Renata Sousa Resende

Leaf Gas Exchange and Oxidative Stress in Sorghum Plants Supplied with Silicon and Infected by Colletotrichum sublineolum

Considering the economic importance of anthracnose, caused by Colletotrichum sublineolum, and silicon (Si) to enhance sorghum resistance against this disease, this study aimed to investigate the effect of this element on leaf gas exchange and also the antioxidative system when infected by C. sublineolum. Plants from sorghum line CMSXS142 (BR 009 [Tx623] - Texas), growing in hydroponic culture with (+Si, 2 mM) or without (-Si) Si, were inoculated with C. sublineolum. Disease severity was assessed at 2, 4, 6, 8, and 10 days after inoculation (dai) and data were used to calculate the area under anthracnose progress curve (AUAPC). Further, the net carbon assimilation rate (A), stomatal conductance to water vapor (g(s)), internal-to-ambient CO₂ concentration ratio (C(i)/C(a)), and transpiration rate (E); the activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR); the electrolyte leakage (EL), and the concentrations of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) were determined. The AUAPC was reduced by 86% for the +Si plants compared with the -Si plants. The values of A, g(s), and E were lower upon inoculation of -Si plants in contrast to inoculated +Si plants with decreases of 31 and 60% for A, 34 and 61% for g(s), and 27 and 57% for E, respectively, at 4 and 8 dai. For the noninoculated plants, there was no significant difference between the -Si and +Si treatments for the values of A, g(s), and E. The C(i)/C(a) ratio was similar between the -Si and +Si treatments, regardless of the pathogen inoculation. The activities of SOD, CAT, APX, and GR tended to be higher in the +Si plants compared with the -Si plants upon inoculation with C. sublineolum. The EL significantly increased for -Si plants compared with +Si plants. The MDA concentration significantly increased by 31 and 38% at 4 and 8 dai, respectively, for the -Si plants compared with the +Si plants. Based on these results, Si may have a positive effect on sorghum physiology when infected by C. sublineolum through the maintenance of carbon fixation and also by enhancing the antioxidant system, which resulted in an increase in reactive oxygen species scavenging and, ultimately, reduced damage to the cell membranes.

Silicon Potentiates Host Defense Mechanisms Against Infection by Plant Pathogens

Several agronomic and horticultural crops, such as barley, cucumbers, oats, rice, sugarcane, and wheat, benefit from applications of silicon. Growth enhancements results, in part, from reductions in the intensities of plant diseases. For the rice-Pyricularia oryzae model pathosystem, the mechanical barrier formed from silicon polymerization below the cuticle and in the cell walls was the first proposed hypothesis to explain how this element reduced the number of blast lesions and the lesion sizes. However, new insights have revealed that silicons effect on plant resistance to a number of diseases may also occur through mediated host plant resistance mechanisms against pathogen infection. Plants supplied with silicon exhibit potentiated activation of the phenylpropanoid pathway resulting in increases in total soluble phenolics and lignin. The activities of defense enzymes, such as chitinases and β-1,3-glucanases, are maintained at higher levels during infection and the transcription of defense related genes occur faster and with greater output. When plants are supplied with silicon and then challenged with a pathogen, there is an enhanced activation in antioxidant metabolism, which in turn, suppresses the damaging cytotoxic effect of the reactive oxygen species that causes lipid peroxidation in the cell membrane. At the physiological level, leaf gas exchange parameters of silicon-treated plants are higher upon pathogen infection for crops, such as common beans, rice, sorghum and wheat, indicating the ameliorating effect of this element on photosynthesis. Although our understanding of how silicon affects plants in response to infection has advanced, the exact mechanism(s) by which silicon modulates plant physiology through the potentiation of host defense mechanisms still requires further investigation at the genomics, proteomics, and metabolomics levels.

Induction of resistance to Pyricularia oryzae in wheat by acibenzolar-S-methyl, ethylene and jasmonic acid

This study investigated the effects of acibenzolar-S-methyl (ASM), ethylene (ET) and jasmonic acid (JA) on the resistance of two wheat cultivars (BRS-229 and BR-18) to infection by Pyricularia oryzae. The treatments included spraying plants with ASM (300 mg L-1), ET (0.5 mM), JA (0.1 mM) and distilled water (control) 48 h before inoculation with P. oryzae. Malondialdehyde concentration, an indicative of oxidative damage to the lipids in the leaf cells, was significantly higher for plants sprayed with ASM compared to plants sprayed with JA and ET. Plants sprayed with JA and ET showed reduced values for the number of lesions per cm2 of leaf area and area under blast progress curve, but these hormones had no effect on the incubation period and lesion size (in mm). Plants sprayed with JA and ET showed reduce blast symptoms in comparison to plants sprayed with ASM due to greater peroxidase, polyphenoloxidase, chitinase and β-1,3-glucanase activities.

Rhizobacteria induces resistance against Fusarium wilt of tomato by increasing the activity of defense enzymes

Fusarium wilt, caused by Fusarium oxysporum f.sp. lycopersici (Fol), is one of the most important diseases that affect tomato yield worldwide. This study investigated the potential of three antagonists, Streptomyces setonii (UFV 618), Bacillus cereus (UFV 592) and Serratia marcescens (UFV 252), and as positive control the hormone jasmonic acid (JA), to reduce Fusarium wilt symptoms and to potentiate the defense enzymes in the stem tissues of tomato plants infected by Fol. The seeds were microbiolized with each antagonist, and the soil was also drenched with them. The plants were sprayed with JA 48 h before Fol inoculation. The area under the Fusarium wilt index progress curve was reduced by 54, 48, 47 and 45% for the UFV 618, JA, UFV 592 and UFV 252 treatments, respectively. The three antagonists, and even the JA spray, efficiently reduced the Fusarium wilt symptoms on the tomato plant stems, which can be explained by the lower malondialdehyde concentration (an indication of oxidative damage to lipids in the plasma membranes) and the greater activities of peroxidases, polyphenoloxidases, glucanases, chitinases, phenylalanine ammonia-lyases and lipoxygenases, which are commonly involved in host resistance against fungal diseases. These results present a novel alternative that can be used in the integrated management of Fusarium wilt on tomatoes.

Silicon reduces bacterial speck development on tomato leaves

This study aimed to evaluate the effect of silicon (Si) on the resistance of tomato plants (cv. Santa Clara) to bacterial speck, caused by Pseudomonas syringae pv. tomato (Pst). Tomatoes were grown in soil without calcium silicate (control), in soil without calcium silicate and sprayed with Supa Silica® (2 mL/L) (SS) and in soil with calcium silicate (0.16 g/kg of soil) (CS). The effect of SS on the growth of Pst was evaluated in vitro. There was no significant difference among the treatments for foliar Si concentration and incubation period. No significant differences were observed between the control and CS for the number of lesions per plant (NLP) and bacterial speck severity estimated by the software QUANT. The NLP was significantly reduced with SS spray. There was a negative linear response of in vitro Pst growth to the SS rates. Peroxidase, polyphenoloxidase and glucanase activities were significantly higher for plants sprayed with SS and grown in soil with CS compared to the control treatment. Phenylalanine ammonia-lyase and lypoxygenase activities were significantly higher for plants grown in soil with CS compared to the control. The concentration of malondialdehyde was significantly higher for plants sprayed with SS compared to the control at 0 and 7 dai. The results of this study indicate that the symptoms of bacterial speck are reduced when plants are sprayed with SS, which can be linked to the direct effect of this product against Pst rather than the potentation of host defense responses.

Influência do magnésio na resistência do arroz à mancha parda

A mancha parda, causada pelo fungo Bipolaris oryzae, e uma das principais doencas da cultura do arroz. Sabe-se que o magnesio (Mg) tem o potencial de reduzir a intensidade de algumas doencas em culturas de importância economica e, portanto, o objetivo desse trabalho foi verificar o efeito de diferentes doses desse macronutriente na resistencia do arroz a mancha parda. Para isto, plantas de arroz (cv. Metica-1) foram cultivadas em solucao nutritiva contendo 0,25; 0,5; 1; 2 e 4 mmol.L-1 de Mg. Foram avaliados o periodo de incubacao (PI), o numero de lesoes (NL) por cm2 de folha, a severidade da mancha parda e a concentracao foliar de Mg. Os dados de severidade foram usados para calcular a area abaixo da curva do progresso da mancha parda (AACPMP). Outro experimento com as doses de 0,25; 2,5 e 4 mmol.L-1 de Mg foi conduzido para determinar a concentracao de aldeido malonico (MDA) e a atividade da quitinase (QUI), β-1,3-glucanase (GLU), peroxidase (POX), polifenoloxidase (PFO) e fenilalanina amonia-liase (FAL) em resposta a infeccao por B. oryzae. Houve efeito linear positivo e linear negativo das doses crescentes de Mg, respectivamente, na concentracao foliar de Mg e na AACPMP. Houve efeito quadratico das doses crescentes de Mg no PI e no NL por cm2 de area foliar. Alta concentracao de MDA e maior atividade da PFO ocorreram com a dose de 4 mmol.L-1 de Mg. As atividades da QUI e da GLU nao foram afetadas pelas doses de Mg. A maior atividade da POX ocorreu para as plantas supridas com as doses de 2,5 e 4 mmol.L-1 de Mg. A FAL apresentou aumento significativo em atividade as 48 e 72 horas apos inoculacao com a dose de 4 mmol.L-1 Mg. Os resultados desse estudo demonstram que a alta concentracao foliar de Mg aumentou a resistencia do arroz a infeccao por B. oryzae principalmente por meio do aumento das atividades das enzimas de defesa.

Antagonistic rhizobacteria and jasmonic acid induce resistance against tomato bacterial spot

Tomato bacterial spot on tomato may be caused by four species of Xanthomonas and among them X. gardneri (Xg) is the most destructive one, especially in areas irrigated using a center pivot system in Minas Gerais state and the midwest region of Brazil. Due to the ineffectiveness of chemical control and the lack of cultivars with high levels of genetic resistance, this study investigated the potential of three antagonists (Streptomyces setonii (UFV618), Bacillus cereus (UFV592) and Serratia marcescens (UFV252)), and the hormone jasmonic acid (JA) as a positive control, to reduce bacterial spot symptoms and to potentiate defense enzymes in the leaves of tomato plants infected by Xg. Tomato seeds were microbiolized with each antagonist, and the soil was drenched with these bacteria. The plants were sprayed with JA 48 h before Xg inoculation. The final average severity on the tomato plants was reduced by 29.44, 59.26 and 61.33% in the UFV592, UFV618 and JA treatments, respectively. The UFV618 antagonist was as effective as JA in reducing bacterial spot symptoms on tomatoes, which can be explained by the greater activities of defense enzymes that are commonly involved in host resistance against bacterial diseases. These results suggest that JA and the UFV618 antagonist can be used in the integrated management of bacterial spot on tomatoes.

Silicon, acibenzolar-S-methyl and potassium phosphite in the control of brown spot in rice

This study investigated the effects of silicon (Si), acibenzolar-S-methyl (ASM), and potassium phosphite (Phi) on the potentiation of rice resistance to infection by Bipolaris oryzae. The treatments included the soil amended with Si (1.25 g of calcium silicate per kg of soil), spraying of plants with ASM (500 mg∙L–1), Phi (5 mL∙L–1), and distilled water (control) 24 h before inoculation with B. oryzae. The treatments Si supply and the spraying of ASM and Phi were effective in reducing the area under brown spot progress curve and the number of lesions per cm2 of leaf. Polyphenoloxidases activity was higher for plants supplied with Si. On plants sprayed with ASM, the activities of polyphenoloxidases, phenylalanine ammonia-lyases, chitinases, and β-1,3-glucanases increased. The spraying of plants with Phi did not increase the activities of the studied defense enzymes. Taken together, the results of this study indicated that brown spot symptoms can be greatly reduced with the use of Si, ASM, and Phi.

A standard area diagram set for severity assessment of botrytis leaf blight of onion

This work aimed to develop and validate a standard area diagram set (SADs) to estimate the severity of botrytis leaf blight of onion caused by Botrytis squamosa. For this purpose, a SADs with nine levels of severity (0.3; 1.0; 2.5; 5.0; 10; 20; 30; 40; and 57.7%) was developed from 193 symptomatic leaves collected in the field. The SADs was validated by 16 raters with no experience in evaluating plant diseases. Both precision and accuracy improved when they used the SADs. The statistical parameters for the 16 raters were: coefficient of bias - Cb (no SADs = 0.592, with SADs = 0.988); correlation coefficient – r (no SADs = 0.864, with SADs = 0.921); and Lin’s concordance correlation coefficient – ρc (no SADs = 0.513, with SADs = 0.909). In addition, estimates were more reliable: coefficient of determination - R2 (no SADs = 0.681, with SAD = 0.796); intra-class correlation coefficient - ρ (no SADs = 0.729, with SADs = 0.876). The SADs proposed here is a useful tool for improving visual assessments of severity of botrytis leaf blight of onion.

Potential of ethyl acetate fractions of Stryphnodendron adstringens shells and fruit extracts of Caesalpinia ferrea to control bacterial leaf speck and on the potentiation of defense enzymes in tomato

Considering the importance of bacterial leaf speck (Pseudomonas syringae pv. tomato) in reducing tomato yield and difficulties in disease control, this study investigated the effects of fractions of shells extract of Stryphnodendron adstringens (Sa) and fruit extract of Caesalpinia ferrea (Cf) compared to Acibenzolar-S-Methyl (ASM) on reducing bacterial leaf speck symptoms and on the potentiation of the activities of defense enzymes. The number of lesions per plant (NLP) was significantly lower in plants treated with the ethyl acetate fraction (EAF) of S. adstringens (Sa) and ASM compared to other treatments (EAF of Cf, n-butanol fractions of Sa and Cf, aqueous fractions of Sa and Cf and sterile distilled water). The bacteria were inhibited by the EAF of Sa and EAF of Cf and n-butanol fractions of Sa and Cf according to the bioautography assay. Saponins and tannins were the two major compounds found in these fractions based on the phytochemical analysis. Peroxidase (POX), polyphenoloxidase (PPO), β-1,3-glucanase (GLU) and phenylalanine ammonia-lyase (PAL) activities were determined on the leaves of plants treated with EAF Sa, ASM and sterile distilled water. Both POX and PAL activities were higher at 3 and 6 days after inoculation (dai), while the PPO and GLU activities were higher from 9 to 12 dai. It is suggested that saponins increased tomato resistance to P. s. pv. tomato because no antimicrobial activity against the bacteria was observed. In conclusion, the EAF Sa was very efficient in reducing bacterial leaf speck symptoms in conditions where the POX, PPO, PAL and GLU activities played a pivotal role in increasing tomato resistance to the disease.