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In most plants [ABSTRACT FROM AUTHOR]

The grape berry provides a model for investigating the physiology of non-climacteric fruits. Increased K+ accumulation in the berry has a strong negative impact on fruit acidity (and quality). In maturing berries, we identified a K+ channel from the Shaker family, Vv K1.2, and two CBL-interacting protein kinase ( CIPK)/calcineurin B-like calcium sensor ( CBL) pairs, Vv CIPK04- Vv CBL01 and Vv CIPK03- Vv CBL02, that may control the activity of this channel. Vv CBL01 and Vv CIPK04 are homologues of Arabidopsis AtCBL1 and AtCIPK23, respectively, which form a complex that controls the activity of the Shaker K+ channel AKT1 in Arabidopsis roots. Vv K1.2 remained electrically silent when expressed alone in Xenopus oocytes, but gave rise to K+ currents when co-expressed with the pairs Vv CIPK03-Vv CBL02 or Vv CIPK04- Vv CBL01, the second pair inducing much larger currents than the first one. Other tested CIPK- CBL pairs expressed in maturing berries were found to be unable to activate Vv K1.2. When activated by its CIPK- CBL partners, Vv K1.2 acts as a voltage-gated inwardly rectifying K+ channel that is activated at voltages more negative than -100 mV and is stimulated upon external acidification. This channel is specifically expressed in the berry, where it displays a very strong induction at veraison (the inception of ripening) in flesh cells, phloem tissues and perivascular cells surrounding vascular bundles. Its expression in these tissues is further greatly increased upon mild drought stress. Vv K1.2 is thus likely to mediate rapid K+ transport in the berry and to contribute to the extensive re-organization of the translocation pathways and transport mechanisms that occurs at veraison. [ABSTRACT FROM AUTHOR]

Summary A functional Shaker potassium channel requires assembly of four α-subunits encoded by a single gene or various genes from the Shaker family. In Arabidopsis thaliana, AtKC1, a Shaker α-subunit that is silent when expressed alone, has been shown to regulate the activity of AKT1 by forming heteromeric AtKC1-AKT1 channels. Here, we investigated whether AtKC1 is a general regulator of channel activity. Co-expression in Xenopus oocytes of a dominant negative (pore-mutated) AtKC1 subunit with the inward Shaker channel subunits KAT1, KAT2 or AKT2, or the outward subunits SKOR or GORK, revealed that the three inward subunits functionally interact with AtKC1 while the outward ones cannot. Localization experiments in plant protoplasts showed that KAT2 was able to re-locate AtKC1 fused to GFP from endomembranes to the plasma membrane, indicating that heteromeric AtKC1-KAT2 channels are efficiently targeted to the plasma membrane. Functional properties of heteromeric channels involving AtKC1 and KAT1, KAT2 or AKT2 were analysed by voltage clamp after co-expression of the respective subunits in Xenopus oocytes. AtKC1 behaved as a regulatory subunit within the heterotetrameric channel, reducing the macroscopic conductance and negatively shifting the channel activation potential. Expression studies showed that AtKC1 and its identified Shaker partners have overlapping expression patterns, supporting the hypothesis of a general regulation of inward channel activity by AtKC1 in planta. Lastly, AtKC1 disruption appeared to reduce plant biomass production, showing that AtKC1-mediated channel activity regulation is required for normal plant growth. [ABSTRACT FROM AUTHOR]

Ectomycorrhizal symbiosis markedly improves plant phosphate uptake, but the molecular mechanisms underlying this benefit are still poorly understood. We identified two ESTs in a cDNA library prepared from the ectomycorrhizal basidiomycete Hebeloma cylindrosporum with significant similarities to phosphate transporters from the endomycorrhizal fungus Glomus versiforme and from non-mycorrhizal fungi. The full-length cDNAs corresponding to these two ESTs complemented a yeast phosphate transport mutant (Δ pho84). Measurements of 33P-phosphate influx into yeast expressing either cDNA demonstrated that the encoded proteins, named HcPT1 and HcPT2, were able to mediate Pi:H+ symport with different affinities for Pi ( Km values of 55 and 4 μm, respectively). Real-time RT-PCR showed that Pi starvation increased the levels of HcPT1 transcripts in H. cylindrosporum hyphae grown in pure culture. Transcript levels of HcPT2 were less dependent on Pi availability. The two transporters were expressed in H. cylindrosporum associated with its natural host plant, Pinus pinaster, grown under low or high P conditions. The presence of ectomycorrhizae increased net Pi uptake rates into intact Pinus pinaster roots at low or high soil P levels. The expression patterns of HcPT1 and HcPT2 indicate that the two fungal phosphate transporters may be involved in uptake of phosphate from the soil solution under the two soil P availability conditions used. [ABSTRACT FROM AUTHOR]

Summary An appreciable number of potassium channels mediating K+ uptake have been identified in higher plants. Promoter-β-glucuronidase reporter gene studies were used here to demonstrate that SKT1, encoding a potato K+ inwardly rectifying channel, is expressed in guard cells in addition to KST1 previously reported. However, whereas KST1 was found to be expressed in essentially all mature guard cells, SKT1 expression was almost exclusively restricted to guard cells of the abaxial leaf epidermis. This suggests that different types of K+ channel subunits contribute to channel formation in potato guard cells and therefore differential regulation of stomatal movements in the two leaf surfaces. The overlapping expression pattern of SKT1 and KST1 in abaxial guard cells indicates that K+in channels of different sub-families contribute to ionic currents in this cell type, thus explaining the different properties of channels expressed solely in heterologous systems and those endogenous to guard cells. Interaction studies had previously suggested that plant K+ inward rectifiers form clusters via their conserved C-terminal domain, KT/HA. K+ channels co-expressed in one cell type may therefore form heteromers, which increase functional variability of K+ currents, a phenomenon well described for animal voltage-gated K+ channels. Co-expression of KST1 and SKT1 in Xenopus oocytes resulted in currents with an intermediate sensitivity towards Cs+, suggesting the presence of heteromers, and a sensitivity towards external Ca2+, which reflected the property of the endogenous K+in current in guard cells. Modulation of KST1 currents in oocytes by co-expressing KST1 with a SKT1 pore-mutant, which by itself was not able to confer activating K+ currents, demonstrated the possibility that KST1 and SKT1 co-assemble to hetero-oligomers. Furthermore, various C-terminal deletions of the mutated SKT1 channel restored KST1 currents, showing that the C-terminal KT motif is essential for heteromeric channel formation. [ABSTRACT FROM AUTHOR]