Md. Tofazzal Islam

Suppression of Damping-Off Disease in Host Plants by the Rhizoplane Bacterium Lysobacter sp. Strain SB-K88 Is Linked to Plant Colonization and Antibiosis against Soilborne Peronosporomycetes

ABSTRACT We previously demonstrated that xanthobaccin A from the rhizoplane bacterium Lysobacter sp. strain SB-K88 suppresses damping-off disease caused by Pythium sp. in sugar beet. In this study we focused on modes of Lysobacter sp. strain SB-K88 root colonization and antibiosis of the bacterium against Aphanomyces cochlioides, a pathogen of damping-off disease. Scanning electron microscopic analysis of 2-week-old sugar beet seedlings from seeds previously inoculated with SB-K88 revealed dense colonization on the root surfaces and a characteristic perpendicular pattern of Lysobacter colonization possibly generated via development of polar, brush-like fimbriae. In colonized regions a semitransparent film apparently enveloping the root and microcolonies were observed on the root surface. This Lysobacter strain also efficiently colonized the roots of several plants, including spinach, tomato, Arabidopsis thaliana, and Amaranthus gangeticus. Plants grown from both sugar beet and spinach seeds that were previously treated with Lysobacter sp. strain SB-K88 displayed significant resistance to the damping-off disease triggered by A. cochlioides. Interestingly, zoospores of A. cochlioides became immotile within 1 min after exposure to a SB-K88 cell suspension, a cell-free supernatant of SB-K88, or pure xanthobaccin A (MIC, 0.01 μg/ml). In all cases, lysis followed within 30 min in the presence of the inhibiting factor(s). Our data indicate that Lysobacter sp. strain SB-K88 has a direct inhibitory effect on A. cochlioides, suppressing damping-off disease. Furthermore, this inhibitory effect of Lysobacter sp. strain SB-K88 is likely due to a combination of antibiosis and characteristic biofilm formation at the rhizoplane of the host plant.

Isolation and Identification of Plant Growth Promoting Rhizobacteria from Cucumber Rhizosphere and Their Effect on Plant Growth Promotion and Disease Suppression

Plant growth promoting rhizobacteria (PGPR) are the rhizosphere bacteria that may be utilized to augment plant growth and suppress plant diseases. The objectives of this study were to identify and characterize PGPR indigenous to cucumber rhizosphere in Bangladesh, and to evaluate their ability to suppress Phytophthora crown rot in cucumber. A total of 66 isolates were isolated, out of which 10 (PPB1, PPB2, PPB3, PPB4, PPB5, PPB8, PPB9, PPB10, PPB11, and PPB12) were selected based on their in vitro plant growth promoting attributes and antagonism of phytopathogens. Phylogenetic analysis of 16S rRNA sequences identified these isolates as new strains of Pseudomonas stutzeri, Bacillus subtilis, Stenotrophomonas maltophilia, and Bacillus amyloliquefaciens. The selected isolates produced high levels (26.78–51.28 μg mL-1) of indole-3-acetic acid, while significant acetylene reduction activities (1.79–4.9 μmole C2H4 mg-1 protein h-1) were observed in eight isolates. Cucumber plants grown from seeds that were treated with these PGPR strains displayed significantly higher levels of germination, seedling vigour, growth, and N content in root and shoot tissue compared to non-treated control plants. All selected isolates were able to successfully colonize the cucumber roots. Moreover, treating cucumber seeds with these isolates significantly suppressed Phytophthora crown rot caused by Phytophthora capsici, and characteristic morphological alterations in P. capsici hyphae that grew toward PGPR colonies were observed. Since these PGPR inoculants exhibited multiple traits beneficial to the host plants, they may be applied in the development of new, safe, and effective seed treatments as an alternative to chemical fungicides.

Chemotaxis of fungal zoospores, with special reference to Aphanomyces cochlioides

Zoospores of phytopathogenic fungi accumulate at the potential infection sites of host roots by chemotaxis. The aggregated spores then adhere, encyst, germinate, and finally penetrate into the root tissues to initiate infection. Some of the host-specific attractants have already been identified. The host-specific attractants also induce cell differentiation of certain zoospores under laboratory conditions. This indicates that a signal released from the roots of the host plant guides the pest propagules for orientation and prepares them for establishing a host-pathogen relationship by necessary physiological changes. Some non-host plant secondary metabolites were found to markedly regulate behavior and viability of zoospores, suggesting that non-host compounds may also play a role in protecting the non-host plants from the attack of zoosporic fungi. We hypothesized that zoospores perceive the host signal(s) by specific G-protein-coupled receptors and translate it into responses by way of the phosphoinositide-Ca2+ signaling cascade. The details of the signal transduction mechanism in fungal zoospores are yet to be discovered. In this report, we review the signaling and communications between phytopathogenic fungal zoospores and host and non-host plants with special reference to Aphanomyces cochlioides.

Isolation and Identification of Potential Phosphate Solubilizing Bacteria from the Rhizoplane of Oryza sativa L. cv. BR29 of Bangladesh

A total of 30 bacteria were isolated from the rhizoplane of rice cv. BR29 cultivated in Mymensingh, Bangladesh and from the seedlings obtained from surface-sterilized seeds of BR29. Upon screening, 6 isolates showed varying levels of phosphate solubilizing activity in both agar plate and broth assays using National Botanical Research Institute’s phosphate medium. The bacterial isolates were identified based on their phenotypic and 16S rRNA genes sequencing data as Acinetobacter sp. BR-12, Klebsiella sp. BR-15, Acinetobacter sp. BR-25, Enterobacter sp. BR-26, Microbacterium sp. BRS-1 and Pseudomonas sp. BRS-2. The BR-25 exhibited highest phosphate solubilizing activity followed by BR-15. They grew rapidly in the liquid medium at pH 5 and 7 but almost no growth occurred at pH 3. The pH value of the culture medium was decreased with bacterial growth suggesting that they might secrete organic acids to solubilize insoluble phosphorus. Scanning electron microscope analysis of two-week-old rice seedlings germinated from seeds previously inoculated with BR-25 and BR-15 revealed dense colonization at the root surfaces presumably using fimbriae on the bacterial cells.

Gageotetrins A-C, noncytotoxic antimicrobial linear lipopeptides from a marine bacterium Bacillus subtilis.

Gageotetrins A-C (1-3), a unique class of linear lipopeptides, consisting of di- and tetrapeptides and a new fatty acid were isolated from a Marine Bacillus subtilis. The structures of 1-3 were assigned by spectroscopic data and their absolute stereochemistries were ascertained by chemical derivatization. Compounds 1-3 displayed good antimicrobial activities with MIC values of 0.01-0.06 μM. However, these compounds failed to register any cytotoxicity (GI50 > 30 μg/ml) against human cancer cell lines.

Host-specific plant signal and G-protein activator, mastoparan, trigger differentiation of zoospores of the phytopathogenic oomycete Aphanomyces cochlioides

We found that the gradient of a host-specific attractant, cochliophilin A (5-hydroxy-6,7-methylenedioxyflavone) isolated from the roots of spinach triggered encystment followed by germination of zoospores of Aphanomyces cochlioidesat a concentration less than micromolar order. This compound did not affect the growth and reproduction of this phytopathogen up to 10−6 M concentration in the culture medium. We also observed that mastoparan, an activator of heterotrimeric G-protein could inhibit the motility of zoospores and then strikingly effect encystment followed by 60–80% germination of cysts. Concomitant application of cochliophilin A and mastoparan showed stronger encystment followed by 100% germination of cysts. In addition, we have observed that chemicals interfering with phospholipase C activity (neomycin) and Ca2+ influx/release (EGTA and loperamide) suppress cochliophilin A or mastoparan induced encystment and germination. These results suggest that G-protein mediated signal transduction mechanism may be involved in the differentiation of the A. cochlioides zoospores. This is the first report on the differentiation of oomycete zoospores initiated by a host-specific plant signal or a G-protein activator.

Khatmiamycin, a motility inhibitor and zoosporicide against the grapevine downy mildew pathogen Plasmopara viticola from Streptomyces sp. ANK313.

In the course of our screening for anti-peronosporomycetal agents, we isolated a new compound khatmiamycin (1), together with five known metabolites, GTRI-02 (3), 4-ethyl-5-methyl-heptanamide (4), aloesaponarin II (5), LL-C10037α (6) and LL-C10037β (7) from the culture broth of a terrestrial Streptomyces sp. ANK313. The structures of these metabolites were assigned on the basis of their spectroscopic data. Khatmiamycin (1) exhibited potent motility inhibitory (100%) and lytic (83±7%) activities against zoospores of the grapevine downy mildew pathogen Plasmopara viticola at 10 μg ml−1, followed by compounds 5 (MIC 25 μg ml−1), 7, 6, 3 in the order of decreasing activity. Khatmiamycin (1) also showed potent antibacterial activity against Staphylococcus aureus and Streptomyces viridochromogenes (Tü57) by causing inhibition zones of 11 and 14 mm diameter, respectively, at the dose of 40 μg per disk. This is the first report on motility inhibitory and lytic activities of metabolites from a terrestrial Streptomyces species against the zoospores of downy mildew pathogen P. viticola.

Morphological studies on zoospores of Aphanomyces cochlioides and changes during interaction with host materials

The behavioral and morphological diversity of intact zoospores of Aphanomyces cochlioides and their dynamic morphological changes when interacting with the host or a host-specific zoospore attractant were studied by light and electron microscopy. The reniform-ovate zoospore has two heterokont flagella, both inserted in a ventral groove. The anterior flagellum possesses two rows of tripartite tubular hairs (TTHs) distributed throughout its length, whereas the posterior flagellum was ornamented with two rows of highly dense fine tubular hairs, except on its tapered terminal part. The tip of the posterior flagellum has a bunch of similar fine hairs like those on the flagellum shaft. The biflagellated zoospores quickly aggregated on the host (spinach) root, adhered to the root surface, encysted by shedding or retracting the flagella, germinated at a fixed point to form germ tubes, and finally invaded the root tissues via the appressoria within 50–60 min. This precise homing response of A. cochlioides zoospores reflects guidance by a host-specific signal(s) for locating potential infection sites, differentiation of zoospores to cystospores, formation of infection structures and/or germ-tube tropism for completing pre-infection events in haste. Prior to the encystment, the posterior flagellum may be involved in successful docking on the root surface by tip contact followed by shedding or retraction.

Mesoporous metallic rhodium nanoparticles

Mesoporous noble metals are an emerging class of cutting-edge nanostructured catalysts due to their abundant exposed active sites and highly accessible surfaces. Although various noble metal (e.g. Pt, Pd and Au) structures have been synthesized by hard- and soft-templating methods, mesoporous rhodium (Rh) nanoparticles have never been generated via chemical reduction, in part due to the relatively high surface energy of rhodium (Rh) metal. Here we describe a simple, scalable route to generate mesoporous Rh by chemical reduction on polymeric micelle templates [poly(ethylene oxide)-b-poly(methyl methacrylate) (PEO-b-PMMA)]. The mesoporous Rh nanoparticles exhibited a ∼2.6 times enhancement for the electrocatalytic oxidation of methanol compared to commercially available Rh catalyst. Surprisingly, the high surface area mesoporous structure of the Rh catalyst was thermally stable up to 400 °C. The combination of high surface area and thermal stability also enables superior catalytic activity for the remediation of nitric oxide (NO) in lean-burn exhaust containing high concentrations of O2.

An antagonistic rhizoplane bacterium Pseudomonas sp. strain EC-S101 physiologically stresses a spinach root rot pathogen Aphanomyces cochlioides

We observed that an antagonistic rhizoplane bacterium Pseudomonas sp. strain EC-S101 induces excessive lateral and apical branching in the hyphae of a root rot phytopathogen Aphanomyces cochlioides AC-5 resulting in radial growth inhibition of hyphae in a dual culture assay. Confocal laser scanning microscopic observations using fluorescent stains indicated an increased quantity of nuclei and lipid bodies in the affected hyphae during the early stage (less affected hyphae) at day 3 of interaction. At a more advanced stage (severely affected hyphae) at day 3, nuclei became smaller and round-shaped compared with the oval shape in AC-5 control hyphae. After 7 days, nuclei disintegrated, and the nuclear materials were released into the disorganized cytoplasm. With transmission electron microscopy at 5 days of interaction, we found that the cell walls of AC-5 hyphae were considerably thicker than those of the control. Enlarged vacuoles, lipid bodies sunk into vacuoles, and vacuoles filled with electron-dense material, followed by an invagination of the AC-5 hyphal cell wall, were commonly observed. Nonmembranous electron-transparent inclusion bodies irregular in size were often distributed in the affected hyphae. By integrating our observations, we conclude that antagonistic effects evoked by strain EC-S101 resulted in the death of AC-5 hyphae, which might contribute to the suppression of A. cochlioides AC-5-linked damping-off disease in its host plants.