A review on plant and pathogen derived carbohydrates, oligosaccharides and their role in plant's immunity

Polysaccharides play a pivotal role throughout the life cycle of plants in a diversified manner which includes the building of a physical structure, storing the energy, participating in metabolic events, signaling, and in defense responses. Many pathogens have evolved to utilize plants as a nutrient source breaching the preformed barriers like tough polysaccharide-made cell wall which acts as a first line of defense in plants. However, the pathogens overcome this primary barrier by secreting hydrolytic enzymes to dissolve the cell walls. In a never-ending war between plants and pathogens, plants developed the ability to recognize the degraded products of cell walls and activate an additional defense response, referred to as an induced defense. Pathogen-associated polysaccharides such as lipopolysaccharides, bacterial-originated peptidoglycan, fungal-derived β-glucan, chitin, chitosan originated oligosaccharides are recognized by the plants and elicit defense responses. Because of the increasing evidence on the utilization of polysaccharides in plant protection, the current review is focused on various nano-polysaccharides and their potential role in sustainable agricultural practices

Harpin_Pss-mediated enhancement in growth and biological control of late leaf spot in groundnut by a chlorothalonil-tolerant Bacillus thuringiensis SFC24

Chemical and biological approaches have been adopted to increase the growth and yield of crops and reduce loss due to diseases. We have adopted an integrated approach, where both direct antagonism and induced resistance were combined to reduce the incidence of late leaf spot (LLS) disease in groundnut caused by Phaeoisariopsis personata. Chitinolytic chlorothalonil-tolerant soil bacterium Bacillus thuringiensis SFC24 (Bt SFC24) was manipulated in vitro to express secretable form of elicitor protein harpin_Pss of Pseudomonas syringae pv. syringae. Severity of the LLS decreased by 65% when the leaves were sprayed with B. thuringiensis expressing harpin_Pss (Bt-pss). As seed treatment, there was an increase in growth of groundnut. Bt and Bt-pss accounted to 13% and 36% increase in shoot length. Expression of a secretable form of harpinPss thus improved the ability of B. thuringiensis SFC24 to promote growth and control LLS in groundnut. In this new approach a chlorothalonil-tolerant chitinolytic bacterium was genetically engineered to secrete elicitor harpin_Pss for dual benefit of growth promotion and disease control

Stomatal Closure and Rise in ROS/NO of Arabidopsis Guard Cells by Tobacco Microbial Elicitors: Cryptogein and Harpin

Plants use stomatal closure mediated by elicitors as the first step of the innate immune response to restrict the microbial entry. We present a comprehensive study of the effect of cryptogein and harpin, two elicitors from microbial pathogens of tobacco, on stomatal closure and guard cell signaling components in Arabidopsis thaliana, a model plant. Cryptogein as well as harpin induced stomatal closure, while elevating the levels of reactive oxygen species (ROS) and nitric oxide (NO) in the guard cells of A. thaliana. Kinetic studies with fluorescent dyes revealed that the rise in ROS levels preceded that of NO in guard cells, when treated with these two elicitors. The restriction of NO levels in guard cells, even by ROS modulators indicates the essentiality of ROS for NO production during elicitor-triggered stomatal closure. The signaling events during elicitor-induced stomatal closure appear to converge at NADPH oxidase and ROS production. Our results provide the first report on stomatal closure associated with rise in ROS/NO of guard cells by cryptogein and harpin in A. thaliana. Our results establish that A. thaliana can be used to study stomatal responses to the typical elicitors from microbial pathogens of other plants. The suitability of Arabidopsis opens up an excellent scope for further studies on signaling events leading to stomatal closure by microbial elicitors

Isolation and purification of microbial community DNA from soil naturally enriched for chitin

We made an attempt to isolate and purify metagenomic DNA from chitin enriched soil. In this communication we report a modified direct lysis method for soil DNA extraction including initial pre-lysis washing of sample, followed by a rapid polyvinylpyrrolidone-agarose-based purification and electroelution of DNA using Gene-capsule^TM assembly. Rapidity was achieved using low molarity conducting media (sodium-borate buffer) for electrophoresis by reducing run time for both the gel electrophoresis and electroelution. Extracted DNA was sufficiently pure and of high quality, evidenced by amplification of 16S rDNA and chitinase genes by PCR. Metagenomic nature of the DNA was confirmed by running V₃ (16S rDNA) region amplicons using denaturing gradient gel electrophoresis. This method requires 30 min for purification, and less than 2 h for complete execution of protocol and becomes the first report on the isolation of metagenomic DNA from soil naturally enriched for chitin

Biophysical investigations on the aggregation and thermal unfolding of harpin<SUB>Pss</SUB> and identification of leucine-zipper-like motifs in harpins

Harpins are heat-stable, glycine-rich proteins secreted by Gram-negative bacteria, which induce hypersensitive response (HR) in non-host plants. In this study, the thermal unfolding and aggregation of harpinPss from Pseudomonas syringae pv. syringae have been investigated by biophysical approaches. Differential scanning calorimetric studies indicate that thermal unfolding of harpinPss is a complex process involving three distinct transitions. CD spectroscopy revealed that the secondary structure of the protein, which is predominantly &#945;-helical, remains unchanged until the onset of transition 2, above which the &#945;-helical content decreases while the &#946;-sheet content increases. Dynamic light scattering measurements yielded the hydrodynamic radius (Rh) of harpinPss as room temperature as 20.54 &#177; 6.19 nm, which decreases to 9.35 nm at 61 &#176;C. These results could be explained in terms of the following thermal unfolding pathway for harpinPss: oligomer &#8594; dimer &#8594; partially unfolded dimer &#8594; unfolded monomer. Sequence analysis indicated the presence of at least two leucine-zipper-like motifs in harpinPss and several other harpins, whereas computational modelling studies suggest that most of them are located on helices present on protein surfaces, suggesting that they can take part in the formation of oligomeric aggregates, which may be responsible for HR elicitation by harpins and their high thermal stability

Biotechnological approaches to develop bacterial chitinases as a bioshield against fungal diseases of plants

Fungal diseases of plants continue to contribute to heavy crop losses in spite of the best control efforts of plant pathologists. Breeding for disease-resistant varieties and the application of synthetic chemical fungicides are the most widely accepted approaches in plant disease management. An alternative approach to avoid the undesired effects of chemical control could be biological control using antifungal bacteria that exhibit a direct action against fungal pathogens. Several biocontrol agents, with specific fungal targets, have been registered and released in the commercial market with different fungal pathogens as targets. However, these have not yet achieved their full commercial potential due to the inherent limitations in the use of living organisms, such as relatively short shelf life of the products and inconsistent performance in the field. Different mechanisms of action have been identified in microbial biocontrol of fungal plant diseases including competition for space or nutrients, production of antifungal metabolites, and secretion of hydrolytic enzymes such as chitinases and glucanases. This review focuses on the bacterial chitinases that hydrolyze the chitinous fungal cell wall, which is the most important targeted structural component of fungal pathogens. The application of the hydrolytic enzyme preparations, devoid of live bacteria, could be more efficacious in fungal control strategies. This approach, however, is still in its infancy, due to prohibitive production costs. Here, we critically examine available sources of bacterial chitinases and the approaches to improve enzymatic properties using biotechnological tools. We project that the combination of microbial and recombinant DNA technologies will yield more effective environment-friendly products of bacterial chitinases to control fungal diseases of crops

Plant growth-promoting chitinolytic paenibacillus elgii responds positively to tobacco root exudates

Bacterial strains from chitin/chitosan-rich soils, from two industries, were screened for their chitinolytic, antifungal, and mineral phosphate solubilization abilities. The isolate SMA-1-SDCH02, positive for all three properties, was selected and identified as Paenibacillus elgii based on morphological and biochemical characters and supported by 16S rRNA gene sequence analysis. P. elgii enhanced the growth of groundnut in terms of shoot height, root length, total chlorophyll, and fresh and dry weight when applied alone or in combination with chitosan. The plant growth-promoting activity of P. elgii was seen in tobacco in a specially designed gnotobiotic setup indicating its capability to promote growth of at least groundnut and tobacco. Metabolite changes in the bacteria, studied using attenuated total reflectance-infrared (ATR-IR) spectroscopy, revealed split bands of amide I at the 1659- and 1636-cm−1 regions when grown in minimal media amended with tobacco root exudates. The difference in ATR-IR bands in the presence of tobacco root exudates indicated production of compounds with differences in functional groups