Soybean resistance to Phakopsora pachyrhizi infection is barely potentiated by boron

Abstract

Asian soybean rust (ASR), caused by the fungus Phakopsora pachyrhizi, is one of the most important diseases affecting soybean production. Nutrients play a crucial role in defending plants against pathogens. Soybean resistance against ASR as affected by boron (B) was investigated through physiological, biochemical, and molecular analyses (chlorophyll a fluorescence, concentrations of photosynthetic pigments, total soluble phenolic compounds (TSP), and lignin-thioglycolic acid (LTGA) derivatives as well as expression of defense- and B-related genes). There was no significant difference for ASR severity and TEF1 (translation elongation factor 1α) expression of P. pachyrhizi for plants grown hydroponically with nutrient solutions containing 0.25 (ideal) or 1.0 (high) mM B. Higher B foliar concentration occurred in inoculated plants than in non-inoculated plants regardless of B supply. Photosynthetic process was impaired during P. pachyrhizi infection regardless of B rates. Concentration of TSP was high at 15 days after inoculation for non-inoculated plants supplied with 1.0 mM B. Concentration of LTGA derivatives was higher for inoculated plants regardless of B supply. Defense-related genes PAL1.1, PAL3.1, CHIA1, CHI1B1, LOX7, and MMP2 were up-regulated in inoculated plants regardless of B supply. Regardless of B supply, fungal infection up-regulated expression of genes encoding nitrate reductase and nitrite reductase and down-regulated genes encoding for rhamnogalacturonan II. Taken together, this study provides molecular and physiological evidences that B was barely involved in the potentiation of soybean resistance against ASR.