The Role of Nitrogen in Plant and Microbe Interactions

The Role of Nitrogen in Plant and Microbe Interactions

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Role of nitrogen in plant and microbe interactions: Roots excrete a different signals and organic nutrients into the soil, which acts as source for different microbial community (Bais et al., 2006; Drogue et al., 2013 Pothier et al., 2007; Shukla et al., 2011). The microbial community with in the root zone is called as the rhizo-microbiome (Chaparro et al., 2013). In the rhizo-microbiome, some microorganisms promote plant growth and while others are pathogenic (Couillerot et al., 2009; Richardson et al., 2009).The plant growth promoting organism are called Plant growth-promoting rhizobacteria (PGPRs) (Bashan and Holguin, 1998).They are able to colonize the root system and to stimulate growth of the plant (Barea et al., 2005; Benizri et al., 2001; Compant et al., 2010; Drogue et al., 2012; Dutta and Podile, 2010).Under high nitrogen levels plants are susceptibility to diseases which again depends on the form of nitrogen available.The competition for N source between the pathogen and host is a complex process.Pathogen attack affects host N metabolism and total nitrogen of plant have a strong influence on pathogen colonization and proliferation (Ali et al., 2009; Divon and Fluhr, 2007; Hoffland et al., 1999; Snoeijers et al., 2000). Nitrogen is required for the production of plant defense proteins like enzymes, alkaloids, and glucosinolates.The vacuolar stored nitrogen is allocated among tissues under pathogen attack which allows plants to cope with infections (Millard and Grelet, 2010).
The different responses of plants to PGPB include stimulation of root growth and development,stimulation of total nutrient uptake,increase in biomass accumulation and stress tolerance. PGPRs promote plant growth through the bacterial production of plant hormones and are able to convert nitrogen (N2) to ammonia (Asghar et al., 2002; Bashan and Holguin, 1998; Lugtenberg & Kamilova, 2009; Postgate, 1982 Spaepen et al., 2007; Vessey, 2003).PGPR interfere with plant nutrition and development pathways to elicit both increased nutrient acquisition rate and plant growth promotion. Inoculation of plants with Achromobacter sp and Bacillus subtilis strain (GB03) resulted in an increase of nitrate uptake per root surface area and a H+efflux was also enhanced (Bertrand et al., 2000; Sondergaard et al., 2004; Zhang et al., 2009).Plant growth-promoting rhizobacteria can directly increase nutrient supply in the rhizosphere and stimulate ion transport systems in root (Bashan and Holguin, 1998). The nutritional status of a plant affects its tolerance against the infections (Modolo et al., 2006).If ammonium is supplied as N source plants are more susceptible to P.

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syringae infection this is due to ammonium inhibits nitric oxide formation which plays a role in plant defence. Nitrate stimulates synthesis of NO (nitric oxide) and provides tolerance against diseases (Modolo et al., 2005; Long et al., 2000).

Under high nitrogen the lateral root growth is inhibited specially under high nitrate. The inhibition of lateral root development by high nitrate is influenced by soil bacteria. The inhibitory effect can be overridden by inoculation with a plant growth-promoting rhizobacterium (PGPR), Phyllobacterium strain STM196 (Mantelin et al 2006). The impairment in the morphological response to high nitrate supply results in an altered expression of nitrate and ammonium transporter genes. Nitrate transporters NRT2.1, NRT2.5 and NRT2.6 and ammonium transporters AMT1; 1 and AMT1; 2 are associated with PGRR induced changes in the plants.

NRT2.1 has a role in pathogen defense against Pseudomonas syringae pv tomato (Camanes et al., 2012).Mutations in the NRT2.1 gene reduced the susceptibility to pathogens. NRT2.1 is abiotic stress sensor that coordinates the development of the root system with environment and nutrition (Little et al., 2005; Tsay et al., 2007). Changes in plant susceptibility to Pst that depend on the nitrogen, nitrate, or ammonium content of the plant (Long et al., 2000; Modolo et al., 2005, 2006).Under early pathogen attack, nrt2 mutants shows up regulation of salicylic acid signalling pathway and down regulation of Jasmonic acid pathway. In addition, these plants present defects in sensitivity to the bacterial effector coronatine and reduced coronatine sensitivity. SA priming is one of the main mechanisms of induced resistance against Pst (Zimmerli et al., 2000; Conrath et al., 2006).There are several priming inducers that reduce bacterial disease symptoms and plant tissue colonization by stimulating the SA pathway (Conrath et al., 2006; Jung et al., 2009). NF1, an elicitin from Phytophthora infestans, induces NRT2.1 expression in tobacco and it stimulates many SA-dependent responses and HR. Thus, SA signaling and NRT2.1 co-ordinately up-regulated by this pathogenic elicitor (Kawamura et al., 2009). One-tenth of the genome is under the control of nitrate and these genes are also controlled by plant hormones (Gutierrez et al., 2003 ;Krouk et al., 2010). NRT2.1 antagonizes the priming of plant defenses against the bacterial pathogen Pseudomonas syringae pv tomato DC3000 (Pst). The nrt2mutant ( NRT2.1 and NRT2.2) displays reduced susceptibility to bacterium. modifying environmental conditions that stimulate the derepression of the NRT2.1 gene influences resistance to Pst independently of the total level of endogenous nitrogen. Additionally, hormonal homeostasis is affected in nrt2, which displays priming of salicylic acid signaling and concomitant irregular functioning of the jasmonic acid and abscisic acid pathways upon infection. Effector-triggered susceptibility and hormonal perturbation by the bacterium seem to be altered in nrt2, probably due to reduced sensitivity to the bacterial phytotoxin coronatine. The main genetic and metabolic targets of coronatine remain largely unstimulated in nrt2 mutants. The reduced susceptibility of nrt2 mutants seems to be a combination of priming of salicylic acid-dependent defenses and reduced sensitivity to the bacterial effector coronatine. NRT2.1 influence plant disease resistance by down-regulating biotic stress defense mechanisms and favoring abiotic stress responses.

NRT2.6 is also involved in the resistance against Erwinia amylovora (Dechorgnat et al., 2012).The expression of NRT2.5 and NRT2.6 is up regulated in response to inoculation with STM196. In the presence of bacteria these two genes have a role in regulating the root architecture of plant under nitrogen.NRT2.5 and NRT2.6 are important genes for STM196 induced lateral root growth, plant growth promotion and regulation of shoot: root biomass allocation and root development through plant N status. This signal and the NRT2.5 and NRT2.6 dependent mechanism responsible for plant growth promotion under PGPR inoculation remain unknown. One possibility would involve the hormone auxin, as STM196 affects auxin distribution within primary and lateral roots (Contesto et al., 2010). NRT2.5 strongly suppressed by nitrate addition in both roots and shoots and plays a role in the transfer of nitrate from stored pools to cytoplasm (Okamoto et al.2002;Orsel et al .2002b).NRT2.5 is structurally closely related to a yeast transporter. The expression patterns are developmentally regulated, in the young plants NRT2.5 is mostly expressed in the roots while in the old plants increased expression was seen in the aerial parts (Orsel et al .2002;Kotur et al.2012). Members of NRT2.6 are constitutive high affinity nitrate transporter acting as cHATS in both roots and shoots involved in nitrate transport (Okamoto etal.2003Orsel et al . 2002).NRT2.6 is highly expressed in roots and its expression patterns were almost same between young and old plants. (Orsel et al. 2002, 2006;Okamoto et al. 2003 and Kotur et al . 2012).
Ammonium transporter AMT1; 1 expression is up regulated under pathogen attack either bacterial (PstV) or fungal (Ec).Pathogen attack also decreases intracellular Glutamine levels in host cells.Expression of AMT1;1, a molecular sensor of cytosolic Glutamine status (Rawat et al., 1999) was induced during pathogen attack, suggesting that the infected host cells have an underlying deficiency in Glutamine. The Glutamine depletion in infected host tissues might also be due to the specific acquisition and usage of amino acids by the pathogen (Hofreuter et al., 2008; Blume et al., 2009).The inoculation of tomato roots with a Azospirillum brasilense induced the ammonium transporterAmt1.2 gene coding for an ammonium transporter (Beckeret al., 2002).This suggests that due to its N2 fixation activity the bacteria excrete NH4+ in quantities consistent with the induction of the AMT1.2 gene (higher concentrations reduces AMT1.2 mRNA levels). NH4+ ions play a signalling role in the interactions between PGPB and plants (Beckeret al., 2002).

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