Several types of K+ permeable channels are known to be presentin the tonoplast (Fig. 2). The most abundant are slow-activating(SV) and fast-activating (FV) vacuolar channels. The SV channelis permeable to both mono- and divalent cations and is activatedby cytosolic Ca2+ and positive vacuolar voltage. The FV channelis selective for monovalent cations only, activated by positivevoltages, and may be blocked by divalent cations (for a review,see Allen and Sanders, 1997). Both SV and FV channels are ubiquitousin plant tissues, including mesophyll and guard cell vacuoles(Allen and Sanders, 1997; Pottosin et al., 1997; Tikhonova etal., 1997). In addition, guard cells possess another channelspecifically selective for K+ (Allen and Sanders, 1997). Thisso-called VK channel is voltage-independent and activated byCa2+ (Schönknecht et al., 2002) as well as by cytosolicalkalization (Allen and Sanders, 1997). Recently, a two-poreA. thaliana KOR channel, named AtKCO1, was cloned and localizedto the tonoplast of both mesophyll cells and guard cells (Czempinskiet al., 2002; Schönknecht et al., 2002). Finally, thereis evidence for mechanosensory SAS channels to be present atthe tonoplast (Alexandre and Lassalles, 1991).
At least two other types of transporters may also contributeto K+ transport across the tonoplast. Firstly, Banuelos et al.(2002) targeted the rice OsHAK10 (a member of KUP/HAK/KT family)gene to the tonoplast and suggested that such a transportermay be needed to release K+ from the vacuole to the cytoplasmwhen the vacuolar concentration is low. Secondly, there is evidencethat NHX1 (a vacuolar Na+/H+ exchanger) has some affinity forK+ and may operate in K+ transport during low Na+ conditions[Blumwald et al. (2000) and references within].
The transport barrier in the chloroplast is the inner membrane,which contains transporters for a selected numbers of low molecularweight substrates. The outer membrane contains specific pore-formingproteins and is permeable to substances with molecular weightof several kDa (Pottosin, 1992). Most of these ‘pores’are also able to conduct ions (for a review, see Neuhaus and Wagner, 2000).
Massive light-driven transport of H+ into the thylakoid lumenis electrically balanced by the counter flow of other ions (Hinnah and Wagner, 1998).This process is mediated by weakly voltage-dependentcation-selective channels, equally permeable to K+ and Mg2+(Pottosin and Schönknecht, 1996). Several types of cation-permeablechannels have been found at thylakoid membranes of differentspecies (Pottosin, 1992; Pottosin and Schonknecht, 1996; Hinnah and Wagner, 1998).All of them belong to the NSCC class. Channelconductance varied greatly from 60 pS (Pottosin and Schonknecht 1996)to very high values (non-selective porin-like maxi channelwith 1016 pS conductance; Pottosin, 1992). Most of thesechannels show bimodal gating (Pottosin, 1992). However, somechannels showed only moderate voltage dependence (Pottosin and Schönknecht, 1996),suggesting that additional mechanismsto regulate the thylakoid cation channel activity might be involved.
channels (mitoKATP) have been reported for
various animal tissues (Ferranti et al., 2003
). For plant mitochondria,
studies on K+
transporters are still at an early stage. It appears
that the only reported evidence comes from Petrussa et al. (2001
who provided evidence that plant mitochondria possess a K+
voltage-dependent channel, which is opened by cyclosporin, regulated
by the redox state and inhibited by nucleotides. The hypothetical
role of this new ATP-sensitive K+
channel was attributed to
mitochondrial volume regulation, thermogenesis, apoptosis and/or
prevention of ROS formation in plants. As studies on ROS have
recently received a great deal of attention from plant scientists
(Demidchik et al., 2003
; Kohler et al., 2003
), there is obviously
a strong need for more studies on the role and mechanisms of
transport across the mitochondrial membranes.