Microscopic pores present in the epidermis of plant aerial organs, called stomata, allow gas exchanges between the inner photosynthetic tissue and the atmosphere. Regulation of stomatal aperture, preventing excess transpirational vapor loss, relies on turgor changes of two highly differentiated epidermal cells surrounding the pore, the guard cells. Increased guard cell turgor due to increased solute accumulation results in stomatal opening, whereas decreased guard cell turgor due to decreased solute accumulation results in stomatal closing. Here we provide direct evidence, based on reverse genetics approaches, that the Arabidopsis GORK Shaker gene encodes the major voltage-gated outwardly rectifying K+ channel of the guard cell membrane. Expression of GORK dominant negative mutant polypeptides in transgenic Arabidopsis was found to strongly reduce outwardly rectifying K+ channel activity in the guard cell membrane, and disruption of the GORK gene (T-DNA insertion knockout mutant) fully suppressed this activity. Bioassays on epidermal peels revealed that disruption of GORK activity resulted in impaired stomatal closure in response to darkness or the stress hormone azobenzenearsonate. Transpiration measurements on excised rosettes and intact plants (grown in hydroponic conditions or submitted to water stress) revealed that absence of GORK activity resulted in increased water consumption. The whole set of data indicates that GORK is likely to play a crucial role in adaptation to drought in fluctuating environments

Sexual reproduction in plants requires elongation of the pollen tube through the transmitting tissues toward the ovary. Tube growth rate is a major determinant of pollen competitive ability. We report that a K + channel of the Shaker family in Arabidopsis, SPIK, plays an important role in pollen tube development. SPIK was found to be specifically expressed in pollen. When SPIK was heterologously expressed in COS cells, its product formed hyperpolarization-activated K + channels. Disruption (T-DNA insertion) of the SPIK coding sequence strongly affected inwardly rectifying K + -channel activity in the pollen-grain plasma membrane. Measurements of membrane potential in growing pollen tubes yielded data compatible with a contribution of SPIK to K + influx. In vitro pollen germination assays were performed, revealing that the disruption results in impaired pollen tube growth. Analysis of the transmission rate of the disrupted allele in the progeny of heterozygous plants revealed a decrease in pollen competitive ability, the probability of fertilization by mutant pollen being ∼1.6 times lower than that by wild-type pollen. The whole set of data supports the hypothesis that functional expression of SPIK plays a role in K + uptake in the growing pollen tube, and thereby in tube development and pollen competitive ability.

The AKT2 K ؉ channel is endowed with unique functional properties, being the only weak inward rectifier characterized to date in Arabidopsis. The gene is expressed widely, mainly in the phloem but also at lower levels in leaf epiderm, mesophyll, and guard cells. The AKT2 mRNA level is upregulated by abscisic acid. By screening a two-hybrid cDNA library, we isolated a protein phosphatase 2C (AtPP2CA) involved in abscisic acid signaling as a putative partner of AKT2. We further confirmed the interaction by in vitro binding studies. The expression of AtPP2CA ( ␤ -glucuronidase reporter gene) displayed a pattern largely overlapping that of AKT2 and was upregulated by abscisic acid. Coexpression of AtPP2CA with AKT2 in COS cells and Xenopus laevis oocytes was found to induce both an inhibition of the AKT2 current and an increase of the channel inward rectification. Site-directed mutagenesis and pharmacological analysis revealed that this functional interaction involves AtPP2CA phosphatase activity. Regulation of AKT2 activity by AtPP2CA in planta could allow the control of K ؉ transport and membrane polarization during stress situations. INTRODUCTIONPotassium is the most abundant cation in the cytoplasm of the living cell, where it is involved in the regulation of ionic strength, osmotic potential, and membrane polarization. K ϩ channels of the so-called Shaker family (nine genes in Arabidopsis) have been shown to play a role in K ϩ uptake by the root periphery (AKT1: Lagarde et al., 1996; Hirsch et al., 1998), K ϩ secretion into the root xylem sap (SKOR: Gaymard et al., 1998), K ϩ transport in the phloem tissues (AKT2: Marten et al., 1999;Lacombe et al., 2000), or K ϩ inward (KAT1: Ichida et al., 1997;Szyroki et al., 2001; KAT2: Pilot et al., 2001) and outward (GORK: Ache et al., 2000) fluxes in guard cells, leading to stomatal opening/closing.To adapt to fluctuating K ϩ availability in the environment and to cope with other stresses, plants need to tightly regulate K ϩ transport at both the whole plant and the cell level (Kochian and Lucas, 1988;Schroeder et al., 1994). Studies aimed at revealing the molecular determinants of these regulations have highlighted mechanisms likely to target Shaker K ϩ channels at both the transcriptional and posttranslational levels. Expression studies have revealed that transcript levels of Shaker channels are sensitive to hormones (Gaymard et al., 1998;Philippar et al., 1999;Lacombe et al., 2000), sugar synthesis and accumulation, and environmental signals (Deeken et al., 2000). At the post-translational level, indications have been found for regulation by ATP and cyclic GMP (Hoshi, 1995), phosphorylation events (Li et al., 1994; Armstrong et al., 1995; Tang and Hoshi, 1999), functional interactions with the cytoskeleton (Hwang et al., 1997), 14-3-3 proteins (Saalbach et al., 1997 Booij et al., 1999), sulfonylurea receptors (Leonhardt et al., 1997), syntaxins (Leyman et al., 1999), and G proteins (Wu andWang et al., 2001).Searches for interacting proteins have been focused on t...

Understanding how mosquito vectors and malaria parasites interact is of fundamental interest, and it also offers novel perspectives for disease control. Both the genetic and environmental contexts are known to affect the ability of mosquitoes to support malaria development and transmission, i.e., vector competence. Although the role of environment has long been recognized, much work has focused on host and parasite genetic effects. However, the last few years have seen a surge of studies revealing a great diversity of ways in which non-genetic factors can interfere with mosquito-Plasmodium interactions. Here, we review the current evidence for such environmentally mediated effects, including ambient temperature, mosquito diet, microbial gut flora, and infection history, and we identify additional factors previously overlooked in mosquito-Plasmodium interactions. We also discuss epidemiological implications, and the evolutionary consequences for vector immunity and parasite transmission strategies. Finally, we propose directions for further research and argue that an improved knowledge of non-genetic influences on mosquito-Plasmodium interactions could aid in implementing conventional malaria control measures and contribute to the design of novel strategies.