Plant can detect and respond to plant pathogens by using light perceiving ability of photoreceptor proteins i.e., specific photoreceptor can sense the intensity and the spectral composition of the light. The light sensing mechanisms in plant cells are localized inside and outside of the chloroplast and the nucleus (Karpinski et al., 2003). The focal spot under epidermal cells is affected by the direction of light that falls on the cells (Vogelmann et al., 1996). Light can influence microbe morphology and reproduction. In fungus, detection of light is facilitated by diverse classes of photoreceptor protein, which in turn coordinate with growth, spore dispersal, stress resistance, primary metabolism and toxin production (Fuller et al., 2016).
Blue light is perceived by two different receptor the cryptochromes and phototropins. These blue light photoreceptors are required for the stability of the R protein and has a role in R-protein mediated resistance to Turnip crinkle virus (Jeong et al., 2010). The blue light receptor cryptochrome and phototropin are not essential for local resistance responses of inoculated Arabidopsis leaves i.e., salicylic acid accumulation, defense gene expression, the hypersensitive response and basal or specific resistance towards Pseudomonas syringae (Griebel and Zeier 2008).
In Arabidopsis blue light photoreceptor cryptochrome 1 (CRY1) regulate inducible resistance to Pseudomonas syringae under continuous light condition. Wu and Yang (2010) result showed CRY1 regulate systemic acquired resistance and that CRY1 influence salicylic acid accumulation and has no effect on hypersensitive cell death. This suggests that CRY1 may regulate R protein-mediated resistance to P. syringae with increased pathogenesis related gene expression(Wu and Yang 2010). Botrytis cinerea causing grey mold in lettuce is suppressed and an increase in antioxidant capacity is observed by blue-light treatment (Kook et al., 2013). The monochromatic blue light reduces infection by modulating Pseudomonas syringae pv. actinidiae virulence more than host plant defense response (Correia et al., 2022). Blue and red-light treatment resulted in accumulation of stilbenic compounds and differential expression of gene involved in defense response (Ahn et al., 2015). Stilbenes are low molecular weight phenolics with antifungal activity enables plant to cope with pathogen attack (Bavaresco et al., 2009). Blue light enhances photosynthesis and increase carbohydrate and flavonoid accumulation (Le et al., 2021).
Antimicrobial blue light in the spectrum of 400 – 470 nm has intrinsic antimicrobial properties resulting from the presence of endogenous photosensitizing chromophores in pathogenic microbe (Wang et al., 2017). Blue light alone can effectively reduce bacterial and fungal (Fusarium graminearum) viability (De Lucca et al., 2012). The mycelial growth of Botrytis cinerea was inhibited by 405-nm light indicating that this wavelength of light has the potential to control the disease caused by B. cinerea (Imada et al., 2014). The effect of 405- and 470-nm blue light in vitro was observed to be bactericidal on Pseudomonas aeruginosa and Staphylococcus aureus but not on Propionibacterium acnes (Guffey and Wilborn 2006). Different studies produce different result, showing, the use of light must be customized according to the plant-pathogen system. Serratia marcescens strain B2is a biocontrol agent that produces antibiotic pigment prodigiosin. Blue light affects the stability of prodigiosin (Someya et al., 2004).
Plants grown under purple or blue light has higher activities of phenylalanine ammonia-lyase and polyphenoloxidase as well as higher level of flavonoids than plant grown under other light (Wang et al., 2010). Blue light radiation inhibits growth of Botrytis cinerea mycelium (Xu et al., 2017). Blue-LED light protects crop plant from pathogenic attack. Mechanistically blue-LED are ascribed into increased production of osmoprotectants and antioxidants including reactive oxygen species scavenging enzymes (Kim et al., 2013). Blue-LED increases proline and phenolic compounds in plant tissues (Kim et al., 2013). Proline provides resistance to plant against pathogenic attack and the proline content depends on light intensity and nutrient concentration (Grote and Claussen 2001). Blue light reduced in vitro fungal growth of Penicillium italicum and Phomopsis citri (Liao et al., 2013). Continuous application of red or blue light at night suppress sporulation by Peronospora belbahrii the causal organism of basil downy mildew (Radetsky et al., 2020).In brief studies indicate signaling pathway are activated under blue light.
References:
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BIOLOGICAL PROCESSES- A NATURE'S GIFT 