MAPK Pathway under Chronic Copper Excess in Green Macroalgae (Chlorophyta): Influence on Metal Exclusion/Extrusion Mechanisms and Photosynthesis

Abstract

There is currently no information regarding the role that whole mitogen activated protein kinase (MAPK) pathways play in counteracting environmental stress in photosynthetic organisms. To address this gap, we exposed Ulva compressa to chronic levels of copper (10 µM) specific inhibitors of Extracellular Signal Regulated Kinases (ERK), c-Jun N-terminal Kinases (JNK), and Cytokinin Specific Binding Protein (p38) MAPKs alone or in combination. Intracellular copper accumulation and photosynthetic activity (in vivo chlorophyll a fluorescence) were measured after 6 h, 24 h, 48 h, and 6 days of exposure. By day 6, when one (except JNK) or more of the MAPK pathways were inhibited under copper stress, there was a decrease in copper accumulation compared with algae exposed to copper alone. When at least two MAPKs were blocked, there was a decrease in photosynthetic activity expressed in lower productivity (ETRmax), efficiency (αETR), and saturation of irradiance (EkETR), accompanied by higher non-photochemical quenching (NPQmax), compared to both the control and copper-only treatments. In terms of accumulation, once the MAPK pathways were partially or completely blocked under copper, there was crosstalk between these and other signaling mechanisms to enhance metal extrusion/exclusion from cells. Crosstalk occurred among MAPK pathways to maintain photosynthesis homeostasis, demonstrating the importance of the signaling pathways for physiological performance. This study is complemented by a parallel/complementary article Rodríguez-Rojas et al. on the role of MAPKs in copper-detoxification.

MAPK Pathway under Chronic Copper Excess in Green Macroalgae (Chlorophyta): Involvement in the Regulation of Detoxification Mechanisms

“…It has been recognised that besides metal exclusion/extrusion mechanisms, the main strategies by which macroalgae counteract metal excess in polluted environments are detoxification of bioavailable metals through the syntheses of chelating compounds and the inactivation of metal-mediated excess of reactive oxygen species (ROS) by the production and activities of antioxidants and antioxidant enzymes [1,2]. In green macroalgae (Ulvophyceae), metal chelation is importantly controlled by cysteine-rich compounds, among which the most important are the gene encoded metallothioneins and the enzymatically synthesised glutathione (GSH) and its poly-oligomers phytochelatins (PCs) [2,3].…”

“…The potential participation of the mitogen-activated protein kinases (MAPKs) has been proposed, but not fully confirmed [2,12]. MAPKs are serine/threonine kinases following a cascade of activations from MAPK kinase kinase (MAPKKK), MAPK kinase (MAPKK) to MAPK, thus finally inducing gene expression [1,16,17]. There are three main sub-pathways within the MAPKs, these ending in the MAPK ERK (extracellular signal regulated kinases), JNK (c-Jun N -terminal kinases), and p38 (cytokinin specific binding protein) [1,16,17].…”

“…MAPKs are serine/threonine kinases following a cascade of activations from MAPK kinase kinase (MAPKKK), MAPK kinase (MAPKK) to MAPK, thus finally inducing gene expression [1,16,17]. There are three main sub-pathways within the MAPKs, these ending in the MAPK ERK (extracellular signal regulated kinases), JNK (c-Jun N -terminal kinases), and p38 (cytokinin specific binding protein) [1,16,17]. While a full description of the mechanistic contribution of these MAPK pathways to tolerance machinery under environmental stress in photoautotrophs has not been elucidated, their presence and induction have been observed in both micro and macroalgae [18,19,20,21].…”

“…The green seaweed U. compressa under 10 µM of copper was exposed to specific inhibitors of MAPK pathways, ERK, JNK, and/or p38, with measurements taken at different time points throughout the experiment (6 h, 24 h, 48 h, and 6 d). In a complementary investigation, we demonstrated that under chronic copper excess when two or more MAPK pathways were inhibited, there was a general trend of decreasing intracellular copper concentrations when compared to exposure only to copper [1]. Moreover, the lowest levels of photosynthetic performance, as determined from measurements of photosynthetic efficiency (α ETR ), productivity (ETR max ), and saturation of the irradiance (Ek ETR ), were under copper in the presence of two or more MAPK inhibitors, [1].…”

Mechanisms of Copper Tolerance, Accumulation, and Detoxification in the Marine Macroalga Ulva compressa (Chlorophyta): 20 Years of Research

“…It was additionally shown that the increase in gene expression involves the mitogen-activated protein kinases (MAPKs) signaling pathway [40]. The MAPKs, JNK, ERK, and p-38 are involved in the regulation of gene expression of antioxidant enzymes such as SOD, CAT, AP, DHAR, and GR [46], photosynthetic parameters, and copper accumulation, and these MAPKs showed a crosstalk among them [47]. Thus, activation of gene expression involves CaMK, CDPK, and MAPK signaling pathways.…”