Md. Motaher Hossain

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.

Plant Probiotics in Phosphorus Nutrition in Crops, with Special Reference to Rice

Low level of soluble soil phosphorus (P) is a serious constraint in crop production in tropical and subtropical soils. Many plant-associated bacteria or plant probiotics can solubilize P from either organic- or inorganic-bound phosphates thereby facilitating plant growth. Understanding the bacterial contribution to plant P nutrition and opportunities for manipulating specific bacterial strain to enhance P availability in soil has, therefore, been of considerable interest over many decades. This interest is accentuated by rising costs of P fertilizer and because of low efficiency of P use by plants from soil and fertilizer sources. Bacteria from diverse taxonomic genera such as Pseudomonas, Bacillus, Klebsiella, Streptomyces, Burkholderia, Pantoea, Enterobacter, etc., can solubilize soil insoluble P and increase growth and yield of crops. The widely recognized mechanisms of phosphate solubilization mediated by these plant-associated bacteria are production of organic acids (such as gluconic, citric, and oxalic) and/or secretion of hydrolytic enzymes (such as phytases, phosphatase, etc.). Despite their potential as low-input practical agents for plant P nutrition, application of phosphate solubilizing bacteria (PSB) has been hampered by their inconsistent performance in the field. Hence, the full potentials of PSB have not yet been achieved for P nutrition in major crop production. Therefore, better understanding on how they interact with roots and other organisms in the rhizosphere is needed. This chapter reviews advances on PSB research and their potential for phosphorus nutrition in crop plants, with special reference to rice. Mode of action of phosphate solubilization by bacteria and their uses as biofertilizer for eco-friendly low-input sustainable crop production are also discussed.

Biological Control of Peronosporomycete Phytopathogen by Bacterial Antagonist

Peronosporomycetes are devastating pathogens to numerous crop, ornamental, and native plants. They are phylogenetically distinct from those of fungi and hence most of the fungicides are ineffective against them. A large body of literature reveals that several bacterial genera such as Pseudomonas, Bacillus, Burkholderia, Lysobacter, Enterobacter, etc. exert antagonistic activities against the peronosporomycete phytopathogens in both in vitro and in vivo conditions. These bacterial strains originated from diverse habitats and some of them showed high promise for biocontrol of plant diseases caused by these notorious pathogens. Mechanisms of biocontrol by these bacterial antagonists include (1) antibiosis, including biosurfactant activity, (2) secretion of lytic enzymes, (3) competition for nutrients (C and Fe), (4) high plant and hyphal colonization, (5) hyperparasitism, and (6) development of induced systemic resistance in the host plants. This chapter comprehensively reviews advances of research on biocontrol of peronosporomycete phytopathogens by bacterial antagonists including the mode of actions of the antagonistic principles against the pathogens. Recent advances on genome sequencing of several peronosporomycetes and biocontrol agents will provide basis for better understanding of bacteria–plant–pathogen interactions and development of improved strains that will potentially function as effective biocontrol agents against the notorious peronosporomycete phytopathogens for low input sustainable agriculture.

Multiple Rpp -gene pyramiding confers resistance to Asian soybean rust isolates that are virulent on each of the pyramided genes

Since Asian soybean rust (ASR) isolates in South America are highly virulent, diverse, and distantly related to Japanese ones, limited numbers of resistance resources are available in soybean breeding in that region. Pyramiding of available ASR resistance genes (Rpp) in a single soybean genotype may provide wider spectrum and higher level of ASR resistance to soybean. However, the desired combinations of genes conferring adequate resistance to highly virulent or distantly related ASR isolates have not yet been studied. In this study, seven pyramided lines carrying multiple Rpp genes have been developed and evaluated for their resistance against one ASR isolate from Japan and two from Brazil. Significantly higher resistance was observed in the pyramided lines, No6-12-B (Rpp4 + Rpp5), Oy49-4 (Rpp2 + Rpp3 + Rpp4), and No6-12-1 (Rpp2 + Rpp4 + Rpp5) compared to the original resistance sources, PI 230970 (Rpp2), Hyuuga (Rpp3), PI 459025 (Rpp4), and Kinoshita (Rpp5) carrying single Rpp genes. Although infection of the resistance sources with the highly virulent Brazilian ASR isolates resulted in susceptible phenotypes with moderate to abundant sporulation, highly resistant phenotypes with almost no sporulation were observed in the three Rpp-pyramided lines. Therefore, pyramided lines carrying these Rpp gene combinations are useful in soybean breeding for conferring broad spectrum, high resistance to ASR isolates that are virulent to the varieties carrying single resistance genes.

Chilli rhizosphere fungus Aspergillus spp. PPA1 promotes vegetative growth of cucumber (Cucumis sativus) plants upon root colonisation

A rhizosphere fungus was isolated from roots of chilli plants and identified as Aspergillus spp. PPA1. The fungus was tested for its ability to promote the growth of cucumber plants in a pot experiment. Cucumber seeds were sown in sterilised field soil amended with wheat grain inoculum (WGI) of PPA 1 at the rate of 0.5, 1 and 1.5% w/w, and plants were grown for 21 days in a net house. The treatment with PPA1 significantly increased shoot length, shoot fresh weight, shoot dry weight, root length, root fresh weight, root dry weight, plant length, leaf area and leaf chlorophyll content of cucumber plants compared to non-treated control. The growth promotion rate increased with the increasing concentration of inoculum of PPA1 applied to the soil. The fungus was re-isolated from the roots of cucumber plants at higher frequencies. These results suggest that Aspergillus spp. PPA1 is a root colonising plant-growth promoting fungus for cucumber.

Molecular mapping of Asian soybean rust resistance in Chinese and Japanese soybean lines, Xiao Jing Huang, Himeshirazu, and Iyodaizu B

Asian soybean rust (ASR), caused by the fungus Phakopsora pachyrhizi, is one of the most serious soybean diseases in South America and other tropical and sub-tropical areas. The soybean lines, ‘Xiao Jing Huang,’ ‘Himeshirazu,’ and ‘Iyodaizu B’ were previously identified for their resistance to ASR fungus, while the genetic basis of the resistance has yet to be known. In this study, we mapped the ASR resistance loci in these three lines using three independent mapping populations derived from crosses with an ASR susceptible variety, BRS184. In each population, resistance to ASR appeared to be primarily controlled by a single major gene. The resistance genes with largest effect from Xiao Jing Huang and Himeshirazu were genetically mapped on chromosome 18 corresponding to the same location of known resistant locus, Rpp1. On the other hand, quantitative trait locus analysis mapped the major ASR resistance locus of Iyodaizu B to the region of chromosome 16 where Rpp2 was previously mapped. Genetic mapping with DNA markers and disease reactions of seven candidates carrying Rpp1 to four Brazilian ASR isolates revealed a significant variation in their ASR resistance reaction, indicating that they share different resistance genes tightly linked to each other or different resistant alleles of a single Rpp1 gene. Therefore, these seven soybean lines could be clearly separated into at least two functional groups.

Growth promotion effect of Fusarium spp. PPF1 from bermudagrass (Cynodon dactylon) rhizosphere on Indian spinach (Basella alba) seedlings are linked to root colonisation

A rhizosphere fungus was isolated from roots of bermudagrass (Cynodon dactylon) and identified as Fusarium spp. PPF1. A pot experiment was conducted to test its ability to promote the vegetative growth of Indian spinach seedlings (Basella alba). Indian spinach seeds were sown in sterilised field soil amended with wheat grain inoculum of PPF1 at the rate of 0.5 and 1.0% w/w, and plants were grown for 21 days in a net house. Significantly, higher germination percentage and vigour index were observed due to application of PPF1 in the potting soil. Treatment with PPF1 also significantly increased shoot length, shoot fresh weight, shoot dry weight, root length, root fresh weight, root dry weight, leaf area and leaf chlorophyll content of cucumber plants compared to non-treated control. The growth promotion rate increased with the increasing concentration of inoculum of PPF1 applied to the soil. The fungus was re-isolated from the roots of cucumber plants at higher frequencies, while a positive co-relation was found between the root colonisation ability and the plant growth enhancement by the isolate. These results suggest that growth promotion effect of Fusarium spp. PPF1 on Indian spinach (B. alba) are linked to root colonisation ability of the fungus.

A comparative screening of hybrid, modern varieties and local rice cultivar for brown leaf spot disease susceptibility and yield performance

Five rice cultivars, one hybrid (WR96), three modern (BR16, BR26, and BRRI Dhan27) and one local (Pari) were screened for reaction to brown leaf spot disease caused by Cochliobolus miyabeanus and performance of yield-related characters. The severity of brown leaf spot varied with growth stages of rice plant as well as different cultivars tested under field condition. Low disease severity was observed at maximum tillering stage compared to moderate to high at dough stage, with hybrid cultivar WR96 showing highest disease, while local cultivar Pari had the lowest. Brown spot disease severity in different cultivars under induced epiphytic condition also followed the similar trend. The results also revealed that most of the yield-contributing characters examined showed wide variations among the cultivars. Modern cultivar BR16 produced the highest panicle length, number of grain per panicle and grain yield per hectare. At the same time as local cultivar Pari generated the lowest number of tiller per plant, panicle length, grain number per panicle and grain yield per hectare. Moreover, hybrid cultivar WR96 produced the highest percentage of spotted grain per panicle and seed yielding C. miyabeanus, and also the lower percentage of seed germination, while the reverse was observed in local cultivar Pari. These findings may allow producers and breeders to select rice cultivar, resistant or tolerant to brown leaf spot disease and to avoid significant reductions in grain yields.

Plant-Associated Bacteria in Nitrogen Nutrition in Crops, with Special Reference to Rice and Banana

Associative and endophytic bacteria that fix atmospheric nitrogen (diazotrophs) are promising alternatives to synthetic nitrogen (N) fertilizers. Application of those diazotrophic bacteria substantially supplements the N requirement and promotes the growth of crop plants. A large body of literature reveals that bacteria from diverse genera such as Acetobacter, Azoarcus, Azospirillum, Bacillus, Burkholderia, Herbaspirillum, Klebsiella, Rhizobium, and Pseudomonas are capable of N2 fixation and thus exert beneficial effects on N nutrition, enhanced uptake of other essential nutrient elements, promotion of root growth, and other physiological activities in economically important nonlegume crop plants like rice and banana. Review of literature on the effects of associative diazotrophic bacteria (e.g., Azospirillum sp.) on rice, wheat, and maize indicates that substantial amount of N can be incorporated into these plants by the bacterial inoculants. Similarly, biofertilization by associative bacteria has been found to promote root growth, nutrient uptake, dry matter yield, and fruit quality of bananas (Musa sp. cv. “Berangan,” AA type). This chapter reviews recent findings on N nutrition, root stimulation, plant growth, and yield by associative and endophytic bacteria in rice and banana. Recent progress in understanding underlying molecular mechanism of N nutrition of plants by the help of diazotrophic bacteria and molecular tools for incorporating N2 fixing gene into plant associated bacterium or plant will create excellent opportunity for increasing crop productivity of economically important nonlegume crops such as rice, wheat, maize and banana.

Plant Growth-Promoting Fungi (PGPF): Phytostimulation and Induced Systemic Resistance

The associations between plants and multipurpose plant growth-promoting fungi (PGPF) have been proven extremely to be beneficial to plants. This review describes new knowledge about the interactions between plants and their associated PGPF in determining improved plant growth and induced systemic resistance (ISR) to invading pathogens. It has been shown that fungi of heterogeneous classes and habitats function as PGPF. The well-known fungal genera Aspergillus, Fusarium, Penicillium, Piriformospora, Phoma, and Trichoderma are the most frequently reported PGPF. On comparing the results of different studies, it appears that plant-PGPF interactions can have positive effects on belowground and aboveground plant organs. The most commonly reported effects are significant improvement in germination, seedling vigor, biomass production, root hair development, photosynthetic efficiency, flowering, and yield. Some strains have the abilities to improve plant biochemical composition. It has now known that PGPF can also control numerous foliar and root pathogens by triggering ISR in the host plants. These capabilities are driven by their abilities to enhance nutrient uptake and phytohormone production as well as to reprogram plant gene expression, through differential activation of plant signaling pathways. The PGPF-triggered plant growth and ISR responses to pathogen attack may work through genetype-dependent manner in plants.