A Tandem Amino Acid Residue Motif in Guard Cell SLAC1 Anion Channel of Grasses Allows for the Control of Stomatal Aperture by Nitrate

The latest major group of plants to evolve were the grasses. These became important in the mid-Paleogene about 40 million years ago. During evolution, leaf CO₂ uptake and transpirational water loss were optimized by the acquisition of grass-specific stomatal complexes. In contrast to the kidney-shaped guard cells (GCs) typical of the dicots such as Arabidopsis, in the grasses and agronomically important cereals, the GCs are dumbbell shaped and are associated with morphologically distinct subsidiary cells (SCs). We studied the molecular basis of GC action in the major cereal crop barley. Upon feeding ABA to xylem sap of an intact barley leaf, stomata closed in a nitrate-dependent manner. This process was initiated by activation of GC SLAC-type anion channel currents. HvSLAC1 expressed in Xenopus oocytes gave rise to S-type anion currents that increased several-fold upon stimulation with >3 mM nitrate. We identified a tandem amino acid residue motif that within the SLAC1 channels differs fundamentally between monocots and dicots. When the motif of nitrate-insensitive dicot Arabidopsis SLAC1 was replaced by the monocot signature, AtSLAC1 converted into a grass-type like nitrate-sensitive channel. Our work reveals a fundamental difference between monocot and dicot GCs and prompts questions into the selective pressures during evolution that resulted in fundamental changes in the regulation of SLAC1 function.

The Venus Flytrap Dionaea muscipula Counts Prey-Induced Action Potentials to Induce Sodium Uptake

Carnivorous plants, such as the Venus flytrap (Dionaea muscipula), depend on an animal diet when grown in nutrient-poor soils. When an insect visits the trap and tilts the mechanosensors on the inner surface, action potentials (APs) are fired. After a moving object elicits two APs, the trap snaps shut, encaging the victim. Panicking preys repeatedly touch the trigger hairs over the subsequent hours, leading to a hermetically closed trap, which via the gland-based endocrine system is flooded by a prey-decomposing acidic enzyme cocktail. Here, we asked the question as to how many times trigger hairs have to be stimulated (e.g., now many APs are required) for the flytrap to recognize an encaged object as potential food, thus making it worthwhile activating the glands. By applying a series of trigger-hair stimulations, we found that the touch hormone jasmonic acid (JA) signaling pathway is activated after the second stimulus, while more than three APs are required to trigger an expression of genes encoding prey-degrading hydrolases, and that this expression is proportional to the number of mechanical stimulations. A decomposing animal contains a sodium load, and we have found that these sodium ions enter the capture organ via glands. We identified a flytrap sodium channel DmHKT1 as responsible for this sodium acquisition, with the number of transcripts expressed being dependent on the number of mechano-electric stimulations. Hence, the number of APs a victim triggers while trying to break out of the trap identifies the moving prey as a struggling Na⁺-rich animal and nutrition for the plant.

Video Abstract

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Carnivorous Dionaea muscipula captures and processes nutrient- and sodium-rich prey

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Via mechano-sensor stimulation, an animal meal is recognized, captured, and processed

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Mechano-electrical waves induce JA signaling pathways that trigger prey digestion

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Number of stimulations controls the production of digesting enzymes and uptake modules

Stomatal guard cells co-opted an ancient ABA-dependent desiccation survival system to regulate stomatal closure

During the transition from water to land, plants had to cope with the loss of water through transpiration, the inevitable result of photosynthetic CO2 fixation on land [1, 2]. Control of transpiration became possible through the development of a new cell type: guard cells, which form stomata. In vascular plants, stomatal regulation is mediated by the stress hormone ABA, which triggers the opening of the SnR kinase OST1-activated anion channel SLAC1 [3, 4]. To understand the evolution of this regulatory circuit, we cloned both ABA-signaling elements, SLAC1 and OST1, from a charophyte alga, a liverwort, and a moss, and functionally analyzed the channel-kinase interactions. We were able to show that the emergence of stomata in the last common ancestor of mosses and vascular plants coincided with the origin of SLAC1-type channels capable of using the ancient ABA drought signaling kinase OST1 for regulation of stomatal closure.

Furosemide inhibits 11 beta-hydroxysteroid dehydrogenase in vitro and in vivo

11 beta-hydroxysteroid dehydrogenase (11 beta-OHSD) protects the non-selective renal mineralocorticoid receptor from the endogeneous glucocorticoid cortisol. Thus, drugs inhibiting 11 beta-OHSD might enhance urinary loss of potassium. In an attempt to find drugs inhibiting 11 beta-OHSD, 23 commonly used agents known to interfere with the potassium metabolism have been screened for inhibitory effect on 11 beta-OHSD. Furosemide appeared as the only inhibitor. Its inhibition constant (Ki) was 19.5 microM when kidney and 21.3 microM when liver microsomes were used as a source of 11 beta-OHSD. The type of inhibition was competitive. For confirmation that furosemide specifically inhibits 11 beta-OHSD, the complementary DNA (cDNA) of 11 beta-OHSD was transfected into COS-1 cells devoid of spontaneous expression of 11 beta-OHSD. In these cells, oxidation of corticosterone (Ki = 17.4 microM) and reduction of dehydrocorticosterone (Ki = 12.5 microM) was inhibited by furosemide. To establish whether this inhibition also occurs in vivo, the 11 beta-hydroxysteroid prednisolone was administered with and without furosemide to rats. The concentration ratio of prednisolone to its 11-ketometabolite prednisone increased in kidney and liver tissue after furosemide administration, indicating inhibition of 11 beta-OHSD. These data suggest that furosemide modulates in vivo the access of 11 beta-OH glucocorticoids to their target organs.