Respostas bioquímicas comparativas de genótipos suscetíveis e resistentes de feijão-de-corda [Vigna unguiculata (l.) Walp.] desafiados com o vírus do mosaico severo do caupi (CPSMV)

Abstract

Cowpea [Vigna unguiculata (L.) WalP.] has great socioeconomic importance in the North and Northeast Brazil, other countries of South and Latin America, Africa, and Asia. Although its rusticity and high natural resistance to most diseases, cowpea severe mosaic virus (CPSMV), which belongs to the family Comoviridae, has great importance because its severity and extensive damage and loss it causes in cowpea. Although there are resistant genotypes to CPSMV, it is not well understood the mechanisms involved in defense. This comparative study was conducted with two resistant and two susceptible cowpea genotypes to shed light on the mechanisms of susceptibility and resistance to CPSMV. Thus, differences in the enzyme activities related to the oxidative stress involving superoxide dismutase (SOD), catalase (CAT), peroxidase ascorbate (APX), in addition to enzymes of the phenol metabolism as phenylalanine ammonia-lyase (PAL), guaiacol peroxidase (POX), and polyphenol oxidase (PPO) were analyzed. Quantitative differences in phenolic compounds, superoxide ion (O2-) and hydrogen peroxide (H2O2) were also evaluated. After inoculation with CPSMV, or only with the abrasive carborundum (controls), the secondary leaves of each genotype were collected at 0, 6, 24, 48, and 72 hours after inoculation (HAI) and the biochemical analyzes performed. In addition, the transcript levels of SOD, CAT, and APX were evaluated at 6 HAI, by quantitative polymerase chain reaction (RT-qPCR). In CPSMV infected resistant cowpea plants compared to the respective controls higher POX, PPO, PAL, SOD, but low CAT and APX activities were observed. Moreover, O2-, H2O2 and phenolic compounds showed higher contents than those of the respective controls at some time points after inoculation of the resistant cowpea genotypes with CPSMV. Opposite responses regarding to these above biochemical parameters were observed for the susceptible genotypes. Expression of the CAT transcript was in agreement with the CAT activity at 6 HAI. However, for only some cowpea genotypes the APX transcript synthesis at 6 HAI was in agreement with the enzyme activity. Nevertheless, the results suggest participation of SOD, CAT, APX, PAL, POX, PPO, associated with phenolic compound, O2-, and H2O2 levels in the mechanisms of resistance/susceptibility of the cowpea to CPSMV infection.