Submitted to: Plant Cell and Environment
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/7/2008
Publication Date: 4/22/2008
Citation: Jones, D.L., Blancaflor, E., Kochian, L.V., Gilroy, S. 2008. Spatial Coordination of Aluminum Uptake, Production of Reactive Oxygen Species, Callose Production and Wall Rigidification in Maize Roots. Plant Cell and Environment. 29:1309-1318. Interpretive Summary: Large areas of land within the U.S. and over 40% of the world’s lands are acidic, where aluminum (Al) toxicity is the primary factor limiting crop production via inhibition of root growth. The physiological and molecular basis for Al tolerance is still poorly understood. Thus, we need a more complete understanding of the mechanisms and underlying Al tolerance if we are going to be able to develop more Al tolerant crop plants for improved cultivation on acid soils. One strategy we are employing to better understand Al tolerance mechanisms is to enhance our knowledge about mechanisms of Al toxicity associated with the primary Al toxicity symptom, a rapid inhibition of root growth. In this study, we investigated the time course for entry of Al ions into the corn root apex (the site of toxicity and tolerance) and the development of a well documented root Al stress response involving the production of the cell wall polymer, callose. It was found that the entry of Al into the cells located at the surface of the root apex was closely correlated with the onset of Al stress in those same cells. Also, it was found that at the same time, toxic reactive oxygen species were generated by Al in these same cells, and this was correlated with increased rigidification of the periphery of the root apex. These findings suggest that the primary symptom of Al toxicity, inhibition of root elongation, occurs at least in part to a rigidification of the cell wall in epidermal layers of the root tip.
Technical Abstract: Aluminum toxicity associated with acid soils represents one of the biggest limitations to crop production worldwide. Although Al specifically inhibits the elongation of root cells, the exact mechanism by which this growth reduction occurs remains controversial. The aim of this study was to investigate the spatial and temporal dynamics of Al migration into roots of maize (Zea mays L.) and the production of the stress response compound callose. Using the Al-specific fluorescent probe morin, we demonstrate the gradual penetration of Al into roots. Al readily accumulates in the root’s epidermal and outer cortical cell layers but does not readily penetrate to the inner cortex. After prolonged exposure times (12 to 24 h), Al had entered all areas of the root apex. The spatial and temporal accumulation of Al within the root is matched by the production of the cell wall polymer callose which is also highly localized to the epidermis and outer cortical region. Exposure to Al induced the rapid production of reactive oxygen species and induced a significant rigidification of the cell wall. Our results suggest that Al-induced root inhibition in maize occurs by rigidification of the epidermal layers.