Under drought stress, abscisic acid (ABA) triggers closure of leaf cell pores called stomata, which are formed by two specialized cells called guard cells in plant epidermis. Two pathways downstream of ABA stimulate phosphorylation of the S-type anion channels SLAC1 (slow anion channel associated 1) and SLAH3 (SLAC1 homolog 3), which causes these channels to open, reducing guard cell volume and triggering stomatal closure. One branch involves OST1 (open stomata 1), a calcium-independent SnRK2-type kinase, and the other branch involves calcium-dependent protein kinases of the CPK (calcium-dependent protein kinase) family. We used coexpression analyses in Xenopus oocytes to show that the calcineurin B-like (CBL) calcium sensors CBL1 and CBL9 and their interacting protein kinase CIPK23 also triggered SLAC1 and SLAH3 opening. We analyzed whether regulation of SLAC1 opening by these different families of kinases involved the same or different sites on SLAC1 by measuring channel conductance of SLAC1 with mutations in the putative phosphorylation sites in the amino or carboxyl termini coexpressed with specific kinases in Xenopus oocytes. SLAC1 mutants lacking the OST1-phosphorylated site were still activated by CPK or by CBL/CIPK complexes. Phosphorylation and activation of SLAC1 by any of the kinases were inhibited by the phosphatase ABI1 (ABA insensitive 1), which is inactivated in response to ABA signaling. These findings identified CBL/CIPK complexes as potential regulators of stomatal aperture through S-type anion channels and indicated that phosphorylation at distinct sites enables SLAC1 activation by both calcium-dependent and calcium-independent pathways downstream of ABA.

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.

Sexual reproduction in animals and plants shares common elements, including sperm and egg production, but unlike animals, little is known about the regulatory pathways that determine the sex of plants. Here we use mutants and gene silencing in a fern species to identify a core regulatory mechanism in plant sexual differentiation. A key player in fern sex differentiation is the phytohormone abscisic acid (ABA), which regulates the sex ratio of male to hermaphrodite tissues during the reproductive cycle. Our analysis shows that in the fern Ceratopteris richardii, a gene homologous to core ABA transduction genes in flowering plants [SNF1-related kinase2s (SnRK2s)] is primarily responsible for the hormonal control of sex determination. Furthermore, we provide evidence that this ABA-SnRK2 signaling pathway has transitioned from determining the sex of ferns to controlling seed dormancy in the earliest seed plants before being co-opted to control transpiration and CO 2 exchange in derived seed plants. By tracing the evolutionary history of this ABA signaling pathway from plant reproduction through to its role in the global regulation of plant-atmosphere gas exchange during the last 450 million years, we highlight the extraordinary effect of the ABA-SnRK2 signaling pathway in plant evolution and vegetation function.OST1 | fern | stomata | evolution | sex determination

In contrast to animal cells, plants use nitrate as a major source of nitrogen. Following the uptake of nitrate, this major macronutrient is fed into the vasculature for long-distance transport. The Arabidopsis thaliana shoot expresses the anion channel SLOW ANION CHANNEL1 (SLAC1) and its homolog SLAC1 HOMOLOGOUS3 (SLAH3), which prefer nitrate as substrate but cannot exclude chloride ions. By contrast, we identified SLAH2 as a nitrate-specific channel that is impermeable for chloride. To understand the molecular basis for nitrate selection in the SLAH2 channel, SLAC1 and SLAH2 were modeled to the structure of HiTehA, a distantly related bacterial member. Structure-guided site-directed mutations converted SLAC1 into a SLAH2-like nitrate-specific anion channel and vice versa. Our findings indicate that two pore-occluding phenylalanines constrict the pore. The selectivity filter of SLAC/SLAH anion channels is determined by the polarity of pore-lining residues located on alpha helix 3. Changing the polar character of a single amino acid side chain (Ser-228) to a nonpolar residue turned the nitrate-selective SLAH2 into a chloride/nitrate-permeable anion channel. Thus, the molecular basis of the anion specificity of SLAC/SLAH anion channels seems to be determined by the presence and constellation of polar side chains that act in concert with the two pore-occluding phenylalanines.

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 CO 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.