Discussion
Result
Effect of carbohydrates on the growth of bacteria in liquid culture:
Pseudomonas fluorescence (CHAO) is one of the most researched PGPR and is known to improve plant growth. Inulin and kappa-carrageenan supplementation significantly improved growth of P. fluorescence (CHAO). After 24 hours of incubation, different concentrations of kappa-carrageenan (0.01, 0.1 and 1%) treatment showed the maximum increment in the growth of P. fluorescence (CHAO). After 48 hours, there was a reduction in the growth of P. fluorescence (CHAO) in all substrate as compared to 24 hours.
Ensifer fredii is a nitrogen-fixing bacteria that promotes plant growth (Rathore et al., 2012). Iota-carrageenan significantly reduced the population of E. fredii. After 48 hours, 0.1% and 1% sucrose showed the maximum increase in the growth of the E. fredii. The growth of the E. fredii in the presence of 1% alginate was at par with growth in 1 % sucrose.
Serratia proteamaculans is known to stimulate plant growth and inhibits the growth of a variety of soil-borne fungal pathogens (Neupane et al., 2013). After 24 hours of incubation, 0.01%, 0.1% and 1% of inulin and kappa-carrageenan significantly improved the growth of S. proteamaculans. However, after 48 hours of incubation, 0.01 and 0.1 % of sucrose induced the growth of S. proteamaculans, as compared to the other treatments, while, 1% of kappa-carrageenan showed the maximum growth of S. proteamaculans as compared to the other treatments.
Growth of bacteria in soil amended with different carbohydrates:
The population of Ensifer fredii (Figure2) and Pseudomonas fluorescence (CHAO) (Figure3) were higher in the soil amended with kappa-carrageenan and inulin, as compared to the sucrose, after 3 days of incubation, while at later time point, the growth of Pseudomonas fluorescence (CHAO) and Ensifer fredii exceeded significantly in the soil amended with sucrose, as compared to the kappa carrageenan and inulin.
The population of Serratia proteamaculans (Figure4) was found higher in the presence of kappa carrageenan and inulin at both the time points. 0.1 % inulin amended soil showed 47.8 % higher population of Serratia proteamaculans as compared to amended soil with sucrose |
The line charts (Figure5) with standard errors, where each line is a carbohydrate treatment. Within the 95% confidence interval, after three days of incubation, the promotion of the population of PGPRs by inulin and iota-carrageenan increased significantly with the increase in concentration, while in the 7th day, only inulin gives the significantly increase in the population of PGPRs with the increase in concentration.
Discussion
Urbanization and ever-increasing world population exerts a pressure to increase yield of crops. This puts ever-increasing pressure on the soil to facilitate greater plant productivity (Shukla et al., 2015). Soil possess highly diverse microbial community, living in intimate association with plants, either in the rhizosphere, or within plant tissues, or as epiphytes attached to aboveground plant tissue, that has a critical impact on plant growth and development (Knief et al., 2012).
Plant growth-promoting rhizobacteria (PGPR) is a group of free-living bacteria that colonize the rhizosphere and contribute to increased growth and yield of crop plants (Shukla et al., 2012). Root exudate plays an important role in colonization of PGPRs. Compounds including the secretion of ions, free oxygen and water, enzymes, mucilage, and a diverse array of carbon-containing primary and secondary metabolites are all root exudation (Bertin et al., 2003 Uren, 2000). Root exudates raise microorganism densities near roots of the plants. In general, the metabolic activity of micro-organisms that live near the roots is often greater than that of soil in bulk (Cunningham et al., 1996).
In this study, I evaluated the influence of polysaccharides alginate (Burdick & Stevens, 2005), inulin(Cui & Roberts, 2009), iota-carrageenan and kappa-carrageenan (Chandrasekaran, 1998) on the growth of PGPRs. The results presented in this study showed that inulin and kappa carrageenan improved the growth of the PGPRs Ensifer fredii, Serratia proteamaculans and Pseudomonas fluorescens CHAO.
While bacteria competing for nutrients and niche in the soil (Lugtenberg & Kamilova, 2009) is a normal phenomena. In 5g of soil, 1% carbohydrate boosted the bacterial population. Soil bacteria require sufficient and available nutrients. Within seven days, the limited nutrients and niche possibly would cause bacteria to overgrow before death.
Similarly, several polysaccharides have been shown to selectively enhance the beneficial bacteria in the soil, thus potentiate the growth of plants.
Sucrose is a common activating agent that increase the activity of most rhizobium and phosphate-solubilizing bacteria (PSB), such as Pseudomonas sp. (Bashan et al., 2014). Solubilization of insoluble phosphate in the rhizosphere is one of the ways that PGPRs promote plant growth (A.E. Richardson et al., 2001; Rodríguez et al., 2006). The addition of sucrose improves the performance of PSB on peanut growth (Taurian et al., 2010).
Acacia exudates carbohydrates which prevents bacteria from desiccation and improve PGPRs survival rate (Deaker et al., 2011). It was further reported that gum Arabic modified the effect of nickel- and zinc-tolerant PGPR Bradyrhizobium sp. on the growth of green gram (Wani et al., 2007). This complex carbohydrate has also been proven to optimize the survival rate of Rhodobacter capsulatus and enhance its promotion of rice growth (Gamal-Eldin & Elbanna, 2011).
Carrageenan is a family of linear sulfated polysaccharides that are extracted from red seaweeds (Kariduraganavar et al., 2014). Shukla et al., (2016) reviewed the role of carrageenan in plant growth promotion and plant defense against plant pathogens. Kappa-carrageenan is used as an effective plant protection agent and growth promoter, especially for chickpea plants (Bi et al., 2011). Kappa-carrageenan is also stronger in larval growth inhibition during plants growth than other carrageenan especially for Arabidopsis plants (Sangha et al., 2010). Similarly, kappa-carrageenan had an activity against tobacco mosaic virus, Pectobacterium carobacterium and Botrytis cinereal (Vera et al., 2011).
Plant growth-promoting rhizobacteria (PGPR) is a group of free-living bacteria that colonize the rhizosphere and contribute to increased growth and yield of crop plants (Shukla et al., 2012). Root exudate plays an important role in colonization of PGPRs. Compounds including the secretion of ions, free oxygen and water, enzymes, mucilage, and a diverse array of carbon-containing primary and secondary metabolites are all root exudation (Bertin et al., 2003 Uren, 2000). Root exudates raise microorganism densities near roots of the plants. In general, the metabolic activity of micro-organisms that live near the roots is often greater than that of soil in bulk (Cunningham et al., 1996).
In this study, I evaluated the influence of polysaccharides alginate (Burdick & Stevens, 2005), inulin(Cui & Roberts, 2009), iota-carrageenan and kappa-carrageenan (Chandrasekaran, 1998) on the growth of PGPRs. The results presented in this study showed that inulin and kappa carrageenan improved the growth of the PGPRs Ensifer fredii, Serratia proteamaculans and Pseudomonas fluorescens CHAO.
While bacteria competing for nutrients and niche in the soil (Lugtenberg & Kamilova, 2009) is a normal phenomena. In 5g of soil, 1% carbohydrate boosted the bacterial population. Soil bacteria require sufficient and available nutrients. Within seven days, the limited nutrients and niche possibly would cause bacteria to overgrow before death.
Similarly, several polysaccharides have been shown to selectively enhance the beneficial bacteria in the soil, thus potentiate the growth of plants.
Sucrose is a common activating agent that increase the activity of most rhizobium and phosphate-solubilizing bacteria (PSB), such as Pseudomonas sp. (Bashan et al., 2014). Solubilization of insoluble phosphate in the rhizosphere is one of the ways that PGPRs promote plant growth (A.E. Richardson et al., 2001; Rodríguez et al., 2006). The addition of sucrose improves the performance of PSB on peanut growth (Taurian et al., 2010).
Acacia exudates carbohydrates which prevents bacteria from desiccation and improve PGPRs survival rate (Deaker et al., 2011). It was further reported that gum Arabic modified the effect of nickel- and zinc-tolerant PGPR Bradyrhizobium sp. on the growth of green gram (Wani et al., 2007). This complex carbohydrate has also been proven to optimize the survival rate of Rhodobacter capsulatus and enhance its promotion of rice growth (Gamal-Eldin & Elbanna, 2011).
Carrageenan is a family of linear sulfated polysaccharides that are extracted from red seaweeds (Kariduraganavar et al., 2014). Shukla et al., (2016) reviewed the role of carrageenan in plant growth promotion and plant defense against plant pathogens. Kappa-carrageenan is used as an effective plant protection agent and growth promoter, especially for chickpea plants (Bi et al., 2011). Kappa-carrageenan is also stronger in larval growth inhibition during plants growth than other carrageenan especially for Arabidopsis plants (Sangha et al., 2010). Similarly, kappa-carrageenan had an activity against tobacco mosaic virus, Pectobacterium carobacterium and Botrytis cinereal (Vera et al., 2011).
Prospects
Applications of PGPR for stress tolerance in crops
At present, excessive input of chemical fertiliser in the agricultural field poses threat to environment. Previously, studies have shown that PGPRs have made great contributions in this field: acting as antagonists and biocontrol agents (Bashan et al., 1999), ameliorating the flooding stress to the deaminase-containing plant (Grichko & Glick, 2001), assisting plants grow in acid environment (Belimov et al., 1998), applying in phytoremediation technologies (Huang et al., 2004) .
Applications in the Forestry
Although the role of PGPRs in forestry is far less than that in agriculture, PGPRs still plays an important role in forestry. Compared with agriculture, the life cycle of trees is long, and the application of PGPRs in forestry aims to increase the biomass in forests, rather than increase the yield of trees (Lucy et al., 2004). PGPRs also plays an important role in the process of seedling transplanting from nursery to field, and reduces the damage during seedling emergence and transplanting (Shishido & Chanway, 2000).
Unlike crops, trees are perennial, and it's inevitable for them to overwinter. The cold winter is a severe test for any tree species. Especially in countries with higher latitudes, such as Canada, Switzerland and Russia, trees will lose their growth momentum due to harsh environment in those places. PGPRs that can survive in winter plays an important role in plant growth. Bacillus polymyxa and Pseudomonas fluorescens have been shown that they are symbiotic with the roots of trees to promote the growth of trees in the cold winter (Chanway et al., 2000). There is a research gap on the application of PGPRs in forestry is still very few, and thereby, there are few conclusions that can be obtained at present.
Unlike crops, trees are perennial, and it's inevitable for them to overwinter. The cold winter is a severe test for any tree species. Especially in countries with higher latitudes, such as Canada, Switzerland and Russia, trees will lose their growth momentum due to harsh environment in those places. PGPRs that can survive in winter plays an important role in plant growth. Bacillus polymyxa and Pseudomonas fluorescens have been shown that they are symbiotic with the roots of trees to promote the growth of trees in the cold winter (Chanway et al., 2000). There is a research gap on the application of PGPRs in forestry is still very few, and thereby, there are few conclusions that can be obtained at present.