The perception of microbes by plants involves highly conserved molecular signatures that are absent from the host and that are collectively referred to as microbe-associated molecular patterns (MAMPs). The Arabidopsis pattern recognition receptors FLAGELLIN-SENSING 2 (FLS2) and EF-Tu receptor (EFR) represent genetically well studied paradigms that mediate defense against bacterial pathogens. Stimulation of these receptors through their cognate ligands, bacterial flagellin or bacterial elongation factor Tu, leads to a defense response and ultimately to increased resistance. However, little is known about the early signaling pathway of these receptors. Here, we characterize this early response in situ, using an electrophysiological approach. In line with a release of negatively charged molecules, voltage recordings of microelectrode-impaled mesophyll cells and root hairs of Col-0 Arabidopsis plants revealed rapid, dose-dependent membrane potential depolarizations in response to either flg22 or elf18. Using ion-selective microelectrodes, pronounced anion currents were recorded upon application of flg22 and elf18, indicating that the signaling cascades initiated by each of the two receptors converge on the same plasma membrane ion channels. Combined calcium imaging and electrophysiological measurements revealed that the depolarization was superimposed by an increase in cytosolic calcium that was indispensable for depolarization. NADPH oxidase mutants were still depolarized upon elicitor stimulation, suggesting a reactive oxygen species-independent membrane potential response. Furthermore, electrical signaling in response to either flg22 or elf 18 critically depends on the activity of the FLS2-associated receptor-like kinase BAK1, suggesting that activation of FLS2 and EFR lead to BAK1-dependent, calcium-associated plasma membrane anion channel opening as an initial step in the pathogen defense pathway.

Some plants can avoid shaded conditions via rapid shoot elongation, thus growing into better lit areas in a canopy. Cell wall-modifying mechanisms promoting this elongation response, therefore, are important regulatory points during shade avoidance. Two major cell wall-modifying protein families are expansins and xyloglucan endotransglucosylase/hydrolases (XTHs). The role of these proteins during shade avoidance was studied in Arabidopsis (Arabidopsis thaliana). In response to two shade cues, low red to far-red light (implying neighbor proximity) and green shade (mimicking dense canopy conditions), Arabidopsis showed classic shade avoidance features: petiole elongation and leaf hyponasty. Measurement of the apoplastic proton flux in green shade-treated petioles revealed a rapid efflux of protons into the apoplast within minutes, unlike white light controls. This apoplastic acidification probably provides the acidic pH required for the optimal activity of cell wall-modifying proteins like expansins and XTHs. Acid-induced extension, expansin susceptibility, and extractable expansin activity were similar in petioles from white light- and shade-treated plants. XTH activity, however, was high in petioles exposed to shade treatments. Five XTH genes (XTH9, -15, -16, -17, and -19) were positively regulated by low red to far-red light conditions, while the latter four and XTH22 showed a significant up-regulation also in response to green shade. Consistently, knockout mutants for two of these XTH genes also had reduced or absent shade avoidance responses to these light signals. These results point toward the cell wall as a vital regulatory point during shade avoidance.

• Vascular wetland plants may substantially increase methane emissions by producing root exudates and easily degradable litter, and by providing a low-resistance diffusion pathway via their aerenchyma. However, model studies have indicated that vascular plants can reduce methane emission when soil oxygen demand is exceeded by oxygen released from roots. Here, we tested whether these conditions occur in bogs dominated by cushion plants. • Root–methane interactions were studied by comparing methane emissions, stock and oxygen availability in depth profiles below lawns of either cushion plants or Sphagnum mosses in Patagonia. • Cushion plants, Astelia pumila and Donatia fascicularis, formed extensive root systems up to 120 cm in depth. The cold soil (< 10°C) and highly decomposed peat resulted in low microbial activity and oxygen consumption. In cushion plant lawns, high soil oxygen coincided with high root densities, but methane emissions were absent. In Sphagnum lawns, methane emissions were substantial. High methane concentrations were only found in soils without cushion plant roots. • This first methane study in Patagonian bog vegetation reveals lower emissions than expected. We conclude that cushion plants are capable of reducing methane emission on an ecosystem scale by thorough soil and methane oxidation.