Photosynthetic and biochemical mechanisms of an EMS-mutagenized cowpea associated with its resistance to cowpea severe mosaic virus

Abstract

The seed treatment of a CPSMV-susceptible cowpea genotype with the mutagenic agent EMS generated mutagenized resistant plantlets that respond to the virus challenge by activating biochemical and physiological defense mechanisms. Cowpea is an important crop that makes major nutritional contributions particularly to the diet of the poor population worldwide. However, its production is low, because cowpea is naturally exposed to several abiotic and biotic stresses, including viral agents. Cowpea severe mosaic virus (CPSMV) drastically affects cowpea grain production. This study was conducted to compare photosynthetic and biochemical parameters of a CPSMV-susceptible cowpea (CE-31 genotype) and its derived ethyl methanesulfonate-mutagenized resistant plantlets, both challenged with CPSMV, to shed light on the mechanisms of virus resistance. CPSMV inoculation was done in the fully expanded secondary leaves, 15 days after planting. At 7 days post-inoculation, in vivo photosynthetic parameters were measured and leaves collected for biochemical analysis. CPSMV-inoculated mutagenized-resistant cowpea plantlets (MCPI) maintained higher photosynthesis index, chlorophyll, and carotenoid contents in relation to the susceptible (CE-31) CPSMV-inoculated cowpea (CPI). Visually, the MCPI leaves did not exhibit any viral symptoms neither the presence of the virus as examined by RT-PCR. In addition, MCPI showed higher SOD, GPOX, chitinase, and phenylalanine ammonia lyase activities, HO, phenolic contents, and cell wall lignifications, but lower CAT and APX activities in comparison to CPI. All together, these photosynthetic and biochemical changes might have contributed for the CPSMS resistance of MCPI. Contrarily, CPI plantlets showed CPSMV accumulation, severe disease symptoms, reduction in the photosynthesis-related parameters, chlorophyll, carotenoid, phenolic compound, and HO contents, in addition to increased β-1,3-glucanase, and catalase activities that might have favored viral infection.

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“…Changes in chlorophyll content are evident in studies that compare susceptible and resistance to the same viruses. For example, a recently published paper involving Cowpea severe mosaic virus (CPSMV, genus Comovirus) infection in susceptible cowpea (Vigna unguiculata) plants evidenced a reduction of 32 and 40% in chlorophyll b and a, respectively, in comparison to resistant plants (Souza et al 2017). In this work, it is clear that the changes in chlorophyll are important steps for symptom development and disease establishment.…”

“…Until 2012, already had been registered 92 genera of plant viruses at the International Committee on Taxonomy of Viruses database (King et al 2012). Plant (+) RNA viruses can infect several cultures with agronomic importance such as beans, soybean, tomato, peanut, cassava, wheat, rice, cucumber, potato, tobacco, barley, and peppers, which corroborate the importance of these pathogens in limitation of food production worldwide (Elvira et al 2008;Scholthof et al 2011;Hill et al 2014;Kenyon et al 2014;Shoup Rupp et al 2016;Dempsey et al 2017;Souza et al 2017).…”

“…Virus-chloroplast interaction is critical for the development of typical disease symptoms during virus infection, such as changes in chloroplast structures, reduction of photosynthesis efficiency and reduction of chlorophyll levels (Shimura et al 2011;Liu et al 2014;Souza et al 2017). The origin of viral symptoms is quite complex and different sources have been implicated in this process; however, it has been suggested that the primary source for symptoms arising are alterations in the normal function of the chloroplast, which produce chlorosis and mosaic (Rahoutei et al 2000).…”

“…Viruses' effects on chlorophyll metabolism are an essential aspect that has to be considered in symptom development during the plant-virus interaction. In most cases, initial symptoms of viral infection on leaves are the appearance of yellow patches, white spots and the mosaic formation (Shimura et al 2011;Liu et al 2014;Souza et al 2017). These symptoms are the result of virus interference in chlorophyll metabolism, leading to its degradation, affecting photosynthesis (Liu et al 2014).…”

“…These symptoms are the result of virus interference in chlorophyll metabolism, leading to its degradation, affecting photosynthesis (Liu et al 2014). Recently published studies with viruses from different families and different plant species report the drastic reduction in chlorophyll levels during compatible plant-virus interaction (Kundu et al 2013;Rys et al 2014;Souza et al 2017).…”