Recognition Of Early Stress Signals

Recognition of early stress signals for stress is an important strategy for plant survival and adaptation. One of the primary events in induction of plant responses is the transient generation of reactive oxygen species (ROS), initiated within minutes of insect herbivory, leading to oxidative stress (Torres, 2010). However, an elevated and sustained phase of ROS generation takes place after hours of this initial response, where superoxide anion (O2-), hydroxyl radical (OH-) and hydrogen peroxide (H2O2) play a predominant role (Bi and Felton, 1995; Miller, 2009; Torres, 2010). These ROS cause oxidation of various biomolecules within the damaged cells, resulting in metabolic disruption and sometimes even cell death (Van Breusegem, 2006). On the other hand, ROS also play a major role in signal transduction pathway during early stress responses thus initiating a cascade of other defence responses (Pei et al., 2000).

In field, plants are regularly faced with various above ground and below ground biotic stress such as insect, microbes or nematodes (Agarwal et al., 1999; van Dam et al., 2018) and abiotic stress such as deficiency, salinity, drought (Kandlbinder et al., 2004; Miller et al., 2009). Insect infestation causes >20% net annual productivity loss in plants (Agrawal 2011). Following insect damage, ROS act as secondary messengers in signal transduction pathway by modulating ion channel activity, leading to the production of antioxidants. These enzymatic and non-enzymatic antioxidants help in maintaining cellular homeostasis (Orozco-Cardenas and Ryan, 1999; Mittler et al., 2011, Xia et al., 2015). Within minutes of ROS generation, Superoxide Dismutase (SOD) (EC.1.15.1.1) catalyses breakdown of superoxide radical (O2−) and leads to the production of relatively stable H2O2 and oxygen (Racchi, 2013) (Figure 1a). H2O2 then spreads systemically via cell-to-cell relay mechanism (Orozco-Cardenas et al., 2001; Miller et al., 2009). Level of H2O2, thus produced, is regulated by a group of Catalases (CAT) (EC.1.11.1.6), Peroxidase (POD) (EC.1.11.1.7) and Ascorbate Peroxidases (APX) (EC.1.11.1.11) that are located in various cellular compartments (Figure 1b, c, d ) (Blokhina et al., 2003, Racchi, 2013).

Both CAT and POD catalyze conversion of H2O2 into water and oxygen, whereas APX catalyses the same reaction using ascorbate as elctron donor. This leads to initiation of a series of reactions that involve ascorbate-glutathione (ACS-GSH) cycle in chloroplast and mitrochondria (López-Vidal et al., 2016). Excess H2O2 leads to increase in CAT and POD activity (Davletova et al., 2004). While acting as electron donor, ascorbate is converted to monodehydroascorbate (MDHA), which is parallelly reduced by the enzyme monodehydroascorbate reductase (Figure 1e)(MDHAR) (EC.1.6.5.4). Excess MDHA is readily converted to dehydroascorbate (DHA) and ascorbate. DHA thus produced, is reduced by DHA reductase with glutathione (GSH) as electron donor. Reduction of oxidized glutathione is the final step of ACS-GSH cycle. It is catalyzed by the enzyme Glutathione Reductase (GR) (EC.1.6.4.2), which uses NADPH as the electron donor (figure 1f) (López-Vidal et al., 2016). This cycle in turn triggers a cascade of reactions leading to induction of responses to counter the insect stress. (concluding sentence).

Following stress, a major share of nutrition is allocated from plant growth to defences in order to sustain these metabolic and physiological changes. Thus, in order to maintain both growth and stress response, plants increase their nutrient uptake from soil following insect damage (Kessler and Baldwin, 2002, Zhou et al., 2015, Peschiutta et al., 2018). However, deficiency of nutrients in soil results in hampered plant growth.

Phosphorus is an essential macronutrient for plant physiology and productivity (Sample and Soper, 1980). Its deficiency leads to PSII impairment, photoinhibition in plant cell, alterations in metabolic pathway, accumulation of biomass and decrease in Calvin cycle (Shin et al., 2005; Xu et al., 2007). However, excessive use of concentrated chemical fertilizers in agricultural systems to supplement phosphorus has led to various long term and cumulative problems (Bates and Lynch, 2000). This has generated world-wide interest in eco-friendly alternates such as Vesicular-Arbuscular Mycorrhiza (VAM), Aspergillus niger, some species of Penicillium and Phosphate Solubilizing Bacteria (PSB), such as Pseudomonas fluorescens and Burkholderia cepacia (Azcon, 1986; Rodrı́guez and Fraga, 1999; Singh, 2011) for making bound soil phosphate available to plants (Mishra et al., 2014; Lhungdim and Devi, 2015).

Earlier studies have found that use of microbial fertilizers also led to improvement in plant physiology under stress conditions (Aliasgharzad et al., 2006; Alavi et al., 2013; Nadeem et al., 2014). Moreover, they also influence production/suppression of various phytohormones such as ethylene (ET) (Glick et al., 1998), jasmonic acid (Nagata et al., 2015) and salicylic acid (Singh et al., 2003), which are the key modulators in signal transduction against herbivory defence.