Submitted to: Biomed Central (BMC) Plant Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/19/2008
Publication Date: 8/19/2008
Citation: Chandran, D., Sharopova, N., Vandenbosch, K.A., Garvin, D.F., Samac, D.A. 2008. Physiological and Molecular Characterization of Aluminum Resistance in Medicago truncatula. Biomed Central (BMC) Plant Biology. 8:89. Available: http://www.biomedcentral.com/1471-2229/8/89. Interpretive Summary: One of the most pressing agricultural problems around the world is soil acidification. When the pH of the soil decreases below 5.5, the aluminum in the soil becomes mobile and can be taken up by plant roots. Aluminum in this form is toxic to most crop plants and causes roots to become deformed and impaired in water and nutrient uptake. Some plant varieties are naturally tolerant of aluminum. In tolerant varieties of wheat and corn, the roots produce compounds that are secreted from roots that bind to the aluminum, preventing uptake. Plants in the bean family (legumes) such as alfalfa and soybean are more sensitive than grain crops to aluminum and although some tolerant plant varieties have been identified, the mechanism of tolerance is not well understood. We are using aluminum tolerant and sensitive lines of the barrel medic, a plant closely related to alfalfa, to understand aluminum tolerance mechanisms. In the tolerant line, root growth was initially inhibited by aluminum but recovered over time. In the susceptible line, root growth showed a steady decline over time. In both tolerant and susceptible lines, examination of the roots showed initial damage to the outer root cells. However, in the tolerant line, the damaged cells detached from the root and root growth continued. These results suggest that tolerance is stimulated by aluminum treatment and involves removal of damaged outer root cells and exclusion of aluminum from inner root cells. This type of tolerance has not been seen previously in legume plants. Expression of 16,000 genes was monitored in tolerant and sensitive plant lines. In both lines the aluminum treatment resulted in expression of genes involved in production of compounds that cause cell death. However, in the tolerant line the expression of these genes was short while expression in sensitive lines was extended. Expression of these genes is consistent with the observed removal of damaged outer cells in the tolerant line and extensive cell damage in the sensitive line. Additionally, we found that expression of a gene likely encoding a multidrug and toxin efflux transporter was highly expressed in the tolerant line and may be involved in detoxification and transport of aluminum inside the plant. Identification of novel aluminum tolerance mechanisms and genes conferring tolerance is useful for enhancing aluminum tolerance in crop plants. Aluminum tolerance is critical for continued production of food, feed and biofuels in agricultural soils that are becoming acidic and for expanding production into new areas with acidic soil.
Technical Abstract: Aluminum (Al) tolerance in the model legume Medicago truncatula Gaertn. was examined using the Al-tolerant line T32 and the Al-sensitive line S70. Hydroponic root growth and hematoxylin staining studies indicated that an inducible Al exclusion mechanism occurs in T32. Molecular events underlying the Al exclusion response were analyzed in T32 and S70 following 12 and 48 h Al treatment using oligonucleotide microarrays. Fewer genes were differentially expressed in T32 compared to S70. Induction of oxidative stress-related genes and expression profiles of these genes suggested that T32 accumulated lower levels of reactive oxygen species (ROS) in response to Al compared to S70. Based on microscopic examination and expression of stress-related genes, the extent of oxidative damage appeared to be higher in S70. Additionally, genes involved in cell death, senescence, and cell wall degradation were induced in both lines after 12 h Al treatment but preferentially in S70 after 48 h. These data suggest that a threshold of ROS sufficient to initiate cell death of Al-accumulating cells is reached in T32 thereby partly contributing to Al exclusion and recovery of root growth. However, significant ROS accumulation in S70 may result in necrosis and irreversible root growth inhibition. Additionally, differential expression of a putative multidrug and toxin efflux (MATE) gene in T32 may be associated with internal detoxification of Al. Our findings suggest that multiple responses likely contribute to Al tolerance in M. truncatula.