prb1-transformed line (P-2) that contains multiple copies of this gene improved biocontrol activity in the greenhouse experiments compared with the nontransformed wild-type strain (WT). All the Trichoderma strains showed the ability to colonize M. javanica-separated eggs and second-stage juveniles (J2) in sterile in vitro assays, whereas P-2 also penetrated the egg masses. This protease-transformed line presented the same nematicidal and overall proteolytic activity as the WT in in vitro tests in which concentrated soil extracts from Trichoderma-treated soils immobilized the infective J2. However, the J2 immobilization and proteolytic activities of both P-2 and the WT were higher than those obtained with strain T-203. Characterization of the activity of all Trichoderma strains soil extracts on J2 showed that it was heat resistant and restricted to the low-molecular-weight fraction (less than 3 kDa). It is suggested that improved proteolytic activity of the antagonist may be important for the biological control of the nematodes.