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Arabidopsis thaliana mitogen-activated protein kinases 3 and 6 (MPK3/6) are activated transiently during pathogen-associated molecular pattern-triggered immunity (PTI) and durably during effector-triggered immunity (ETI). The functional differences between these two kinds of activation kinetics and how they coordinate the two layers of plant immunity remain poorly understood. Here, by suppressor analyses, we demonstrate that ETI-mediating nucleotide-binding domain leucine-rich repeat receptors (NLRs) and the NLR signaling components NDR1 and EDS1 can promote the salicylic acid sector of defense downstream of MPK3 activity. Moreover, we provide evidence that both sustained and transient MPK3/6 activities positively control the expression of several NLR genes, including AT3G04220 and AT4G11170. We further show that NDR1 and EDS1 contribute to the up-regulation of these two NLRs in both an ETI and a PTI context. Remarkably, whereas in ETI MPK3/6 activities are dependent on NDR1 and EDS1, they are not in PTI, suggesting crucial differences in the two signaling pathways. Finally, we demonstrate that expression of the NLR AT3G04220 is sufficient to induce expression of defense genes from the salicylic acid branch. Overall, this study expands our knowledge of MPK3/6 functions during immunity and provides new insights into the intricate interplay of PTI and ETI. [ABSTRACT FROM AUTHOR]

Summary: Root endophytes establish beneficial interactions with plants, improving holobiont resilience and fitness, but how plant immunity accommodates beneficial microbes is poorly understood. The multi‐stress tolerance‐inducing endophyte Enterobacter sp. SA187 triggers a canonical immune response in Arabidopsis only at high bacterial dosage (>108 CFUs ml−1), suggesting that SA187 is able to evade or suppress the plant defence system at lower titres. Although SA187 flagellin epitopes are recognized by the FLS2 receptor, SA187‐triggered salt tolerance functions independently of the FLS2 system. In contrast, overexpression of the chitin receptor components LYK4 and LYK5 compromised the beneficial effect of SA187 on Arabidopsis, while it was enhanced in lyk4 mutant plants. Transcriptome analysis revealed that the role of LYK4 is intertwined with a function in remodelling defence responses with growth and root developmental processes. LYK4 interferes with modification of plant ethylene homeostasis by Enterobacter SA187 to boost salt stress resistance. Collectively, these results contribute to unlock the crosstalk between components of the plant immune system and beneficial microbes and point to a new role for the Lys‐motif receptor LYK4 in beneficial plant–microbe interaction. [ABSTRACT FROM AUTHOR]