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1. Contrasting effects of high‐intensity photosynthetically active radiation on two bloom‐forming dinoflagellates

   https://doi.org/10.1111/jpy.12890  

   Cooney, Elizabeth C. ; Fredrickson, Kerri A. ; Bright, Kelley J. ; Strom, Suzanne L. ; Raven, ed., J. ( July 2019 , Journal of Phycology)

   While light limitation can inhibit bloom formation in dinoflagellates, the potential for high‐intensity photosynthetically active radiation (PAR) to inhibit blooms by causing stress or damage has not been well‐studied. We measured the effects of high‐intensityPARon the bloom‐forming dinoflagellatesAlexandrium fundyenseandHeterocapsa rotundata. Various physiological parameters (photosynthetic efficiencyF_v/F_m, cell permeability, dimethylsulfoniopropionate [DMSP], cell volume, and chlorophyll‐acontent) were measured before and after exposure to high‐intensity natural sunlight in short‐term light stress experiments. In addition, photosynthesis‐irradiance (P‐E) responses were compared for cells grown at different light levels to assess the capacity for photophysiological acclimation in each species. Experiments revealed distinct species‐specific responses to highPAR. While high light decreasedF_v/F_min both species,A. fundyenseshowed little additional evidence of light stress in short‐term experiments, although increased membrane permeability and intracellularDMSPindicated a response to handling. P‐E responses further indicated a high light‐adapted species with Chl‐ainversely proportional to growth irradiance and no evidence of photoinhibition; reduced maximum per‐cell photosynthesis rates suggest a trade‐off between photoprotection and C fixation in high light‐acclimated cells.Heterocapsa rotundatacells, in contrast, swelled in response to high light and sometimes lysed in short‐term experiments, releasingDMSP. P‐E responses confirmed a low light‐adapted species with high photosynthetic efficiencies associated with trade‐offs in the form of substantial photoinhibition and a lack of plasticity in Chl‐acontent. These contrasting responses illustrate that high light constrains dinoflagellate community composition through species‐specific stress effects, with consequences for bloom formation and ecological interactions within the plankton.

    

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2. Na + /K + ‐ ATP ase gene duplications in clitellate annelids are associated with freshwater colonization

   https://doi.org/10.1111/jeb.13439  

   Horn, Kevin M. ; Williams, Bronwyn W. ; Erséus, Christer ; Halanych, Kenneth M. ; Santos, Scott R. ; Creuzé des Châtelliers, Michel ; Anderson, Frank E. ( April 2019 , Journal of Evolutionary Biology)

   Major habitat transitions, such as those from marine to freshwater habitats or from aquatic to terrestrial habitats, have occurred infrequently in animal evolution and may represent a barrier to diversification. Identifying genomic events associated with these transitions can help us better understand mechanisms that allow animals to cross these barriers and diversify in new habitats. Study of theCapitella telataandHelobdella robustagenomes allows examination of one such habitat transition (marine to freshwater) in Annelida. Initial examination of these genomes indicated that the freshwater leechH. robustacontains many more copies (12) of the sodium–potassium pump alpha‐subunit (Na⁺/K⁺‐ATPase) gene than does the marine polychaeteC. telata(2). The sodium–potassium pump plays a key role in maintenance of cellular ionic balance and osmoregulation, and Na⁺/K⁺‐ATPase duplications may have helped annelids invade and diversify in freshwater habitats. To assess whether the timing of Na⁺/K⁺‐ATPase duplications coincided with the marine‐to‐freshwater transition in Clitellata, we used transcriptomic data from 18 annelid taxa, along with the two genomes, to infer a species phylogeny and identified Na⁺/K⁺‐ATPase gene transcripts in order to infer the timing of gene duplication events using tree‐based methods. The inferred timing of Na⁺/K⁺‐ATPase duplication events is consistent with the timing of the initial marine‐to‐freshwater transition early in the history of clitellate annelids, supporting the hypothesis that gene duplications may have played a role in the annelid diversification into freshwater habitats.

    

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3. TaADF4 , an actin‐depolymerizing factor from wheat, is required for resistance to the stripe rust pathogen Puccinia striiformis f. sp. tritici

   https://doi.org/10.1111/tpj.13459  

   Zhang, Bing ; Hua, Yuan ; Wang, Juan ; Huo, Yan ; Shimono, Masaki ; Day, Brad ; Ma, Qing ( February 2017 , The Plant Journal)

   Actin filament assembly in plants is a dynamic process, requiring the activity of more than 75 actin‐binding proteins. Central to the regulation of filament assembly and stability is the activity of a conserved family of actin‐depolymerizing factors (ADFs), whose primarily function is to regulate the severing and depolymerization of actin filaments. In recent years, the activity ofADFproteins has been linked to a variety of cellular processes, including those associated with response to stress. Herein, a wheatADFgene,TaADF4,was identified and characterized.TaADF4encodes a 139‐amino‐acid protein containing five F‐actin‐binding sites and two G‐actin‐binding sites, and interacts with wheat (Triticum aestivum) Actin1 (TaACT1),in planta. Following treatment of wheat, separately, with jasmonic acid, abscisic acid or with the avirulent race,CYR23, of the stripe rust pathogenPuccinia striiformisf. sp.tritici, we observed a rapid induction in accumulation ofTaADF4mRNA. Interestingly, accumulation ofTaADF4mRNAwas diminished in response to inoculation with a virulent race,CYR31. Silencing ofTaADF4resulted in enhanced susceptibility toCYR23, demonstrating a role forTaADF4in defense signaling. Using a pharmacological‐based approach, coupled with an analysis of host response to pathogen infection, we observed that treatment of plants with the actin‐modifying agent latrunculin B enhanced resistance toCYR23, including increased production of reactive oxygen species and enhancement of localized hypersensitive cell death. Taken together, these data support the hypothesis thatTaADF4 positively modulates plant immunity in wheat via the modulation of actin cytoskeletal organization.

    

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4. The Arabidopsis heterotrimeric G‐protein β subunit, AGB 1, is required for guard cell calcium sensing and calcium‐induced calcium release

   https://doi.org/10.1111/tpj.14318  

   Jeon, Byeong Wook ; Acharya, Biswa R. ; Assmann, Sarah M. ( April 2019 , The Plant Journal)

   Cytosolic calcium concentration ([Ca²⁺]_cyt) and heterotrimeric G‐proteins are universal eukaryotic signaling elements. In plant guard cells, extracellular calcium (Ca_o) is as strong a stimulus for stomatal closure as the phytohormone abscisic acid (ABA), but underlying mechanisms remain elusive. Here, we report that the sole Arabidopsis heterotrimeric Gβ subunit,AGB1, is required for four guard cell Ca_oresponses: induction of stomatal closure; inhibition of stomatal opening; [Ca²⁺]_cytoscillation; and inositol 1,4,5‐trisphosphate (InsP3) production. Stomata in wild‐type Arabidopsis (Col) and in mutants of the canonical Gα subunit,GPA1, showed inhibition of stomatal opening and promotion of stomatal closure by Ca_o. By contrast, stomatal movements ofagb1mutants andagb1/gpa1double‐mutants, as well as those of theagg1agg2 Gγ double‐mutant, were insensitive to Ca_o. These behaviors contrast withABA‐regulated stomatal movements, which involveGPA1 andAGB1/AGG3 dimers, illustrating differential partitioning of G‐protein subunits among stimuli with similar ultimate impacts, which may facilitate stimulus‐specific encoding.AGB1knockouts retained reactive oxygen species andNOproduction, but lostYC3.6‐detected [Ca²⁺]_cytoscillations in response to Ca_o, initiating only a single [Ca²⁺]_cytspike. Experimentally imposed [Ca²⁺]_cytoscillations restored stomatal closure inagb1. Yeast two‐hybrid and bimolecular complementation fluorescence experiments revealed thatAGB1 interacts with phospholipase Cs (PLCs), and Ca_oinduced InsP3 production in Col but not inagb1. In sum, G‐protein signaling viaAGB1/AGG1/AGG2 is essential for Ca_o‐regulation of stomatal apertures, and stomatal movements in response to Ca_oapparently require Ca²⁺‐induced Ca²⁺release that is likely dependent on Gβγ interaction withPLCs leading to InsP3 production.

    

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5. Mitogen‐activated protein kinases MPK 4 and MPK 12 are key components mediating CO 2 ‐induced stomatal movements

   https://doi.org/10.1111/tpj.14087  

   Tõldsepp, Kadri ; Zhang, Jingbo ; Takahashi, Yohei ; Sindarovska, Yana ; Hõrak, Hanna ; Ceciliato, Paulo H. O. ; Koolmeister, Kaspar ; Wang, Yuh‐Shuh ; Vaahtera, Lauri ; Jakobson, Liina ; et al ( October 2018 , The Plant Journal)

   Respiration in leaves and the continued elevation in the atmosphericCO₂concentration causeCO₂‐mediated reduction in stomatal pore apertures. Several mutants have been isolated for which stomatal responses to both abscisic acid (ABA) andCO₂are simultaneously defective. However, there are only few mutations that impair the stomatal response to elevatedCO₂, but not toABA. Such mutants are invaluable in unraveling the molecular mechanisms of earlyCO₂signal transduction in guard cells. Recently, mutations in the mitogen‐activated protein (MAP) kinase,MPK12, have been shown to partially impairCO₂‐induced stomatal closure. Here, we show thatmpk12plants, in whichMPK4is stably silenced specifically in guard cells (mpk12 mpk4GChomozygous double‐mutants), completely lackCO₂‐induced stomatal responses and have impaired activation of guard cell S‐type anion channels in response to elevatedCO₂/bicarbonate. However,ABA‐induced stomatal closure, S‐type anion channel activation andABA‐induced marker gene expression remain intact in thempk12 mpk4GCdouble‐mutants. These findings suggest thatMPK12 andMPK4 act very early inCO₂signaling, upstream of, or parallel to the convergence ofCO₂andABAsignal transduction. The activities ofMPK4 andMPK12 protein kinases were not directly modulated byCO₂/bicarbonatein vitro, suggesting that they are not directCO₂/bicarbonate sensors. Further data indicate thatMPK4 andMPK12 have distinguishable roles in Arabidopsis and that the previously suggested role ofRHC1 in stomatalCO₂signaling is minor, whereasMPK4 andMPK12 act as key components of early stomatalCO₂signal transduction.

    

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