Light is an energy source and provides signal for defense responses against plant pathogens. To colonize and infect the host the pathogen needs to overcome the defensive barrier. Plants can sense different intensity, quality and duration of light.
Photosynthesis occurs in chloroplast and generates adenosine triphosphate, nicotinamide adenine dinucleotide phosphate hydrogen and carbohydrate. These are further utilized in the synthesis of primary metabolites, hormones such as salicylic acid, jasmonic acid, ethylene and abscisic acid as well as antimicrobial compounds. Besides this chloroplast also generates reactive oxygen species (ROS), nitric oxide and a site for calcium signaling (Delprato et al., 2015; Lu and Yao 2018). ROS including hydrogen peroxide produced during plant defense response may have a destructive and toxic effect on pathogens (Chen and Schopfer 1999), toughens cell wall in plants (Bradley et al., 1992), mediates lipid peroxidation (to play a role in execution of cell death) (Montillet et al., 2005), triggers defense genes activation and phytoalexin accumulation (Thoma et al., 2003), induces hypersensitive response (HR) and this HR is accompanied by the development of systemic acquired resistance in establishment of plant defense response against the virulent pathogen (Alvarez et al., 1998) and is required in ligand-induced endocytosis of patten recognition receptors (Serrano et al., 2007). Chloroplast also interacts with other ROS-producing organelles such as the apoplast, peroxisomes and mitochondria to establish a ROS network that triggers subset of defense responses (Delprato et al., 2015).
Green light (480 – 570 nm) induce disease resistance. The green light irradiation inhibited strawberry anthracnose caused by Glomerella cingulata incidence in field (Kudo et al., 2011). Green and red light suppressed infection caused by Pseudomonas cichorii JBC1 thereby increasing resistance of tomato plant against the pathogen (Rajalingam and Lee 2015). Phytotoxic lipopeptides and siderophores production in Pseudomonas cichorii JBC1were reduced by green light which may affect this pathogen survival (Rajalingam and Lee 2018). Green light reduced disease severity in tomato plants by modulating defense-related gene expression (Rajalingam and Lee 2015). The mycelial growth of Colletotrichum acutatum causing anthracnose in pepper plant was reduced by red and green light as compared to blue, white light and dark condition (Yu et al., 2013). While the conidial germination rate was insignificant under white, red, green light and in dark (Yu et al., 2013).
Responses to light begins with activation of photoreceptors that are specific to wavelength of light (Folta and Carvalho 2015). Carvalho and Folta (2016) study showed that anthocyanin accumulation is induced with far-red light and green light cannot reverse it and may enhance their accumulation under low fluence rate condition. Responses of green light share a tendency to oppose blue- or red-light-induced responses including stem growth rate inhibition, anthocyanin accumulation or chloroplast gene expression (Zhang and Folta 2012). Anthocyanin absorbs more green light than red light and prevent plant defense secondary metabolite such as tharubine-A, which may otherwise degrade if exposed to visible light or ultraviolet radiation (Gould et al., 2010; Rajalingam and Lee 2015). Green light treatment elevated both jasmonic acid and salicylic acid level in wild type plants (Sato et al., 2015).
Fungal photoreceptor comprises the green light sensing microbial rhodopsins (Schumacher 2017). Green light retards germination and germ tube growth in vitro (Schumacher 2017). Fusarium fujikuroi cause bakanae-disease in rice. The two different rhodopsin present in F. fujikuroi are OpsA and CarO. The CarO is a green light-activated proton pump distributed in plasma membrane of conidia which retards the growth of conidia and hyphal development in fungus (Garcia-Martinez et al., 2015). Green light suppresses conidial germination and mycelial growth of Botrytis cinerea (Zhu et al., 2013). High peroxidase and superoxide dismutase activity was observed in green light in callus culture of Lepidium sativum (Ullah et al., 2019). These antioxidant enzymes have a role in sustaining cell viability during metabolic reaction by protecting them from the free radical toxicity and spectral lights have a role in regulation of enzymatic activities (Suzuki and Mittler 2005; Ullah et al., 2019).
Insect pests, damage plant by feeding and often transmit pathogenic viruses to plants. The lime green light emitting diodes equipped yellow sticky card trap is used in green house for insect detection and control (Chu et al., 2008; Ben-Yakir et al., 2012). Green light is effective against spider mite in field (Kudo et al., 2011).
Light influence plant pathogen interaction hence can help in designing strategy to reduce plant disease.
References:
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BIOLOGICAL PROCESSES- A NATURE'S GIFT 