Since years, research on SnRK1, the major cellular energy sensor in plants, has tried to define its role in energy signalling. However, these attempts were notoriously hampered by the lethality of a complete knockout of SnRK1. Therefore, we generated an inducible amiRNA::SnRK1α2 in a snrk1α1 knock out background (snrk1α1/α2) to abolish SnRK1 activity to understand major systemic functions of SnRK1 signalling under energy deprivation triggered by extended night treatment. We analysed the in vivo phosphoproteome, proteome and metabolome and found that activation of SnRK1 is essential for repression of high energy demanding cell processes such as protein synthesis. The most abundant effect was the constitutively high phosphorylation of ribosomal protein S6 (RPS6) in the snrk1α1/α2 mutant. RPS6 is a major target of TOR signalling and its phosphorylation correlates with translation. Further evidence for an antagonistic SnRK1 and TOR crosstalk comparable to the animal system was demonstrated by the in vivo interaction of SnRK1α1 and RAPTOR1B in the cytosol and by phosphorylation of RAPTOR1B by SnRK1α1 in kinase assays. Moreover, changed levels of phosphorylation states of several chloroplastic proteins in the snrk1α1/α2 mutant indicated an unexpected link to regulation of photosynthesis, the main energy source in plants.

The lipid biopolymer suberin plays a major role as a barrier both at plant-environment interfaces and in internal tissues, restricting water and nutrient transport. In potato (Solanum tuberosum), tuber integrity is dependent on suberized periderm. Using microarray analyses, we identified ABCG1, encoding an ABC transporter, as a gene responsive to the pathogenassociated molecular pattern Pep-13. Further analyses revealed that ABCG1 is expressed in roots and tuber periderm, as well as in wounded leaves. Transgenic ABCG1-RNAi potato plants with downregulated expression of ABCG1 display major alterations in both root and tuber morphology, whereas the aerial part of the ABCG1-RNAi plants appear normal. The tuber periderm and root exodermis show reduced suberin staining and disorganized cell layers. Metabolite analyses revealed reduction of esterified suberin components and hyperaccumulation of putative suberin precursors in the tuber periderm of RNA interference plants, suggesting that ABCG1 is required for the export of suberin components.

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In nature, plants are subject to various stresses that are often accompanied by wounding of the aboveground tissues. As wounding affects plants locally and systemically, we investigated the impact of leaf wounding on interactions of Medicago truncatula with root-colonizing microorganisms, such as the arbuscular mycorrhizal (AM) fungus Glomus intraradices, the pathogenic oomycete Aphanomyces euteiches and the nitrogen-fixing bacterium Sinorhizobium meliloti. To obtain a long-lasting wound response, repeated wounding was performed and resulted in locally and systemically increased jasmonic acid (JA) levels accompanied by the expression of jasmonate-induced genes, among them the genes encoding allene oxide cyclase 1 (MtAOC1) and a putative cell wall-bound invertase (cwINV). After repeated wounding, colonization with the AM fungus was increased, suggesting a role of jasmonates as positive regulators of mycorrhization, whereas the interaction with the rhizobacterium was not affected. In contrast, wounded plants appeared to be less susceptible to pathogens which might be caused by JA-induced defence mechanisms. The effects of wounding on mycorrhization and pathogen infection could be partially mimicked by foliar application of JA. In addition to JA itself, the positive effect on mycorrhization might be mediated by systemically induced cwINV, which was previously shown to exhibit a regulatory function on interaction with AM fungi.

Summary• The oomycete Phytophthora infestans is the causal agent of late blight, the most devastating disease of potato. The importance of vesicle fusion processes and callose deposition for defense of potato against Phytophthora infestans was analyzed.• Transgenic plants were generated, which express RNA interference constructs targeted against plasma membrane-localized SYNTAXIN-RELATED 1 (StSYR1) and SOLUBLE N-ETHYLMALEIMIDE-SENSITIVE FACTOR ADAPTOR PROTEIN 33 (StSNAP33), the potato homologs of Arabidopsis AtSYP121 and AtSNAP33, respectively.• Phenotypically, transgenic plants grew normally, but showed spontaneous necrosis and chlorosis formation at later stages. In response to infection with Phytophthora infestans, increased resistance of StSYR1-RNAi plants, but not StSNAP33-RNAi plants, was observed. This increased resistance correlated with the constitutive accumulation of salicylic acid and PR1 transcripts. Aberrant callose deposition in Phytophthora infestans-infected StSYR1-RNAi plants coincided with decreased papilla formation at penetration sites. Resistance against the necrotrophic fungus Botrytis cinerea was not significantly altered. Infiltration experiments with bacterial solutions of Agrobacterium tumefaciens and Escherichia coli revealed a hypersensitive phenotype of both types of RNAi lines.• The enhanced defense status and the reduced growth of Phytophthora infestans on StSYR1-RNAi plants suggest an involvement of syntaxins in secretory defense responses of potato and, in particular, in the formation of callose-containing papillae.