Inhibition of Elongation in Pellia Setae by the Monovalent Ionophore Monensin

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Abstract
Segments 5 mm long, cut from young setae of the liverwort Pellia, elongated when floated on an aqueous solution. Growth rates were constant for at least 20 h and in the presence of auxin approached those of intact setae. Elongation was inhibited by 60% by the monovalent ionophore monensin at concentrations of 0.5 .mu.M or greater. Monensin, however, did not inhibit preferentially elongation of the segments during the first 4 h of auxin-promoted growth. As a result, the percentages of inhibition were nearly constant at all concentrations of monensin tested over the range from 5 nM to 50 .mu.M either with or without auxin. Blockage of Golgi apparatus function occurred within 30-60 min of monensin treatment. Inhibition was observed as a failure to produce normal secretory vesicles. In their place, accumulations of vacuoles presumed to represent swollen cisternal elements and secretory vesicles were found. Within the first 4 h, the monensin effects were partially reversible but not thereafter. After 4 h the segments became increasingly less able to support auxin growth. Some stopped elongating and became flaccid, especially when treated with monensin plus auxin. The findings indicate that Golgi apparatus function was essential to support sustained elongation growth, either endogenous or auxin promoted. However, neither normal Golgi apparatus function in secretion nor delivery of Golgi apparatus vesicles to the cell surface appeared to be essential for short-term growth responses or for the mechanism whereby auxin growth was controlled and/or mediated.