Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/1995
Publication Date: N/A
Citation: Interpretive Summary: There are wide regions of acid soils throughout the United States and the world; approximately 40% of the worlds lands are acidic. In these acid soils, aluminum (Al) toxicity is the major factor limiting growth and crop productivity. Thus, this is a serious agricultural problem throughout the world. In order to develop aluminum resistant crop varieties, we need to gain an understanding of the cellular mechanisms of aluminum toxicity, which are still poorly understood. In this study, we investigated the role of ion transport processes in controlling cellular growth in plants and Al interference with these transport systems that might result in inhibition of growth. It was found in growing root hairs that calcium (Ca2+) uptake into the root hair tip was correlated with growth, but did not control tip growth. This contradicts previous statements in the literature. Also, we found that of all of the ion transport systems studied, Al specifically blocked Ca2+ influx. However, this blockage was not responsible for the Al induced inhibition of root hair growth, because at low Al levels root hair growth was inhibited but Ca influx was unaffected. These results are significant, for they clarify a controversial aspect of Al toxicity that previously had implicated Ca2+ transport systems as key sites of Al toxicity. From this work, it is clear that Al blockage of root Ca2+ transport systems is not a critical aspect of Al toxicity.
Technical Abstract: The magnitude and spatial localization of Ca2+, K+ and H+ fluxes in growing and non-growing Limnobium stoloniferum root hairs was determined using non- invasive, ion selective vibrating microelectrodes. Both the spatial pattern and magnitude of the ionic flux was dependent on the particular ion in question. A distinct localization of H+ and Ca2+ influx was observed at the tips of growing root hairs indicating that these fluxes may be involve in directing growth. K+ influx showed no distinct localization and uptake appeared uniform along the length of the root hair. Competitive inhibition studies performed with Ca2+ and Mg2+ indicated that the magnitude of the Ca2+ influx into the root hair tip did not determine growth rate, however, the presence of Ca2+ on the external face of the membrane was implicit for root hair integrity. Al proved to be a potent inhibitor of root hair growth. At an exogenous Al concentration of 20 uM a complete blockage of root hair tip Ca2+ influx was observed, indicating that this may be one of the sites of Al toxicity. At a lower Al concentration (2 uM), Ca2+ fluxes were unaffected; however, inhibition of growth was still observed with a distinct swelling apparent at the tip of the root hairs. The swelling at the root hair tips was identical in appearance to that seen in the presence of microtubule inhibitors, suggesting that Al has multiple sites of toxicity. The role of Ca2+ and H+ fluxes in directing tip growth is discussed.