A plant cation-chloride co-transporter promoting auxin-independent tobacco protoplast division

Although auxin plays a central role in plant development, little is known about the signal transduction pathways triggered by auxin regulating cell elongation, division and differentiation. We describe the molecular analysis of the mutant tobacco line axi 4/1, which was regenerated from an auxin-independent callus created by activation T-DNA tagging. Transcriptional enhancer-mediated deregulated expression of the tagged plant gene axi 4 uncouples division of axi 4/1 protoplasts from external auxin stimuli, whereas in untransformed protoplasts expression of axi 4 and cell division are auxin dependent. axi 4 encodes a 109 kDa protein with significant homology to a family of electroneutral cation-chloride co-transporters (CCCs). We show that overexpression of AXI 4 or another member of the CCC family, the Na+/K+/2Cl--co-transporter from shark, triggers auxin-independent growth of tobacco protoplasts. We suggest that Na+/K+/2Cl--co-transporters may play a role in signalling cell division and that this function is highly conserved between AXI 4 and the shark Na+/K+/2Cl--co-transporter. We also demonstrate that a C-terminal fragment of AXI 4 is sufficient to promote auxin-independent cell division, showing that the C-termini of CCCs are functional subunits triggering cell division. This may allow a molecular dissection of this process not only in plants but also in animal cells.