Illumination changes elicit modifications of thylakoid proteins and reorganization of the photosynthetic machinery. This involves, in the short term, phosphorylation of photosystem II (PSII) and light-harvesting (LHCII) proteins. PSII phosphorylation is thought to be relevant for PSII turnover 1,2 , whereas LHCII phosphorylation is associated with the relocation of LHCII and the redistribution of excitation energy (state transitions) between photosystems 3,4 . In the long term, imbalances in energy distribution between photosystems are counteracted by adjusting photosystem stoichiometry 5,6 . In the green alga Chlamydomonas and the plant Arabidopsis, state transitions require the orthologous protein kinases STT7 and STN7, respectively 7,8 . Here we show that in Arabidopsis a second protein kinase, STN8, is required for the quantitative phosphorylation of PSII core proteins. However, PSII activity under high-intensity light is affected only slightly in stn8 mutants, and D1 turnover is indistinguishable from the wild type, implying that reversible protein phosphorylation is not essential for PSII repair. Acclimation to changes in light quality is defective in stn7 but not in stn8 mutants, indicating that short-term and long-term photosynthetic adaptations are coupled. Therefore the phosphorylation of LHCII, or of an unknown substrate of STN7, is also crucial for the control of photosynthetic gene expression.STT7 and STN7 are orthologous protein kinases required for LHCII phosphorylation and for state transitions in Chlamydomonas and Arabidopsis, respectively 7,8 . In Arabidopsis, another STT7/STN7-like protein (STN8) exists that is not required for state transitions 8 . STN8 is located in the chloroplast, as shown by in vivo subcellular localization of its amino-terminal region fused to the dsRED protein and by the import of, and transit peptide removal from, STN8 translated in vitro (Fig. 1a, b). Chloroplast subfractionation after import revealed that the protein is associated, like STT7 and STN7, with thylakoids ( Fig. 1c) (refs 7, 8).Insertion mutants for STN8 and STN7 were obtained from the Salk collection 9 , and for each gene two independent mutant alleles lacking the respective transcript were identified (Supplementary Fig. S1). The stn7 stn8 double mutant was generated by crossing stn7 and stn8 single knockouts and screening the resulting F 2 generation for homozygous double mutants. All mutants were indistinguishable from the wild type with regard to the timing of seed germination and growth rate in the greenhouse ( Supplementary Fig. S1). In stn7 and stn7 stn8 mutants, a slight decrease in the levels of neoxanthin, lutein and total chlorophyll was found (Supplementary Table S1). These subtle changes can be attributed to a minor decrease in LHCII content, not detectable by polyacrylamide-gel electrophoresis (PAGE) analysis ( Supplementary Fig. S2).Photosynthetic electron flow, measured on the basis of chlorophyll fluorescence, was not altered in the mutants (Supplementary Table S2). State transitions w...

Plants and animals deploy intracellular immune receptors that perceive specific pathogen effector proteins and microbial products delivered into the host cell. We demonstrate that the ADR1 family of Arabidopsis nucleotide-binding leucine-rich repeat (NB-LRR) receptors regulates accumulation of the defense hormone salicylic acid during three different types of immune response: (i) ADRs are required as "helper NB-LRRs" to transduce signals downstream of specific NB-LRR receptor activation during effector-triggered immunity; (ii) ADRs are required for basal defense against virulent pathogens; and (iii) ADRs regulate microbial-associated molecular pattern-dependent salicylic acid accumulation induced by infection with a disarmed pathogen. Remarkably, these functions do not require an intact P-loop motif for at least one ADR1 family member. Our results suggest that some NB-LRR proteins can serve additional functions beyond canonical, P-loop-dependent activation by specific virulence effectors, extending analogies between intracellular innate immune receptor function from plants and animals.nucleotide-binding domain and leucine-rich repeat-containing protein receptors | plant immune system | effector-triggered immunity | microbial-associated molecular pattern-triggered immunity P lants respond to attempted microbial infection with a twotiered immune system. In the first tier, extracellular pattern recognition receptors (PRRs) bind conserved microbial-associated molecular pattern (MAMP) ligands, activating a complex host response that results in MAMP-triggered immunity (MTI). Successful pathogens deploy suites of virulence effectors that delay or suppress MTI, allowing infection. In the second tier, plant intracellular immune receptors of the nucleotide-binding leucinerich repeat (NB-LRR) protein family can be activated either by direct binding of effectors or, alternatively, by effector action on an associated target protein that generates a "modified-self" molecule (1, 2). Effector-mediated NB-LRR activation results in effectortriggered immunity (ETI), a rapid and high-amplitude output significantly overlapping with MTI (1). ETI is typically accompanied by the hypersensitive cell death response (HR), limited to the site of pathogen attack. Both MTI and some cases of NB-LRRmediated ETI require the salicylic acid (SA)-signaling molecule as a downstream mediator of transcriptional output responses (3, 4).Plant NB-LRR proteins belong to the STAND (signal transduction ATPases with numerous domains) superfamily, which includes the animal apoptotic proteins Apaf-1/CED4 and innate immune receptors of the nucleotide-binding domain and leucinerich repeat-containing proteins (NLR) family (5). Animal NLRs are activated by MAMPs and by modified-self molecules in the form of danger-associated molecular patterns (6) and regulate inflammasome activation, autophagy, and cell death (7). STAND protein functions require an intact P-loop motif (GxxxxGKT/S) that coordinates ATP binding. STAND proteins are molecular switches that toggle fr...

Flowering plants control energy allocation to their photosystems in response to light quality changes. This includes the phosphorylation and migration of light-harvesting complex II (LHCII) proteins (state transitions or short-term response) as well as long-term alterations in thylakoid composition (long-term response or LTR). Both responses require the thylakoid protein kinase STN7. Here, we show that the signaling pathways triggering state transitions and LTR diverge at, or immediately downstream from, STN7. Both responses require STN7 activity that can be regulated according to the plastoquinone pool redox state. However, LTR signaling does not involve LHCII phosphorylation or any other state transition step. State transitions appear to play a prominent role in flowering plants, and the ability to perform state transitions becomes critical for photosynthesis in Arabidopsis thaliana mutants that are impaired in thylakoid electron transport but retain a functional LTR. Our data imply that STN7-dependent phosphorylation of an as yet unknown thylakoid protein triggers LTR signaling events, whereby an involvement of the TSP9 protein in the signaling pathway could be excluded. The LTR signaling events then ultimately regulate in chloroplasts the expression of photosynthesis-related genes on the transcript level, whereas expression of nuclear-encoded proteins is regulated at multiple levels, as indicated by transcript and protein profiling in LTR mutants.

The nucleotide-binding domain leucine-rich repeat proteins (NLRs) represent the major class of intracellular innate immune receptors in plants and animals. Understanding their functions is a major challenge in immunology. This review highlights recent efforts toward elucidating NLR functions in human and plants. We compare unconventional aspects of NLR proteins across the two kingdoms. We review recent advances describing P-loop independent activation, nuclear-cytoplasmic trafficking, oligomerization and multimerization requirements for signaling, and for expanded functions beyond pathogen recognition by several NLR proteins.

Plants react to pathogen attack via recognition of, and response to, pathogen-specific molecules at the cell surface and inside the cell. Pathogen effectors (virulence factors) are monitored by intracellular nucleotide-binding leucine-rich repeat (NB-LRR) sensor proteins in plants and mammals. Here, we study the genetic requirements for defense responses of an autoactive mutant of ADR1-L2, an Arabidopsis coiled-coil (CC)-NB-LRR protein. ADR1-L2 functions upstream of salicylic acid (SA) accumulation in several defense contexts, and it can act in this context as a “helper” to transduce specific microbial activation signals from “sensor” NB-LRRs. This helper activity does not require an intact P-loop. ADR1-L2 and another of two closely related members of this small NB-LRR family are also required for propagation of unregulated runaway cell death (rcd) in an lsd1 mutant. We demonstrate here that, in this particular context, ADR1-L2 function is P-loop dependent. We generated an autoactive missense mutation, ADR1-L2D484V, in a small homology motif termed MHD. Expression of ADR1-L2D848V leads to dwarfed plants that exhibit increased disease resistance and constitutively high SA levels. The morphological phenotype also requires an intact P-loop, suggesting that these ADR1-L2D484V phenotypes reflect canonical activation of this NB-LRR protein. We used ADR1-L2D484V to define genetic requirements for signaling. Signaling from ADR1-L2D484V does not require NADPH oxidase and is negatively regulated by EDS1 and AtMC1. Transcriptional regulation of ADR1-L2D484V is correlated with its phenotypic outputs; these outputs are both SA–dependent and –independent. The genetic requirements for ADR1-L2D484V activity resemble those that regulate an SA–gradient-dependent signal amplification of defense and cell death signaling initially observed in the absence of LSD1. Importantly, ADR1-L2D484V autoactivation signaling is controlled by both EDS1 and SA in separable, but linked pathways. These data allows us to propose a genetic model that provides insight into an SA–dependent feedback regulation loop, which, surprisingly, includes ADR1-L2.