Diverse & rapidly evolving pathogens and global climate changes threaten crop yield and food security. The increased use of synthetic pesticides and fertilizers provides immediate solutions for the alleviating plant disease and crop yield but drastically affect human and environment health. Although bio-pesticides, bio-fertilizers, and bio-control agents derived from living microbes are becoming suitable replacements for the hazardous synthetic pesticides and fertilizers, their reduced efficiency, high costs and inconsistent field performance generally relegate them to niche products.
Bacterial inoculants are reported to modulate plant growth, metabolic & defense gene expression and development through the emission of mVOCs. The impact of VOCs produced by rhizospheric bacteria on plants and their important role in plant-bacterial interactions has been well-documented. It is reported that VOCs act both as plant growth promoters and inhibitors, of which 2,3-butanediol and acetoin (3-hydroxy-2-butanone) produced by Bacillus sps., are the best-known growth-promoting VOCs (Rath et al.,2018). In addition, VOCs such as 2-pentylfuran, 13-tetradecadien-1-ol, 2-butanone, 2-methyl-n-1-tridecene, from various bacterial species have also been reported to promote plant growth, whereas hydrogen cyanide (HCN), dimethyl sulfide and inorganic volatiles, are reported to be phytotoxic to plants. VOCs also trigger induced systemic resistance in several plant species in response to pathogen challenge.
The present study reports the growth promoting; antagonistic; & gene elicitation potential of Sg8, through its ability to produce VOCs. The data recorded on the inhibition of fungal pathogens through the production of volatile and non-volatile metabolites by Sg8, show that it significantly inhibited the growth of all the pathogens tested. Maximum inhibition of mycelia growth of M. phaseolina was observed followed by M. oryzae, B. cinerea, Colletotrichum sp., and F. oxysporum. The inhibitory effects observed can be mainly attributed to the antibiosis effect of the volatile metabolites and induction of defense responsive genes in plants. Thus Sg8 can be considered as a promising bio-control agent for control of root-rot/wilt diseases, as it is able to colonize in advance to the pathogens and stimulate PR genes & promote plant growth under biotic stress
The test pathogen B. cinerea is a non-specific, necrotrophic pathogen that reportedly cause heavy crop loss due high disease incidence in most vegetable and oil crops (Elad et al. 1994; Cowan et al. 2005; Swartzberg et al. 2008; Petrasch et al. 2019). Invitro evaluation of the mVOC’s released from Sg8 showed that the inhibitory effect of the isolate from marine algae was substantial (73%) and cogitates its evaluation in open field.
The inhibitory activity of mVOC’s released by Sg8 was effective in controlling the growth (60%) of F. oxysporum, a fungal pathogen, which is a wide spread threat to a variety of economical important crops like cotton, chickpea, banana, melon and tomato(Michielse and Rep 2009; Gawehns et al. 2015).
The mVOC’s of Sg8 showed significant inhibitory effect (67%), on the phyto pathogen Colletotrichum geosporioides, a causal agent of anthracnose disease in vegetables, fruits, legumes and ornamental plants (Luis Fernando Zepeda-Giraud et al. 2016). Similarly the mVOCs showed a significant effect on the mycelial growth of Magnoporthae oryzae (a causative agent for rice blast) and M. phaseolina (a causative agent of seedling blight, collar rot, stem rot & root rot). Review of the literature show that this is the first report on the inhibitory potential of Shewanella algae strain Sg8 against M. oryzae (77%) and M. phaseolina (88%), thus underlining the potential of the strain as a biological input in Agriculture. This is also the first study investigating the growth promoting potential of MVOC’s of SG8 and it demonstrates considerable growth modulation effect on tomato seedlings when the bacterium is grown on culture media beneath the plant system.
GS enzyme is responsible for the production of glutamine in the leaf and generally GS activity is lower during fruit development, due to low metabolic activity, high requirement of carbohydrate, sugar and protein demand for the fruit formation. Glutamine, the major transported amino acid, generally increases in the senescing leaves compared to the early phase of the harvest, as the GS that synthesize, glutamine, is transferred to the growing tissues in plants. The expression of leaf GS activity in the leaves of treated & control plants was monitored to assess if the VOCs produced by Sg8 could activate metabolic machinery, through up-regulation of the glutamine synthase.
The genes encoding for GS and CS did not show any increased expression in the plants exposed to volatiles emitted by Sg8.
The genes coding for pathogen resistance (1, 2 and 5) showed higher levels of expression after 14 days in plants induced with FOC infection. The accumulation of PR RNAs correlates with the synthesis of PR proteins that are commonly observed to escalate in response to pathogen attack by the plant (Van Loon et al. 2006). Reports show an abundance of specific tomato proteins, including PR proteins, with changes in the xylem sap in Fusarium infected plants (Rep et al. 2002; Houterman et al. 2007).
Plants have evolved the ability to systemically defend to pathogens, through systemic acquired resistance (SAR) and induced systemic resistance (ISR) (Berendsen et al. 2015; Radhesh et al. 2020). A set of PR proteins, such as PR1, PR2 and PR5, were identified as the basic characteristic of SAR signaling pathway (Molinari et al. 2014), and PR3, as the marker of ISR signaling pathway (Yu et al. 2018). The marker genes, PR1, PR2 and PR5 induced by validamycin to enhance plant resistance are considered as markers of SAR (Khunnamwong et al. 2020).
Sg8 mVOC-induced defense responses in tomato in terms of the expression of salicylic acid (SA) dependent marker genes were analysed by qPCR. The expression level of the SA inducible gene PR-1 (unknown anti-fungal function), PR-2 (β-1,3-glucanase) and PR-5 (thaumatin-like protein) genes (Cao Hui et al. 1994; Cao et al. 1997, 1998), showed higher expression ( 9.5 fold) of PR-5 gene in plants with the application of mVOCs from Sg8 which could induce enhanced disease-resistance. This investigation indicates that the reported S. algae strain Sg8 has the potential to control Fusarium wilt disease and promote growth and induce resistance to pathogens in tomato.
Several studies demonstrate mVOCs can inhibit a range of plant pathogens, highlighting their suitability as a potential sustainable alternative to pesticides. One of the first examples demonstrating an inhibitory role for mVOCs against plant pathogens were those produced by Pseudomonas species isolated from soybean and canola, in the inhibition of Sclerotinia sclerotiorum; a fungal pathogen with a broad host range of over 400 plant species (Fernando et al. 2005). Of 23 VOCs identified from Pseudomonas species, six significantly reduced mycelial growth of S. sclerotiorum. Similarly, VOC production by two strains of Bacillus endophytes significantly reduced the weight and number of the vegetative, long-term survival structures (sclerotia) of S. sclerotiorum (Tahir et al. 2017a). VOCs from Burkholderia ambifaria and a range of other rhizobacterial isolates (Groenhagen et al. 2013) have also demonstrated the ability to inhibit growth of the ubiquitous soil‐borne pathogen Rhizoctonia solani. MVOCs can also display inhibitory activity against bacterial pathogens. Exposure of Clavibacter michiganensis, the causal agent of bacterial ring rot of potato, to VOCs from Bacillus subtilis led to significant inhibition of pathogen growth, with benzaldehyde, nonanal, benzothiazole and acetophenone specifically demonstrating inhibitory activities (Tahir et al. 2017b). Bacillus VOCs also inhibited the growth of Xanthomonas oryzae, the causal agent of bacterial leaf blight of rice, with decyl alcohol and 3,5,5‐trimethylhexanol specifically inhibiting pathogen growth (Fernando et al. 2005; Srinivasan et al. 2017).
Recent advances on the research aspects of microbial volatile & its interactions with plants has on the whole unfolded the understanding of the dynamic nature of mVOCs, & their potential role in enhancing crop protection and productivity in a sustainable way. It is observed that exposing plants to mVOCs, results in a significant modulation of plant metabolomics, physiology, and transcriptional status, which confirms that plants have the ability to perceive and respond to microbial volatile compounds. Most of the studies have, however, been conducted under lab conditions, though recently few studies been performed in open field conditions to demonstrate efficient adoption of mVOCs for sustainable crop protection and production (Gong et al. 2015; Tahir et al. 2017b; Fincheira and Quiroz 2018). These studies clearly demonstrate the need for implementation of MVOCs application in Agriculture taking advantage of the multiple functions exhibited such as increase in pathogen resistance, protection against herbivores ; enhancement in plant growth and control of disease & pests of plants. In addition the application of mVOCs in agriculture ensures a sustainable crop protection and production strategy as a possible substitute for hazardous and synthetic chemical pesticides and fertilizers.
According to Lemfack et al., (Lemfack et al. 2014), the application of mVOCs as plant defense and growth modulators is yet to be established, as out of the 10,000 microbial species described the mVOCs released by 400 bacteria and fungi have been described in the literature.