Publikationen - Molekulare Signalverarbeitung

The decision between defence and symbiosis signalling in plants involves alternative and modular plasma membrane‐localized receptor complexes. A critical step in their activation is ligand‐induced homo‐ or hetero‐oligomerization of leucine‐rich repeat (LRR)‐ and/or lysin motif (LysM) receptor‐like kinases (RLKs). In defence signalling, receptor complexes form upon binding of pathogen‐associated molecular patterns (PAMPs), including the bacterial flagellin‐derived peptide flg22, or chitin. Similar mechanisms are likely to operate during the perception of microbial symbiont‐derived (lipo)‐chitooligosaccharides. The structurally related chitin‐oligomer ligands chitooctaose and chitotetraose trigger defence and symbiosis signalling, respectively, and their discrimination involves closely related, if not identical, LysM‐RLKs. This illustrates the demand for and the challenges imposed on decision mechanisms that ensure appropriate signal initiation. Appropriate signalling critically depends on abundance and localization of RLKs at the cell surface. This is regulated by internalization, which also provides a mechanism for the removal of activated signalling RLKs. Abundance of the malectin‐like domain (MLD)‐LRR‐RLK Symbiosis Receptor‐like Kinase (SYMRK) is additionally controlled by cleavage of its modular ectodomain, which generates a truncated and rapidly degraded RLK fragment. This review explores LRR‐ and LysM‐mediated signalling, the involvement of MLD‐LRR‐RLKs in symbiosis and defence, and the role of endocytosis in RLK function.

Plants form root symbioses with fungi and bacteria to improve their nutrient supply. SYMBIOSIS RECEPTOR-LIKE KINASE (SYMRK) is required for phosphate-acquiring arbuscular mycorrhiza, as well as for the nitrogen-fixing root nodule symbiosis of legumes [1] and actinorhizal plants [2, 3], but its precise function was completely unclear. Here we show that the extracytoplasmic region of SYMRK, which comprises three leucine-rich repeats (LRRs) and a malectin-like domain (MLD) related to a carbohydrate-binding protein from Xenopus laevis [4], is cleaved to release the MLD in the absence of symbiotic stimulation. A conserved sequence motif—GDPC—that connects the MLD to the LRRs is required for MLD release. We discovered that Nod factor receptor 5 (NFR5) [5, 6, 7, 8] forms a complex with the SYMRK version that remains after MLD release (SYMRK-ΔMLD). SYMRK-ΔMLD outcompeted full-length SYMRK for NFR5 interaction, indicating that the MLD negatively interferes with complex formation. SYMRK-ΔMLD is present at lower amounts than MLD, suggesting rapid degradation after MLD release. A deletion of the entire extracytoplasmic region increased protein abundance, suggesting that the LRR region promotes degradation. Curiously, this deletion led to excessive infection thread formation, highlighting the importance of fine-tuned regulation of SYMRK by its ectodomain.

Symbiosis Receptor-like Kinase (SYMRK) is indispensable for the development of phosphate-acquiring arbuscular mycorrhiza (AM) as well as nitrogen-fixing root nodule symbiosis, but the mechanisms that discriminate between the two distinct symbiotic developmental fates have been enigmatic. In this study, we show that upon ectopic expression, the receptor-like kinase genes Nod Factor Receptor 1 (NFR1), NFR5, and SYMRK initiate spontaneous nodule organogenesis and nodulation-related gene expression in the absence of rhizobia. Furthermore, overexpressed NFR1 or NFR5 associated with endogenous SYMRK in roots of the legume Lotus japonicus. Epistasis tests revealed that the dominant active SYMRK allele initiates signalling independently of either the NFR1 or NFR5 gene and upstream of a set of genes required for the generation or decoding of calcium-spiking in both symbioses. Only SYMRK but not NFR overexpression triggered the expression of AM-related genes, indicating that the receptors play a key role in the decision between AM- or root nodule symbiosis-development.

The Lotus japonicus SYMBIOSIS RECEPTOR-LIKE KINASE (SYMRK) is required for symbiotic signal transduction upon stimulation of root cells by microbial signaling molecules. Here, we identified members of the SEVEN IN ABSENTIA (SINA) E3 ubiquitin-ligase family as SYMRK interactors and confirmed their predicted ubiquitin-ligase activity. In Nicotiana benthamiana leaves, SYMRK–yellow fluorescent protein was localized at the plasma membrane, and interaction with SINAs, as determined by bimolecular fluorescence complementation, was observed in small punctae at the cytosolic interface of the plasma membrane. Moreover, fluorescence-tagged SINA4 partially colocalized with SYMRK and caused SYMRK relocalization as well as disappearance of SYMRK from the plasma membrane. Neither the localization nor the abundance of Nod-factor receptor1 was altered by the presence of SINA4. SINA4 was transcriptionally upregulated during root symbiosis, and rhizobia inoculated roots ectopically expressing SINA4 showed reduced SYMRK protein levels. In accordance with a negative regulatory role in symbiosis, infection thread development was impaired upon ectopic expression of SINA4. Our results implicate SINA4 E3 ubiquitin ligase in the turnover of SYMRK and provide a conceptual mechanism for its symbiosis-appropriate spatio-temporal containment.

Plant receptor-like kinases (RLKs) function in diverse signaling pathways, including the responses to microbial signals in symbiosis and defense. This versatility is achieved with a common overall structure: an extracytoplasmic domain (ectodomain) and an intracellular protein kinase domain involved in downstream signal transduction. Various surfaces of the leucine-rich repeat (LRR) ectodomain superstructure are utilized for interaction with the cognate ligand in both plant and animal receptors. RLKs with lysin-motif (LysM) ectodomains confer recognitional specificity toward N-acetylglucosamine-containing signaling molecules, such as chitin, peptidoglycan (PGN), and rhizobial nodulation factor (NF), that induce immune or symbiotic responses. Signaling downstream of RLKs does not follow a single pattern; instead, the detailed analysis of brassinosteroid (BR) signaling, innate immunity, and symbiosis revealed at least three largely nonoverlapping pathways. In this review, we focus on RLKs involved in plant-microbe interactions and contrast the signaling pathways leading to symbiosis and defense.

Prosopis strombulifera, a common legume in high-salinity soils of Argentina, is a useful model for elucidation of salt tolerance mechanisms and specific biochemical pathways in halophytes, since its NaCl tolerance exceeds the limit described for most halophytic plants. We analyzed the effects of the increasing concentration of two main soil salts, Na2SO4 and NaCl, on growth parameters of P. strombulifera, chlorophyll levels, and content of jasmonates (JAs) and polyamines (PAs), which are key molecules involved in stress responses. P. strombulifera showed a halophytic response (growth promotion) to NaCl, but strong growth inhibition by iso-osmotic solutions of Na2SO4. Chlorophyll levels, number of leaves and leaf area were also differentially affected. An important finding was the partial alleviation of SO42− toxicity by treatment with two-salt mixture. JAs are not directly involved in salt tolerance in this species since its levels decrease under all salt treatments. Beneficial effects of Putrescine (Put) accumulation in NaCl treated plants maybe inferred probably associated with the antioxidative defense system. Another novel finding is the accumulation of the uncommon PA cadaverine in roots under high Na2SO4, which may be related to SO42− toxicity.

There is considerable evidence suggesting that jasmonates (JAs) play a role in plant resistance against abiotic stress. It is well known that in Angiosperms JAs are involved in the defense response, however there is little information about their role in Gymnosperms. Our proposal was to study the involvement of JAs in Pinus pinaster Ait. reaction to cold and water stress, and to compare the response of two populations of different provenances (Gredos and Bajo Tiétar) to these stresses. We detected 12-oxo-phytodienoic acid (OPDA), jasmonic acid (JA), and the hydroxylates 11-hydroxyjasmonate and 12-hydroxyjasmonate in foliage and shoots of P. pinaster plants. The response of the Gredos population to cold stress differed from that of Bajo Tiétar. Gredos plants showed a lower JA-basal level than Bajo Tiétar; under cold stress JA increased twofold at 72 h, while it decreased in Bajo Tiétar plants. The hydroxylates slightly increased in both populations due to cold stress treatment. Under water stress, plants from Gredos showed a remarkable JA-increase; thus the JA-response was much more prominent under water stress than under cold stress. In contrast, no change was found in JA-level in Bajo Tiétar plants under water stress. The level of JA-precursor, OPDA, was very low in control plants from Gredos and Bajo Tiétar. Under water stress OPDA increased only in plants from Bajo Tiétar. Therefore, we inform here of a different JAs-accumulation pattern after the stress treatment in P. pinaster from two provenances, and suggest a possible correlation with adaptations to diverse ecological conditions.

In this study, we characterized two sunflower (Helianthus annuus L.) lines with differential sensitivity to drought, the sensitive line B59 and the tolerant line B71. Using both lines, we compared the content of endogenous jasmonates (JAs) in dry and imbibed seeds from plants grown under irrigation and drought. Jasmonic acid (JA), 12-oxo-phytodienoic acid (OPDA), 11-hydroxyjasmonate (11-OH-JA) and 12-hydroxyjasmonate (12-OH-JA) were detected in dry and imbibed sunflower seeds. Seeds from plants grown under drought had a lower content of total JAs and exhibited higher germination percentages than seeds from irrigated plants, demonstrating that environmental conditions have a strong influence on the progeny. OPDA and 12-OH-JA were the main compounds found in dry seeds of both lines. Imbibed seeds showed an enhanced amount of total JAs with respect to dry seeds produced by plants grown in both soil moisture conditions. Imbibition triggered a dramatic OPDA increase in the embryo, suggesting a role of this compound in germination. We conclude that JAs patterns vary during sunflower germination and that the environmental conditions experienced by the mother plant modify the hormonal content of the seed progeny.

Although jasmonates (JAs) are involved in germination and seedling development, the regulatory mechanism of JAs, and their relation with endogenous level modifications in these processes, is not well understood. We report here the detection of 12-oxo-phytodienoic acid (OPDA), jasmonic acid (JA), 11-hydroxyjasmonate (11-OH-JA), 12-hydroxyjasmonate (12-OH-JA) and methyljasmonate (JAME) in unimbibed seeds and seedlings of tomato Lycopersicon esculentum Mill cv. Moneymaker (wild type) and tss1, tss2, tos1 mutants. The main compounds in wild-type and tss1, tss2, tos1 seeds were the hydroxylate-JAs; 12-OH-JA was the major component in dry seeds of the wild type and in tss2 and tos1. The amounts of these derivatives were higher in seeds than in seedlings. Changes in JAs during wild-type and tss1 imbibition were analysed in seeds and the imbibition water. In wild-type imbibed seeds, 11-OH-JA content was higher than in tss1. 12-OH-JA showed a different tendency with respect to 11-OH-JA, with high levels in the wild type at early imbibition. In tss1, levels of 12-OH-JA rose from 24 to 48 h of imbibition. At 72 h of imbibition, when radicles had emerged, the amounts of both hydroxylates in wild-type and tss1 seeds were minimal. An important release of the hydroxylate forms was observed in the imbibition water. 11-OH-JA decreased in the imbibition water of wild-type seeds at 48 h. On the contrary, a high and sustained liberation of this compound was observed in tss1 after 24 h. 12-OH-JA increased in wild-type as well in tss1 until 24 h. Thereafter, a substantial reduction in the content of this compound was registered. NaCl-treated wild-type seedlings increased their 12-OH-JA, but tss1 seedlings increased their JA in response to salt treatment. In tss2 seedlings, NaCl caused a slight decrease in 11-OH-JA and JAME, whereas tos1 seedlings showed a dramatic OPDA and 12-OH-JA decrease in response to salt treatment. Under salt stress the mutant seedlings showed different patterns of JAs according to their differential hypersensitivity to abiotic stress. The JA-hydroxylate forms found, and the differential accumulation of JAs during germination, imbibition and seedling development, as well as their response to NaCl stress, provide new evidence about the control of many developmental processes by JA.

Various octadecanoids and derived compounds have been identified in potato leaves. However, information regarding jasmonate hydroxylated forms in stolons or tubers is scarce. We investigated endogenous jasmonates in stolon material ofSolarium tuberosum cv. Spunta. Stolons and incipient tubers were collected from 8 weeks old plants. The material was cut into apical regions and stolons. We identified jasmonic acid (JA), methyl jasmonate, 11-OH-JA, 12-OH-JA, 12-oxo-phytodienoic acid (OPDA) and a conjugate. The content of JA and 12OH-JA decreased in the apical region but remained high in stolons during tuberization. Thus the apical region might be a site of JAs-utilization or metabolization and stolons might supply JAs to that region. The content of 12-OH-JA was higher than that of 11-OH-JA in all stages analyzed, both in apical regions and stolons. However, these compounds showed a different time-course in the apical region: while 11-OH-JA increased, 12-OH-JA decreased. Thus, JA from leaves or roots could be transported as 12-OH-JA to the apical region, stimulating tuber formation.