No abstract

A protease was isolated from potato (Solanum tuberosum L. cv. Huinkul) tuber disks after 24 h of aeration when proteolysis is markedly increased. Purification was performed by ammonium sulfate precipitation, ion exchange chromatography, and affinity chromatography. A size of 40 kDa was estimated by sodium dodecyl sulphate‐polyacrylamide gel electrophoresis (SDS‐PAGE) and gel filtration, it is monomeric and its properties are consistent with those of aspartic proteinases (EC 3.4.23): it had a pH optimum between 4 and 5 and it was inhibited by pepstatin. Partial homology with other plant aspartic proteinases was observed in two sequenced tryptic fragments. It binds to Sepharose‐concanavalin A and can be eluted with α‐methyl mannoside, indicating that it is possibly glycosylated. Unlike other aspartic proteinases from Solanaceae that degrade pathogenesis‐related proteins, it is unable to cleave a basic chitinase from potato. Moreover, this aspartic protease is strongly inhibited by the basic chitinase; the 50% inhibition is obtained when the molar ratio approaches 1, the same as with pepstatin. The interaction between this aspartic protease and a new type of endogenous inhibitor may be an interesting starting point to study the regulation of these aspartic proteases during stress.

The utilization of phosphites (Phi) could be considered as another strategy to be included in integrated disease management programmes to reduce the intensive use of fungicides and production costs.

The use of biocompatible chemical compounds that enhance plant disease resistance through Induced Resistance (IR) is an innovative strategy to improve the yield and quality of crops. Phosphites (Phi), inorganic salts of phosphorous acid, are environment friendly, and have been described to induce disease control. Phi, similar to other plant inductors, are thought to be effective against different types of biotic and abiotic stress, and it is assumed that the underlying signaling pathways probably overlap and interact. The signaling pathways triggered by UV-B radiation, for instance, are known to crosstalk with other signaling routes that respond that biotic stress. In the present work, the effect of potassium phosphite (KPhi) pre-treatment on UV-B stress tolerance was evaluated in potato leaves. Plants were treated with KPhi and, after 3 days, exposed to 2 h/day of UV-B (1.5 Watt m(-2)) for 0, 3 and 6 days. KPhi pre-treatment had a beneficial effect on two photosynthetic parameters, specifically chlorophyll content and expression of the psbA gene. Oxidative stress caused by UV-B was also prevented by KPhi. A decrease in the accumulation of hydrogen peroxide (H2O2) in leaves and an increase in guaiacol peroxidase (POD) and superoxide dismutase (SOD) activities were also observed. In addition, the expression levels of a gene involved in flavonoid synthesis increased in UV-B-stressed plants only when pre-treated with KPhi. Finally, accumulation of glucanases and chitinases was induced by UV-B stress and markedly potentiated by KPhi pre-treatment. Altogether, this is the first report that shows a contribution of KPhi in UV-B stress tolerance in potato plants.

Phosphite (Phi) compounds are salts derived from phosphorous acid. These compounds have the ability to protect plants against different pathogens. The aim of the present research was to assess the effect of Phi compounds on components of potato tuber periderm and cortex and to assess their effects on pathogen resistance in the postharvest stage. In a series of field experiments, potassium phosphite (KPhi) was applied to seed potato tubers and foliage. After harvest, several variables were analyzed in tubers obtained from these plants. An increase in pectin content was observed in both periderm and cortex tissue in tubers originating from KPhi-treated plants. After wounding and infection with Fusarium solani, a higher amount of pectin accumulation in cortical tissues was observed in tubers following treatment with KPhi. The content and/or activity of polygalacturonase and proteinase inhibitor also increased in tubers from KPhi-treated plants. A new isoform of chitinase was detected in the tuber periderm of treated plants.These results suggest that KPhi applied to seed tuber and foliage induces defense responses in tuber periderm and cortex and that these reactions are associated with structural and biochemical changes in these tissues.