Nicholas Smirnoff

SUMMARY Stomatal defences are important for plants to prevent pathogen entry and further colonisation of leaves. Apoplastic reactive oxygen species (ROS) generated by NADPH oxidases and apoplastic peroxidases play an important role in activating stomatal closure upon perception of bacteria. However, downstream events, particularly the factors influencing cytosolic hydrogen peroxide (H 2 O 2 ) signatures in guard cells are poorly understood. We used the H 2 O 2 sensor roGFP2‐Orp1 and a ROS‐specific fluorescein probe to study intracellular oxidative events during stomatal immune response using Arabidopsis mutants involved in the apoplastic ROS burst. Surprisingly, the NADPH oxidase mutant rbohF showed over‐oxidation of roGFP2‐Orp1 by a pathogen‐associated molecular pattern (PAMP) in guard cells. However, stomatal closure was not tightly correlated with high roGFP2‐Orp1 oxidation. In contrast, RBOHF was necessary for PAMP‐mediated ROS production measured by a fluorescein‐based probe in guard cells. Unlike previous reports, the rbohF mutant, but not rbohD , was impaired in PAMP‐triggered stomatal closure resulting in defects in stomatal defences against bacteria. Interestingly, RBOHF also participated in PAMP‐induced apoplastic alkalinisation. The rbohF mutants were also partly impaired in H 2 O 2 ‐mediated stomatal closure at 100 μ m while higher H 2 O 2 concentration up to 1 m m did not promote stomatal closure in wild‐type plants. Our results provide novel insights on the interplay between apoplastic and cytosolic ROS dynamics and highlight the importance of RBOHF in plant immunity.

A number of experimental studies have demonstrated that phytoplankton can display rapid thermal adaptation in response to warmed environments. While these studies provide insight into the evolutionary responses of single species, they tend to employ different experimental techniques. Consequently, our ability to compare the potential for thermal adaptation across different, ecologically relevant, species remains limited. Here, we address this limitation by conducting simultaneous long‐term warming experiments with the same experimental design on clonal isolates of three phylogenetically diverse species of marine phytoplankton; the cyanobacterium Synechococcus sp., the prasinophyte Ostreococcus tauri and the diatom Phaeodoactylum tricornutum . Over the same experimental time period, we observed differing levels of thermal adaptation in response to stressful supra‐optimal temperatures. Synechococcus sp. displayed the greatest improvement in fitness (i.e., growth rate) and thermal tolerance (i.e., temperature limits of growth). Ostreococcus tauri was able to improve fitness and thermal tolerance, but to a lesser extent. Finally, Phaeodoactylum tricornutum showed no signs of adaptation. These findings could help us understand how the structure of phytoplankton communities may change in response to warming, and possible biogeochemical implications, as some species show relatively more rapid adaptive shifts in their thermal tolerance.