Endophytes are microorganisms which live symbiotically with almost all varieties of plant and in turn helping the plant in a number of ways. Instead of satisfactory surface sterilization approaches, repeatedly occurring bacterial growth on in vitro rootstock cultures of peach and pear was identified and isolated as endophytic bacteria in our present study. Five different isolates from peach rootstocks were molecularly identified by 16S rRNA gene sequencing as Brevundimonas diminuta, Leifsonia shinshuensis, Sphingomonas parapaucimobilis Brevundimonas vesicularis, Agrobacterium tumefaciens while two endophytic isolates of pear were identified as Pseudoxanthomonas mexicana, and Stenotrophomonas rhizophilia. Identified endophytes were also screened for their potential of plant growth promotion according to indoleacetic acid (IAA) production, nitrogen fixation, solubilization of phosphate and production of siderophore. All seven endophytic isolates have shown positive results for IAA, nitrogen fixation and phosphate solubilization tests. However, two out of seven isolates showed positive results for siderophore production. On the basis of these growth promoting competences, isolated endophytes can be presumed to have significant influence on the growth of host plants. Future studies required to determine the antimicrobial susceptibility profile and potential application of these isolates in biological control, microbial biofertilizers and degradative enzyme production.
Introduction Use of bacteria as a biofertilizer or biocontrol agent in agriculture became widespread in the late 1990s. Recently, the plant-growth-promoting and productivity enhancement properties of rhizobacteria have been studied worldwide. Agricultural policies that promote the use of environment friendly products, such as biofertilizers, have gained increasing importance nowadays (Chauhan et al., 2015). Commercial fertilizers containing strains of the genus Bacillus have been given considerable importance because they are more tolerant of extreme abiotic conditions, such as temperature, pH, and pesticides. Bacillus-based biofertilizers became a candidate for commercial production as they are not harmful to the environment and humans, play a role in biocontrol against pathogenic fungus, grow rapidly in the soil, and increase the nutrient uptake of plants to improve the productivity of plants (Kumar et al., 2011). Phosphorus is a main vital macronutrient for plants known to perform many functions in their growth and metabolism. Numerous important cellular, metabolic, and reproductive mechanisms depend on adequate phosphorus supply (Chen et al., 2006; Kaymak, 2010). Although soils contain sufficient amounts of phosphate, only a very small quantity is accessible to plants. The ability of the microorganisms to solubilize phosphate is a key character related to plant phosphate nutrition (Hayat et al., 2010; Bhattacharrya and Jha, 2012; Sharma et al., 2013). The process of phosphate solubilization by phosphatesolubilizing bacteria (PSB) strains is linked with the production of low molecular weight organic acids, via which hydroxyl and carboxyl groups chelate cations bind to phosphate; thus soluble phosphate is obtained (Chen et al., 2006). Various types of microorganisms have been used as phosphate-solubilizing biofertilizer (Malboobi et al., 2009). Production is a major property of plant growthpromoting bacteria (Mohite, 2013). IAA is one of the most physiologically active auxins. Bacteria synthesize auxins in order to perturb host physiological processes for their own benefit. The microorganisms isolated from the rhizosphere region of various crops can produce IAA. IAA
The intrinsic ability of microalgae to accumulate high amounts of carotenoids has made them the preferred aquatic organisms of biotechnological exploration for carotenoid production. To continuously innovate and modify microalgal bioprocesses, aquaculture scientists have been working hard for the past decades in a forwardlooking way, and mixotrophic cultivation of microalgae is deemed as a promising strategy to decrease production cost. This review is intended to summarise the recent research advancement of carotenoids production from mixotrophically cultivated microalgae, starting from the structure, biosynthesis, physiological roles and applications of carotenoids and followed by the production processes both currently established and under development. Most importantly, the microalgal physiology of mixotrophic cultivation is reviewed in depth both in general and specifically for the most studied species, and the prospects of commercially viable mixotrophic microalgal processes for carotenoid production along with the insight of future research are of course discussed. Finally, we conclude that mixotrophy might be a promising strategy for large-scale cultivation of microalgae to produce carotenoids although some technical obstacles need to be overcome.
Although Haematococcus lacustris has been developed for astaxanthin production for decades, the production cost is still high. In order to modify the production processes, we proposed a novel strategy of cultivation, featured by sequential indoor continuous mixotrophic cultivation for the production of green cells followed by outdoor phototrophic induction for astaxanthin accumulation. The continuous mixotrophic cultivation was first optimized indoor, and then the seed culture of mixotrophic cultivation was inoculated into outdoor open raceway ponds for photoinduction. The results showed that mixotrophically grown cultures could efficiently grow without losing their photosynthetic efficiency and yielded higher biomass concentration (0.655 g L−1) and astaxanthin content (2.2% DW), compared to phototrophically grown seed culture controls. This novel strategy might be a promising alternative to the current approaches to advance the production technology of astaxanthin from microalgae.
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