Pathogate: Stoma-Antwort auf Mikrobenbefall
Zusammenfassung der Projektergebnisse
Plants utilize small pores in the leaf surface to take up the greenhouse gas carbon dioxide (CO2) from the atmosphere. However, these stomatal pores make a plant vulnerable to drought and infections, as they enable transpiration of water and the entry of microorganisms into leaves. These threats are counteracted by two guard cells that surround the pore and close the stomata when they recognize the drought hormone Abscisic acid (ABA), or molecular structures of microorganisms (MAMPs). Upon recognition of these signals, guard cells activate anion channels in their cell membrane, which induces closure of stomatal pores. The objective of our “Pathogate” project was to compare the signaling pathways of ABA and MAMPs in guard cells. Guard cells express six PYR/PYL/RCAR receptor-proteins that can recognize ABA and were thought to have a redundant function in drought-induced stomatal closure. However, experiments with mutants that had lost five of these proteins, showed that the induction of stomatal closure is predominately due to one receptor isoform. Other ABA-receptor isoforms seem to provoke other ABA-dependent adaptations in guard cells, such as an enhancement of CO2-sensitivity during drought. Upon recognition of ABA, the ABA receptor complex addresses the central regulatory protein OST1, which in turn activates the anion channel SLAC1. Due to the activity of the SLAC1 anion channels the guard cells deflate and the stomatal pores close. In contrast to the multitude of ABA-receptors located in the cytoplasm of guard cells, the fugal cell wall component “chitin” is recognized at the cell surface by a pair of receptor proteins (CERK1 and LYK5). Upon binding of chitin, these receptor proteins stimulate the protein kinase (PBL27) within the guard cells that activates the SLAC1 anion channel homolog SLAH3. Apparently PBL27 cause activation of the anion channel SLAH3, while OST1 has SLAC1 as its target. Nevertheless, the two regulatory kinases PBL27 and OST1 seem to have very similar functions in pathogen- and drought signaling, respectively. Whereas the core signaling pathways for drought and chitin lack common components, it is likely that both signaling chains will interact in guard cells. We found that the cytosolic Ca2+ concentration transiently increases when stomata are closing in response to ABA. It is likely, that these “Ca2+ signals” are evoked, when guard cells rapidly lose osmotically active ions, during stomatal closure. The rapid loss of ions causes swelling of intracellular compartments, which can trigger release of Ca2+ into the cytosol. In turn, the elevation of the cytosolic Ca2+ level is sensed by Ca2+-dependent protein kinases, that activate the SLAC1 and SLAH3 anion channels. It is very likely that this accessory, Ca2+-dependent, signaling pathway boosts stomatal closure of stomata when leaves are experiencing a sudden loss of water to the atmosphere. In agriculture, there is a demand for crop plants that can resist drought and need less protection agrochemicals to prevent infections by microorganisms. Our project has pointed to the importance of distinct regulatory protein kinases in responses of stomata to pathogens and drought. This information can be used in future breeding programs, to obtain crop plants that are superior in pathogen defense and tolerance to drought. “Wie Pflanzen sich gegen Bakterien abschotten” (http://www.presse.uni-wuerzburg.de/aktuell/einblick/single/news/wie-pflanzen-sich-gegen-bakterien-abschotten)
Projektbezogene Publikationen (Auswahl)
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(2016). Current injection provokes rapid expansion of the guard cell cytosolic volume and triggers Ca2+ signals. Mol. Plant. 9:471-480
Voss, LJ; Hedrich, R; and Roelfsema, MRG
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(2018). Mycorrhizal lipochitinoligosaccharides (LCOs) depolarize root hairs of Medicago truncatula. Plos One 13:14
Hürter, AL; Fort, S; Cottaz, S; Hedrich, R; Geiger, D; and Roelfsema, MRG
