Research Interests:
My research focuses on the molecular and genetic aspects of aluminum tolerance in maize. In other crop species such as wheat and sorghum, aluminum tolerance is genetically determiined by a major locus that controls the release of organic acids from the root (malate and citrate in the case of wheat and sorghum, respectively). On the other hand Al tolerance in maize, although strongly associated with hight rates of root citrate release, appears to be a rather complex phenomenon involving multiple genes and probably multiple physiological mechanisms. For example, Piñeros et al. (2005) observed a clear correlation between root tip Al exclusion and Al tolerance across a panel of six maize genotypes. However, Al-activated root citrate release was not as well correlated with Al tolerance, suggesting that although Al-activated root citrate release plays an important role, it is likely that other tolerance mechanisms are also operating in maize roots. These physiological observations are supported by several genetic studies that have described maize Al tolerance as a quantitative trait, subject to additive gene effects (Borrero et al., 1995; Pandey et al., 1994; Magnavaca et al., 1987). A recent study on QTL mapping identified five distinct genomic regions with importance to Al tolerance in maize (Ninamango-Cárdenas et al., 2003).
Temporal changes in gene expression are major determinants of normal metabolic and physiological processes, and are also the primary mediators of altered cellular properties that define various stress- and disease-related states (Jiang et al., 2000). The identification of genes and determination of their expression patterns in response to stress should improve our understanding of their functions and provide the basis for effective strategies to improve stress adaptation. In particular, a number of genes have been shown to be differentially regulated by Al stress in different plant species (Mao et al., 2004; Hamel et al., 1998; Richards et al., 1998; Ezaki et al., 1995). However, genes identified as Al-responsive so far have turned out to be mostly related to a general stress response resulting from the toxic effects of Al, and are unlikely to play a significant role in Al tolerance (Kochian et al., 2004). In a recently published study (Maron et. al 2008, see publication list below) , we presented the first comprehensive survey of global transcriptional regulation in maize roots under Al stress. The use of short Al exposure periods in a time course study allowed us to observe the effects of Al on gene expression prior to the onset of severe toxicity symptoms, as indicated by the increase in the number of differentially regulated genes over time in roots of the Al-sensitive genotype. In addition, using a comparative approach we were able to identify a number of genes displaying different patterns of expression in response to Al between an Al-tolerant and an Al-sensitive maize genotype.
Even though Al-activated citrate release is an important mechanism of Al tolerance in maize, other mechanisms are likely to be operating in this species and have yet to be characterized. The recent cloning of the first Al tolerance genes is starting to shed light into the regulation of the Al tolerance response in plants. In the case of organic acid transporters, it is their expression level that plays a key role in differential Al tolerance. It is therefore plausible to assume that differential expression might also play a role in other unknown mechanisms of Al tolerance. Consequently, the results of this study are likely to become a valuable resource to help further our understanding on the mechanisms of Al toxicity and tolerance in maize, and on how these mechanisms are regulated at a transcriptional level.
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