Potentiation of soybean resistance against Phakopsora pachyrhizi infection using phosphite combined with free amino acids

Considering the importance of Asian soybean rust (ASR), caused by Phakopsora pachyrhizi, in the decrease in soybean yield, this study investigated the potential of using phosphite combined with l-α-

Free amino acids (referred to as induced resistance [IR] stimulus hereafter) to boost defence responses of soybean plants against P. pachyrhizi infection. Plants were sprayed with water (control), acibenzolar-S-methyl (ASM) or IR stimulus and noninoculated or inoculated with P. pachyrhizi. Urediniospore germination was not affected by the IR stimulus in vitro. Reduced ASR severity, lower malondialdehyde concentration and less colonization of leaf tissues by P. pachyrhizi (lower TEF-1α expression from 1 to 15 days after inoculation [dai]) occurred for IR stimulus-sprayed

plants. The pattern of gene expression for IR stimulus-sprayed and infected plants was strikingly similar but sometimes more remarkable than that in ASM-sprayed and infected plants. Higher production of phenolics and lignin along with stronger up-regulation of PAL1.3 (5 and 10 dai), PAL2.2 (3 dai), PAL3.1 (1, 3 and 5 dai), ICS1 (5 dai), CHIA1 (1, 5 and 10 dai), CHI1B1 (5 dai), PR-1A (5 and 10 dai), NR1-2 (5 and 10 dai) and INR-2 (5 and 10 dai) for IR stimulus-sprayed plants increased their resistance against ASR. In addition, IR stimulus-sprayed and infected plants showed less impairment of the photosynthetic apparatus and maintained high concentrations of chlorophyll a + b and carotenoids. These findings highlight the potential of using this IR stimulus for developing a well-tuned and effective defensive strategy in soybean plants against P. pachyrhizi infection.