A critical role for Arabidopsis MILDEW RESISTANCE LOCUS O2 in systemic acquired resistance

Abstract

Members of the MILDEW RESISTANCE LOCUS O (MLO) gene family confer susceptibility to powdery mildews in different plant species, and their existence therefore seems to be disadvantageous for the plant. We recognized that expression of the Arabidopsis MLO2 gene is induced after inoculation with the bacterial pathogen Pseudomonas syringae, promoted by salicylic acid (SA) signaling, and systemically enhanced in the foliage of plants exhibiting systemic acquired resistance (SAR). Importantly, distinct mlo2 mutant lines were unable to systemically increase resistance to bacterial infection after inoculation with P. syringae, indicating that the function of MLO2 is necessary for biologically induced SAR in Arabidopsis. Our data also suggest that the close homolog MLO6 has a supportive but less critical role in SAR. In contrast to SAR, basal resistance to bacterial infection was not affected in mlo2. Remarkably, SAR-defective mlo2 mutants were still competent in systemically increasing the levels of the SAR-activating metabolites pipecolic acid (Pip) and SA after inoculation, and to enhance SAR-related gene expression in distal plant parts. Furthermore, although MLO2 was not required for SA- or Pip-inducible defense gene expression, it was essential for the proper induction of disease resistance by both SAR signals. We conclude that MLO2 acts as a critical downstream component in the execution of SAR to bacterial infection, being required for the translation of elevated defense responses into disease resistance. Moreover, our data suggest a function for MLO2 in the activation of plant defense priming during challenge by P. syringae.

l‐lysine metabolism to N‐hydroxypipecolic acid: an integral immune‐activating pathway in plants

“…MLO2 is required for biologically induced SAR and proper resistance induction by exogenous SA and Pip, but not for SAR‐related metabolite accumulation or defense gene expression. This indicates that MLO2 functions at a very downstream position of both SA and Pip/NHP signaling during SAR (Gruner et al ., ).…”

UGT76B1, a promiscuous hub of small molecule-based immune signaling, glucosylates N-hydroxypipecolic acid and controls basal pathogen defense

“…Lower (1°) rosette leaves of a subset of plants were therefore pre-inoculated with Psm to induce SAR, whereas the 1° leaves of other plants were mock-treated for the examination of an appropriate non-induced control. Two days later, all the plants were challenge-infected with a bioluminescent Psm strain (Psm lux, Fan et al, 2008) and resistance to the bacterial infection was assessed by bioluminescence measurements 2.5 days later (Hartmann et al, 2017;Gruner et al, 2018). Inducing inoculations with Psm limited bacterial growth in challenged 2° leaves of the wild type compared to those of mock-control plants by about 16-fold, indicating a strong SAR establishment ( Figure 7B).…”

A Role for Tocopherol Biosynthesis in Arabidopsis Basal Immunity to Bacterial Infection

“…would be functionally relevant for immunity against bacterial infection. To determine plant basal resistance to P. syringae, we leaf inoculated the tocopherol biosynthetic mutants and the corresponding wild-type accessions with a compatible Psm strain that expresses the luxCDABE operon from Photorhabdus luminescens (Psm lux) and assessed bacterial growth via the quantification of bioluminescence (Fan et al, 2008;Gruner et al, 2018;. Bacterial growth in the leaves of vte1, VTE3/vte3-3, vte4-2, and tat1-2 was similar to the respective wild type, but the vte2-2 mutant allowed significantly higher bacterial proliferation than the wild-type Col-0 and the other lines under investigation ( Fig.…”

“…Sixty hours later, leaf discs (10 mm in diameter) were punched out from inoculated leaves and bacterial numbers were assessed by determining relative light units of the bioluminescent Psm lux strain with a Sirius luminometer (Berthold Detection Systems). Colony-forming units, which linearly correlate with relative light unit values, were calculated by an experimentally determined calibration line (Gruner et al, 2018).…”