Changes in the antioxidant status in leaves of Solanum species in response to elicitor from Phytophthora infestans

SNF1-Related Protein Kinases SnRK2.4 and SnRK2.10 Modulate ROS Homeostasis in Plant Response to Salt Stress

In response to salinity and various other environmental stresses, plants accumulate reactive oxygen species (ROS). The ROS produced at very early stages of the stress response act as signaling molecules activating defense mechanisms, whereas those produced at later stages in an uncontrolled way are detrimental to plant cells by damaging lipids, DNA, and proteins. Multiple systems are involved in ROS generation and also in ROS scavenging. Their level and activity are tightly controlled to ensure ROS homeostasis and protect the plant against the negative effects of the environment. The signaling pathways responsible for maintaining ROS homeostasis in abiotic stress conditions remain largely unknown. Here, we show that in Arabidopsis thaliana, two abscisic acid- (ABA)-non-activated SNF1-releted protein kinases 2 (SnRK2) kinases, SnRK2.4 and SnRK2.10, are involved in the regulation of ROS homeostasis in response to salinity. They regulate the expression of several genes responsible for ROS generation at early stages of the stress response as well as those responsible for their removal. Moreover, the SnRK2.4 regulate catalase levels and its activity and the level of ascorbate in seedlings exposed to salt stress.

Changes in the Antioxidant System in Soybean Leaves Infected by Corynespora cassiicola

Considering the importance of target spot, caused by the fungus Corynespora cassiicola, to reduce soybean yield in Brazil and that more basic information regarding the soybean-C. cassiicola interaction is needed, the present study aimed to investigate whether the cellular damage caused by C. cassiicola infection could activate the antioxidant system and whether a more efficient antioxidant system could be associated with an increase in soybean resistance to target spot. The activities of the antioxidant enzymes superoxide dismutase, catalase, peroxidase, ascorbate peroxidase, glutathione peroxidase, glutathione reductase, glutathione S-transferase as well as the concentrations of ascorbate (AsA), hydrogen peroxide (H2O2), superoxide (O2•-), and malondialdehyde (MDA) were measured in soybean plants from two cultivars differing in resistance to the pathogen. The number of lesions per square centimeter was significantly reduced by 14% in plants from cultivar Fundacep 59 compared with plants from cultivar TMG 132. The area under the disease progress curve was significantly lower, by 15%, in plants from Fundacep 59 than in plants from TMG 132. Generally, antioxidant enzyme activities and AsA concentration significantly increased in response to C. cassiicola infection in plants from both cultivars, however more prominent increases were recorded for plants from Fundacep 59. The concentrations of MDA, H2O2, and O2•- also increased, particularly for plants from TMG 132. The results from this study highlight the importance of a more efficient antioxidative system in the removal of reactive oxygen species generated in soybean plants during C. cassiicola infection, contributing to the resistance to target spot.

Interspecific somatic hybrids Solanum villosum (+) S. tuberosum, resistant to Phytophthora infestans

The interspecific somatic hybrids 4x S. villosum (+) 2x S. tuberosum clone DG 81-68 (VT hybrids) were obtained and characterized molecularly and cytogenetically. The morphology of fusion-derived plants was intermediate in relation to the parental species. The expected ploidy level of the regenerants was 6x for the VT hybrids, but the real ploidy of the hybrids varied, with some of them being euploids, and others - aneuploids. The hybridity of the regenerants was verified by random amplified polymorphic DNA (RAPD) analysis. Despite the variation in ploidy, the RAPD patterns of the hybrids were mostly uniform, suggesting similarity of the genotypes of the VT clones. Genomic in situ hybridization (GISH) analysis discriminated between the chromosomes of both parental genomes in VT somatic hybrids and also confirmed their hybridity. The resistance of VT somatic hybrids to Phytophthora infestans was evaluated and all of the hybrids proved to be highly resistant. In search of the mechanisms involved in resistance of the Solanum species to P. infestans, the biochemical reactions occurring early after elicitor treatment were studied. The production of reactive oxygen species (ROS), as one of the earliest reactions induced by pathogens or their elicitors, was examined in the resistant wild species S. villosum, susceptible S. tuberosum clone DG 81-68 and in the VT hybrid, resistant to P. infestans. After treatment of the leaves with elicitor, the relative increase in ROS production was higher in leaves of the susceptible potato clone than in the resistant plants of S. villosum and the somatic hybrid.

Changes in the initial phase of lipid peroxidation induced by elicitor from Phytophthora infestans in Solanum species

The initial phase of the lipid peroxidation process in leaves of Solanum nigrum var. gigantea, Solanum tuberosum cv Bzura and clone H-8105, which represent non-host resistance, field resistance and susceptibility, respectively, against Phytophthora infestans, was investigated. Based on quantitative and qualitative high-performance liquid chromatography (HPLC) analyses of free and esterified fatty acid hydroperoxides (FAHs), we characterized the lipid peroxidation process induced by the pathogen-derived elicitor, culture filtrate (CF), in leaves of the studied genotypes. In all plants, FAHs generated due to 13-lipoxygenase (LOX) action dominated over those from the non-enzymatic pathway. The FAHs derived from 9-LOX activity were found only in CF-treated leaves of the non-host resistant S. nigrum. However, experiments in vitro and in planta with exogenous linoleic acid (LA) as a substrate for LOX revealed high constitutive activity of 9-LOX in all genotypes, which increased in response to CF treatment. The time course changes in polyunsaturated fatty acid (PUFA) pools in the total lipid fractions as well as the degree of their oxidation suggested that CF-induced PUFA peroxidation was enhanced mostly in S. nigrum, less so in Bzura and least in the susceptible clone H-8105. The obtained results are discussed in light of the overall biochemical cell status of plants in the studied interactions.

Changes in oxylipin synthesis after Phytophthora infestans infection of potato leaves do not correlate with resistance

Oxylipins constitute a class of molecules notably involved in host-pathogen interactions. In the potato-Phytophthora infestans (Mont.) De Barry (P. infestans) relationships, the role of colneleic and colnelenic acids, two oxylipins resulting from the consecutive action of lipoxygenase (EC 1.13.11.12) and divinyl ether synthase (EC 1.-) on respectively linoleic and linolenic acids have been previously reported. In the present paper, five potato cultivars with contrasting resistance to P. infestans were submitted to infection. Lipoxygenase pathway response was studied at both transcriptional and metabolic levels. A Northern blot preliminary study revealed that lipoxygenase (lox1 and lox3) and divinyl ether synthase genes were clearly up-regulated 96h after leaf inoculation with P. infestans. Profiling of free and esterified oxylipins performed 24h, 48h, 72h and 96h after inoculation, showed that esterified oxylipins are mainly produced with 9-derivatives in higher concentrations (esterified forms of colnelenic acid, 9-hydroxy octadecatrienoic acid, 9-hydroperoxy octadecatrienoic acid). Oxylipin accumulation is undetectable 24h after infection, slightly detectable after 48h, reaching highest concentrations after 96h. Cultivars show slightly different oxylipin profiles but the concentration of individual oxylipins differs markedly 96h after infection. No correlation was found between P. infestans resistance levels and oxylipin synthesis rates or concentration. To assess local and systemic effects of colneleic acid application before P. infestans infection, Bintje cultivar was sprayed with colneleic acid 72h before inoculation. Both application modes (local and systemic) resulted in lipoxygenase pathway activation without affecting the resistance level to the pathogen.