The Influence Of Microbial Fertilizers On The Plants Growth

Microbial fertilizers not only facilitate plant nutrient uptake but also influence its hormonal cross talk. However, whether these microbial fertilizers boost both plant growth and development or whether biofertilizer-facilitated growth results in immunity trade-off still remains unknown. Moreover, interconnection of this phosphorus deficiency with temporal dynamics of the antioxidant enzymes is still unclear. In order to address these questions, we studied the impact of insect herbivory induced oxidative stress in Brown/Indian mustard (Brassica juncea) (Brassicaceae) when it is supplemented by Phosphate-Solubilizing Bacteria (PSB) and Vesicular-Arbuscular Mycorrhiza (VAM), which are established microbial fertilizers. B. juncea is the third most important oilseed crop worldwide, with India contributing about 23.1% of its total production (Meena et al., 2016). It has a high phosphorus requirement for optimum growth and development (Mathur et al., 2011). As major mustard producing areas in India have low phosphorus content (Shekhawat et al., 2012), application of VAM and PSB was found to increase the growth and biomass of B. juncea (Diyansah et al., 2013; Puttaradder and Lakshman, 2015).

However, both PSB and VAM have different mode of action with which they make phosphorus available to plants. PSB, which usually colonize within the plant rhizosphere, directly/indirectly influence plant growth by secreting various chemicals such as organic acids and phytohormones (Sharma et al., 2013). Production of organic acid is desribed to be an important mechanism of phosphate solubilization resulting in acidification of microbial cells and their surroundings (Rodrı́guez and Fraga, 1999). This sometimes leads to inhibition of phytopathogenic fungal mycelium (Park et al., 2009). Moreover, PSBs mediate induced systemic response (ISR) in plants (Chakraborty et al., 2006). On the other hand, VAM’s mechanism of action is quite similar to that of fungal pathogens. VAM penetration in the host roots is initiated due to various secondary metabolite cues of the host (Azcón-Aguilar and Barea, 1997; Vierheilig, 2004).

During early stage of colonization in roots, arbuscular mycorrhizal (AM) fungus causes a transient increase in salicylic acid (SA) production and ROS (Blilou et al., 1999; Fester and Hause, 2005; Jung et al., 2012). However during later stages, a decrease in SA levels was found, while jasmonic acid (JA) levels become predominant. Thus, when mechanical wounding or insect feeding occurs, expression of JA and related defence signals are also amplified in AM supplemented plants (Jung et al., 2012). However, whether these microbial fertilizers help the plants in case of insect induced responses is still unclear. Therefore, we studied the temporal induction of six antioxidants, viz. POD, APX, MDHAR, GR, CAT and SOD in microbial fertilizers supplemented B. juncea following herbivory by the generalist insect Spodoptera litura (Lepidoptera: Noctuidae).