Ca^2 CHANNELS Depolarization-activated, Ca^2 -selective channels were described [44]. Ca^2
channels have been implicated in signaling evoked by various environmental stimuli, including mechan-
ical stimuli [18] (see reviews in Refs. 45 and 46).
NONSELECTIVE CATION CHANNELS A depolarization-activated nonselective cation channel
that conducts Ca^2 was demonstrated in guard cells [47], in seed endosperm cells [48], and in roots of ce-
reals [49]. These channels are probably the major pathways for entry of Naand Ca^2 into cells. When
the external Naconcentration is high (saline conditions) and the electrochemical potential gradient of
Nais inward, depolarization-activated cation channels may conduct considerable fluxes of Nainto the
cytosol [49]. A tobacco plant overexpressing a putative nonselective cation channel became sensitive to
Pb^2 [50].
ANION CHANNELS Two types of Clchannels have been described in the guard cell plasma mem-
brane: rapidly gated anion channels (R type) and slowly gated ones (S type) [51]. It has not been resolved
whether they are indeed separate molecular entities or two interconverting facets of the same channel
[52]. They are permeable to various additional anions, including nitrate, malate, and sulfate [53–55]. The
gating of the S-type anion channel of guard cells is further regulated by these anions [56], as well as by
the plant hormones auxin and abscisic acid [57,58] (see also review in Ref. 59).
In addition to their obvious function in nutrient uptake, anion channels are implicated in signaling in
photomorphogenesis, in osmoregulation of stomatal guard cells, and in interactions with pathogens and
symbionts [57–62]. A gene homologous to the animal Clchannel has been cloned from Arabidopsis
[63,64], but its identification with a native (in situ) anion channel remains uncertain [37].
- Tonoplast Channels
Vacuoles make up over 90% of the cell volume in most plant cells and serve as a major storing compart-
ment for minerals. Nevertheless, our knowledge of the tonoplast channels is much less complete. A total
of two channels, both Ca^2 activated, have been identified that are capable of conducting K; these are
the voltage-independent vacuolar channels specific for K(VK) and the slow vacuolar (SV) channels
[65]. The SV channels are slowly activated by depolarization (of the cytosolic side relative to the vacuo-
lar side) and are permeable to Kand Ca^2 . They may be involved in the mechanism of “Ca^2 -activated
Ca^2 -release” [66]. The fast-activating (FV) channels are cation-conducting channels and dominate the
ion conductance of the vacuolar membrane at physiological Ca^2 concentrations. They may serve for the
uptake of NH 4 and Kinto the vacuole [67] (see also reviews in Refs. 68–70).
Two distinct types of vacuolar calcium channels facilitating Ca^2 release from the vacuole to the cy-
tosol are activated by two types of second messengers: 1P 3 [17] and cADP-ribose [71]. Both have been
implicated in various abscissic acid responses [72–74] (see also reviews in 75–78). 1P 3 is produced by the
phosphatidylinositol cascade [79]. cADP-ribose is produced from NADby ADP-ribosyl-cyclase (see
references in Ref. 77).
Much less is known about anion channels. Malate and chloride channels have been described in the
vacuole [80,81].
B. Carriers
- Transport Kinetics
Carriers catalyze the transport of specific solutes across the membrane. This transport is often vectorial
(unidirectional). The relation between solute concentration and unidirectional flux in plant tissues was de-
scribed by Epstein and Hagen [82]. They used terms introduced by Michaelis and Menten [83] for en-
zyme kinetics (the relation between the chemical activity of the substrate and the velocity of enzyme-cat-
alyzed reactions). Carrier-catalyzed transport (or enzyme-catalyzed reaction) is modeled as follows:
k 1 k 3
[E][S^0 ]↔[ES]↔[E][Si] (12)
k 2where [S], [E], and [ES] are the concentrations of the transported solute (or substrate), the carrier (or en-
zyme), and the carrier-solute (or enzyme-substrate) complex, respectively, and k 1 ,k 2 , and k 3 are the rate
344 JACOBY AND MORAN