Physiological and biochemical aspects of silicon and magnesium interaction and of the acids beta-aminobutyric and gamma-aminobutyric in the potentiation of wheat resistance against blast

Abstract

Epidemics of blast, caused by Pyricularia oryzae, have caused great yield losses in wheat and disease control has relied on using resistant cultivars combined with fungicides spray. Other control strategies need to be investigated and the use of inducers of host resistance may become an environmentally friendly and viable strategy for blast management. In the first study, the hypothesis that wheat plants with higher foliar concentrations of silicon (Si) and magnesium (Mg) could had their resistance against blast maximized was tested. An in-depth analysis of the photosynthetic apparatus (parameters of chlorophyll (Chl) a fluorescence and pool of photosynthetic pigments) and changes in enzymes activities involved in host defense and in the antioxidant metabolism in the leaves of wheat plants non-supplied (0 mM) or supplied (2 mM) with Si combined to lower (0.25 mM) or adequate (0.6 mM) Mg rates and challenged with P. oryzae were investigated. Blast symptoms were reduced for Si-supplied plants due to higher foliar Si concentration. Magnesium or its possible synergistic effect with Si did not contribute to reduce blast severity even though there was higher foliar Mg concentration for plants supplied with 0.6 mM Mg compared to 0.25 mM Mg. Higher values of variable-to-maximum chlorophyll a fluorescence ratio (F v /F m ), photochemical yield (Y(II)), and yield for dissipation by down-regulation (Y(NPQ)) parameters, great Chl a+b concentration, and less production of malondialdehyde (MDA), hydrogen peroxide (H 2 O 2 ), and superoxide anion radical (O 2 • - ) were noticed for Si-supplied and infected plants due to less blast symptoms. In general, Si-supplied and infected plants, regardless of Mg rate, displayed higher activities of defense (chitinase (CHI), β-1,3-glucanase (GLU), phenylalanine ammonia-lyase (PAL), peroxidase (POX), and polyphenoloxidase (PPO)) and antioxidative (ascorbate peroxidase (APX), catalase (CAT), glutathione reductase (GR), and superoxide dismutase (SOD)) enzymes as well as more lignin concentration reconfirming, therefore, the potential of Si to increase wheat resistance against blast. No evidence of synergism between Si and Mg or a possible effect of higher foliar Mg concentration to maximize wheat resistance to blast, linked to antioxidative metabolism robustness, was obtained as illustrated by the three independent groups (0.25 mM Mg +Si; 0.6 mM Mg +Si; as well as 0.25 mM Mg -Si and 0.6 mM Mg -Si) generated from principal component analysis. The fact that Mg was not able to reduce blast symptoms cannot discard its indirect and barely participation in the potentiation of host defense responses considering its key functions in plant metabolism even though the physiological and biochemical analysis performed were not able to bring this up for discussion. In the second study, the potential of using β-aminobutyric acid (BABA) and γ-aminobutyric acid (GABA) to induce wheat resistance against blast was investigated. This goal was achieved by performing Chl a fluorescence measurements, determining Chl a+b and carotenoids concentrations, host defense responses (CHI, GLU, POX, PAL, PPO, and LOX activities along with concentrations of phenolics and lignin), antioxidative metabolism (SOD, CAT, APX, and GR activities) as well as oxidative stress (concentrations of MDA, H 2 O 2 , and O 2 • - ) of non-inoculated and inoculated plants from cultivar BRS Guamirim (susceptible to blast) sprayed with water or with solutions (100 mM) of BABA and GABA. Plants were sprayed with BABA and GABA solutions at 48 h before inoculation with B. oryzae. Blast progressed much faster for water and GABA-sprayed plants than for BABA-sprayed plants. The area under blast progress curve was significantly higher by 52 and 47% for plants from control and GABA treatments, respectively, compared to BABA-sprayed plants. Values of Chl a fluorescence parameters F v /F m , Y(II), and Y(NPQ)) linked to Chl a+b and carotenoids concentrations were higher for BABA-sprayed plants than for those of control and GABA treatments. Activities of CHI, GLU, PAL, PPO, and LOX were more remarkable for BABA-sprayed plants than for those from control and GABA treatments. Greater APX, CAT, and SOD activities for BABA-sprayed plants helped to alleviate the stress imposed by higher H 2 O 2 and O 2 • - concentrations in contrast from control and GABA treatments. Taken together, the results of the present study allowed to conclude that supplying BABA to wheat plants increased their resistance against blast in a scenario where the photosynthetic apparatus was preserved along with a boosted defense response and a more robust antioxidative metabolism to counteract the harmful effect imposed by P. oryzae infection. Keywords: Amino acids. Antioxidative metabolism. Fungal disease. Hemibiotrophic pathogen. Host defense responses. Induced resistance. Plant nutrition. Reactive oxygen species.