Plants encounter beneficial and detrimental microorganisms both above- and belowground and the health status of the plant depends on the composition of this pan-microbiome. Beneficial microorganisms contribute to plant nutrition or systemically or locally protect plants against pathogens, thus facilitating adaptation to a variety of environments. Induced systemic resistance, caused by root-associated microbes, manifests as aboveground resistance against necrotrophic pathogens and is mediated by jasmonic acid/ethylene-dependent signaling. By contrast, systemic acquired resistance relies on salicylic acid (SA) signaling and confers resistance against secondary infection by (hemi)biotrophic pathogens. To investigate whether symbiotic rhizobia that are ubiquitously found in natural ecosystems are able to modulate resistance against biotrophs, we tested the impact of preestablished nodulation of Medicago truncatula and pea (Pisum sativum) plants against infection by the powdery mildew fungus Erysiphe pisi. We found that root symbiosis interfered with fungal penetration of M. truncatula and reduced asexual spore formation on pea leaves independently of symbiotic nitrogen fixation. Improved resistance of nodulated plants correlated with elevated levels of free SA and SA-dependent marker gene expression upon powdery mildew infection. Our results suggest that nodulation primes the plants systemically for E. pisi-triggered SA accumulation and defense gene expression, resulting in increased resistance.

Many biotrophic fungal plant pathogens develop feeding structures, haustoria, inside living plant cells, which are essential for their success. Extrahaustorial membranes (EHMs) surround haustoria and delimit the extrahaustorial matrices (EHMxs). Little is known about transport mechanisms across EHMs and what properties proteins and nutrients need in order to cross these membranes. To investigate this further, we expressed fluorescent proteins in the cytosol of infected barley leaf epidermal cells after particle bombardment and investigated properties that influenced their localisation in the powdery mildew EHMx. We showed that this translocation is favoured by a neutral isoelectric point (pI) between 6.0 and 8.4. However, for proteins larger than 50 kDa, pI alone does not explain their localisation, hinting towards a more complex interplay between pI, size, and sequence properties. We discuss the possibility that an EHM translocon is involved in protein uptake into the EHMx.

Excess cellular sterol is harmful in mammals and plants, but the mechanisms why are awaiting clarification. Here we find a strong autoimmune response to be associated to excess endoplasmic reticulum (ER) sterols. This was obtained by studying a plant peroxisome lipase, SSD5, required for the lesion phenotype of the Arabidopsis syntaxin mutant, pen1 syp122. SSD5 is a lipase with a catalytic triad including a GxSxG motif localized to a subdomain of the peroxisome periphery. Lipidomics revealed reduced steryl ester levels in pen1 syp122 when SSD5 is mutated. This involvement in sterol homeostasis was confirmed by a requirement of SSD5 for the lesions of hise1 psat1 that suffers from excess ER sterol. These data suggest SSD5 is contributing to a peroxisome-located segment of the sterol biosynthesis pathway. SSD5’s contribution to the pen1 syp122 autoimmunity is not associated with nine highly diverse down-stream immune components, and SSD5 does not influence general plant disease levels and immunity. Therefore, our data indicated SSD5 as well as ER sterol functions up-stream of immune activation. This in turn suggests plant excess ER sterol to activate one or more immune receptors.