pH effects on plant calcium fluxes: lessons from acidification-mediated calcium elevation induced by the γ-glutamyl-leucine dipeptide identified from Phytophthora infestans

Abstract

Cytosolic Ca2+ ([Ca2+]cyt) elevation is an early signaling response upon exposure to pathogen-derived molecules (so-called microbe-associated molecular patterns, MAMPs) and has been successfully used as a quantitative read-out in genetic screens to identify MAMP receptors or their associated components. Here, we isolated and identified by mass spectrometry the dipeptide γ-Glu-Leu as a component of a Phytophthora infestans mycelium extract that induces [Ca2+]cyt elevation. Treatment of Arabidopsis seedlings with synthetic γ-Glu-Leu revealed stimulatory effects on defense signaling, including a weak enhancement of the expression of some MAMP-inducible genes or affecting the refractory period to a second MAMP elicitation. However, γ-Glu-Leu is not a classical MAMP since pH adjustment abolished these activities and importantly, the observed effects of γ-Glu-Leu could be recapitulated by mimicking extracellular acidification. Thus, although γ-Glu-Leu can act as a direct agonist of calcium sensing receptors in animal systems, the Ca2+-mobilizing activity in plants reported here is due to acidification. Low pH also shapes the Ca2+ signature of well-studied MAMPs (e.g. flg22) or excitatory amino acids such as glutamate. Overall, this work serves as a cautionary reminder that in defense signaling studies where Ca2+ flux measurements are concerned, it is important to monitor and consider the effects of pH.

The PTI to ETI Continuum in Phytophthora-Plant Interactions

“…Ca 2+ ion influx is one of the earliest events that has been reported to trigger within 30 s of PAMP perception. This Ca 2+ influx opens up the membrane transporters for the influx of H + and efflux of K + , Cl − , and NO − 3 that leads to changes in extracellular pH and depolarization of plasma membrane within 1 to 3 min of PAMP perception (Bigeard et al, 2015;Westphal et al, 2019). PAMP induced Ca 2+ ion homeostasis in the host has been tested for only a few Phytophthora spp.…”

Effect of glutamate on Pyricularia oryzae infection of rice monitored by changes in photosynthetic parameters and antioxidant metabolism

“…The glutamate metabolism undergoes expressive change in diseased plants due to its participation in two distinct defense mechanisms depending on the pathogen's lifestyle (Seifi et al ). Westphal et al () speculated that glutamate would induce the calcium signaling due to cytosolic acidification, which revealed stimulatory effects on defense response against Phytophthora infestans in Arabidopsis plants. Rice plants supplied with glutamate through the roots showed a reduction in blast severity due to an increase in the expression of defense genes (Kadotani et al ).…”

Humic acids trigger the weak acids stress response in maize seedlings

“…The WRKY is a class of DNA-binding protein TFs that play a major role in plant defence to abiotic and biotic stresses. WRKY33 is one of the upregulated genes found in transcriptome of plants exposed to acidic apoplastic pH [58] as well as by cytosolic acidification [59]. These transcription factors were down regulated by maize seedling treatment with humic acids (Fig.…”