Abstract
Hormones orchestrate the physiology of organisms. Measuring the activity of defense hormone-responsive genes can help understanding immune signaling and facilitate breeding for plant health. However, different from model species like Arabidopsis, genes that respond to defense hormones salicylic acid (SA) and jasmonic acid (JA) have not been disclosed in the soybean crop. We performed global transcriptome analyses to fill this knowledge gap. Upon exogenous application, endogenous levels of SA and JA increased in leaves. SA predominantly activated genes linked to systemic acquired resistance and defense signaling whereas JA mainly activated wound response-associated genes. In general, SA-responsive genes were activated earlier than those responding to JA. Consistent with the paradigm of biotrophic pathogens predominantly activating SA responses, free SA and here identified most robust SA marker genes GmNIMIN1, GmNIMIN1.2 and GmWRK40 were induced upon inoculation with Phakopsora pachyrhizi, whereas JA marker genes did not respond to infection with the biotrophic fungus. Spodoptera exigua larvae caused a strong accumulation of JA-Ile and JA-specific mRNA transcripts of GmBPI1, GmKTI1 and GmAAT whereas neither free SA nor SA-marker gene transcripts accumulated upon insect feeding. Our study provides molecular tools for monitoring the dynamic accumulation of SA and JA, e.g. in a given stress condition.
Highlights
IntroductionSynergistic and antagonistic crosstalk of the salicylic acid (SA) and jasmonic acid (JA)/ET signaling pathways adds additional complexity and flexibility to the plant immune s ystem[3,4,5]
To increase the chance of detecting possible transient accumulation of the hormone in question, we focused on marker genes in category 1, because genes in this category are induced for a longer period (Figs. 2 and 3)
Selecting marker genes exclusively by their sequence homology to known hormone-responsive genes in other species can be misleading. This is true for the paleopolyploid soybean, which has undergone at least two rounds of genome duplication and which possesses ~ 70% more protein-coding genes than A rabidopsis[51]
Summary
Synergistic and antagonistic crosstalk of the SA and JA/ET signaling pathways adds additional complexity and flexibility to the plant immune s ystem[3,4,5] In their crosstalk, SA mostly overrules JA6–9, for example by inhibition of transcription factors involved in JA signaling and/or by inducing transcription-repressive epigenetic marks to histones (e.g. trimethylation of lysine 27 in histone H3) in JA-responsive defense genes (e.g. PDF1.2)[10]. SA and JAs are both present in the soybean crop[29,30] but, despite soybean’s importance as a food and feed source[31], little is known about the SA/JA signaling pathways and the identity of SA and JA target genes in this plant Knowledge of their identity may help providing stress-resilient soybean cultivars. We validated usability of the genes for use as diagnostic tools to determine SA or JA responsiveness in soybean upon exposure to two distinct biotic stresses, that is inoculation with Phakopsora pachyrhizi (Pp), which causes Asian soybean r ust[32,33] and leaf feeding of Spodoptera exigua Hübner[34]
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