Plant ion channels that select poorly between cations have been investigated over the last decade because they may act as pathways for sodium entry into plant cells under salinity stress. In this issue and in the December issue, five papers address aspects of nonselective cation channels that includes their roles in salinity stress, biophysical properties in heterologous expression systems, and properties and potential roles in biotrophic interfaces. The channels investigated have been referred to as nonselective cation channels (NSCCs) or voltage-independent channels (VICs). The latter reflects the relatively low dependence of gating on membrane voltage in these channels. However, the truly distinguishing feature is their lack of selectivity; hence, I prefer the term NSCC. Despite the descriptor VIC being applied in some reports, the channels can still display a 2- to 3-fold change in open probability with voltage. However, the VIC descriptor does emphasize that other factors are likely to control the activity of these channels such as cyclic nucleotides (Maathuis and Sanders, 2001) or divalent cations (Roberts and Tyerman, pp. 370-378).

NSCCs have been identified from patch-clamp studies on cereals (for review, see Amtmann and Sanders, 1999; Tyerman and Skerrett, 1999). The similarity between the sensitivities to external Ca²⁺ for Na⁺ influx into roots and Na⁺ currents, either through single channels or across the plasma membrane, suggested that a major component of Na⁺ influx could be accounted for by NSCCs (Davenport and Tester, 2000). However, until now we had very little information on the role played by NSCCs in Na⁺ uptake in the model plant Arabidopsis. In this issue, Demidchik and Tester (pp. 379-387) describe the NSCCs present in the protoplasts derived from Arabidopsis roots. The sodium currents across whole-cell membranes show the typical partial block by Ca²⁺ observed for the cereals. This partial block is also observed in single channel records. Some interesting pharmacological features are revealed in this study that have not been observed in previous studies. One in particular is the dramatic sensitivity to pH with external acid pH causing substantial inhibition. Inhibition that is also caused by diethyl pyrocarbonate (a His modifier) may suggest that the pH sensitive site is a His residue. This information will allow test of function in whole plants by examination of the pharmacological effects on Na⁺ uptake and salinity tolerance.

There may be more than one type of NSCC involved in Na⁺ influx in roots cells and some transporters may also contribute (e.g. HKT1). In a previous issue, Maathuis and Sanders (2001) show that approximately one-half of the Na⁺ currents observed in Arabidopisis root cells could be inhibited directly by cyclic nucleotides. There were also differences in the responses to different cyclic nucleotides, suggesting that there may be more than one type of cyclic nucleotide-dependent NSCC. The inhibition of Na⁺ uptake and improved salt tolerance evoked by cyclic nucleotides in intact Arabidopsis plants adds weight to these channels being involved in salinity stress. It is not known if these channels are the same as those that are also sensitive to external Ca²⁺.

Another approach to establishing the function of the NSCCs in salinity stress will be to eliminate various candidates such as HKT1 using knockout mutations. There are, however, 20 putative cyclic nucleotide-gated channels in the Arabidopsis genome and 20 Glu receptor family genes that may form nonselective ion channels (Lacombe et al., 2001). In this issue, two members in the cyclic nucleotide gated family from Arabidopsis and one from tobacco (Nicotiana tabacum) have been investigated using heterologous expression systems by Berkowitz's group. The heterologous expression of these channels is very difficult because of the toxic effects on the expressing cells. In this issue, Leng et al. (pp. 400-410) demonstrate that AtCNGC2, although nonselective between several univalent cations, is actually very selective for K⁺ over Na⁺ that is similar in degree to the highly selective KAT1 channel. Leng et al. point out that AtCNGC2 lacks the "GYG" motif considered to be important in conferring the high selectivity between K⁺ and Na⁺ in most K⁺-selective channels. The channel is also permeable to Ca²⁺ and is blocked by it in a voltage-dependent manor. The AtCNGC2 and AtCNGC1 channel are probably not among the channels characterized by Maathuis and Sanders (2001) because AtCNGC2 and AtCNGC1 are activated by cyclic nucleotides and AtCNGC2 has different selectivity to the channels studied by Maathuis and Sanders. AtCNGC2 may play a role in programmed cell death (Clough et al., 2000; Köhler et al., 2001).

Root cells may seem to be an odd place for NSCCs to be situated given that, as the first line of defense, cation uptake by roots would be expected to be selective. However, roots do aquire cations nonselectively when they are cation deficient (Rains, 1972) and perhaps osmotic stress also has this effect. In other types of cells and membranes where selectivity is provided by "upstream" transporters, the function of NSCCs may be as a general nutrient release mechanism. This may occur in root xylem parenchyma cells where NSCCs have been characterized (Wegner and DeBoer, 1997). In developing seeds of bean (Phaseolus vulgaris), the cells lining the maternal seed coat release all the nutrients that are required by the developing seed into a common apoplasm surrounding the seed. The nutrients arrive in the seed coat cells from the phloem via the symplasm; thus, some selectivity has already been exerted. In the plasma membrane of these seed coat cells, there are two types of NSCCs (Zhang et al., pp. 388-399) that show different but low selectivity between univalent cations and Ca²⁺ and also between univalent cations and anions. The low selectivity for a wide range of cations and anions, including large organic ions, suggests that the channels may mediate a component of the efflux of phloem-imported ions (see also van Dongen et al., 2001). Another biotrophic interface of importance is the membrane that surrounds nitrogen-fixing bacteria within the cytoplasm of cells in legume nodules. This membrane (symbiosome membrane) displays similarities to plasma membranes. Roberts and Tyerman (pp. 370-378) describe a unique NSCC of very low conductance (not resolved by standard patch clamp) from the model legume Lotus japonicus that is permeable to NH₄⁺. The high density of this channel and the relatively high flux that results makes it a strong candidate for the pathway of nitrogen efflux to the cytoplasm for assimilation. Like all the NSCCs described in this issue, the symbiosome channel is also sensitive to Ca²⁺, but it also shows a finite permeability to Ca²⁺ that sets it apart from its relative in soybean (Glycine max). Unique among plant ion channels is the way in which Ca²⁺ and Mg²⁺ can change the direction of ion flux through the symbiosome channel. The divalent ions block from the side of the membrane that the univalent cations enter the channel under the prevailing electrical and voltage gradient. The cytoplasm contains about 2 mM Mg²⁺, which will block univalent cation flow into the symbiosome, but will allow exit from the symbiosome of the predominant cation NH₄⁺ produced by nitrogen fixation. The activity of the proton pump balances charge across the membrane and provides protons to form NH₄⁺ from NH₃.

This year, a review article from Mark Tester's group on NSCCs will appear in the Annual Review of Plant Physiology and Plant Molecular Biology that promises to reveal many other insights on the properties and functions of NSCCs. Several labs also promise some interesting results for cyclic nucleotide-gated channels. The plant Glu receptor family of putative ion channels also is being investigated in several labs and Christine Cheffings reported ion currents induced by expression of a plant Glu receptor channel in Xenopus sp. oocytes at the recent 12th International Workshop on Plant Membrane Biology. Research on NSCCs is revealing new biophysical aspects of ion channels and diverse functions in salinity stress, signaling, and nutrient relations.