|Gantt, J. Stephen|
|Samac, Deborah - Debby|
Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/28/2008
Publication Date: 5/23/2008
Citation: Chandran, D., Sharopova, N., Ivashuta, S., Gantt, J., VandenBosch, K.A., Samac, D.A. 2008. Transcriptome profiling identified novel genes associated with aluminum toxicity, resistance and tolerance in Medicago truncatula. Planta. 228:151-166. Interpretive Summary: Aluminum is toxic to most crop plants causing stunting and deformation of roots, which leads to poor crop growth. Although aluminum is abundant in most soils, it becomes toxic when soil has an acidic pH (below pH 5). The effect of aluminum on plant roots has been well studied in cereal crops, but less is known about its effects on legume crops. We measured expression of more than 16,000 genes in the legume plant barrel medic, a close relative of alfalfa, after exposure to aluminum to determine which genes are affected by aluminum treatment. After 12 hours of aluminum treatment, we found that many genes involved in plant cell wall synthesis and modification were turned on. This is likely because aluminum binds to the cell wall and reduces the wall's ability to expand. Paradoxically, the action of some of the genes exposes additional cell wall components that bind aluminum, further increasing wall stiffening. Indeed, when one gene was prevented from being turned on, the plants showed a small increase in aluminum tolerance. Aluminum treatment also turned on expression of genes involved in reducing the toxic effects of oxygen free radicals. Production of these toxic compounds is likely due to the damaging effect of aluminum on plant cell membranes and other cellular components. Additionally, aluminum treatment turned on genes involved in plant cell death, presumably to remove cells damaged by aluminum. We identified several novel genes turned on by aluminum treatment that may be involved in transport or sequestration of metals. We showed that although the root tip is the target of aluminum toxicity, many of these genes are turned on in older portions of the root as well. Understanding the molecular underpinnings of a plant's response to aluminum is key to developing new plant varieties that can tolerate aluminum in soil. Aluminum tolerant crops are needed to expand production in new areas and to sustain production in current crop production areas where soils are becoming acidic.
Technical Abstract: Oligonucleotide microarrays corresponding to more than 16,000 genes were used to analyze changes in transcript accumulation in root tips of the Al-sensitive Medicago truncatula cultivar Jemalong genotype A17 in response to Al stress treatment. Out of 2,782 genes with significant changes in transcript accumulation, 324 genes were up-regulated and 267 genes were down-regulated greater than or equal to 2.0-fold by Al. Genes involved in cell-wall modification and abiotic and biotic stress responses were up-regulated in response to Al toxicity while genes involved in primary metabolism, secondary metabolism, protein synthesis and processing, and cell cycle were down-regulated. Transcript accumulation data revealed putative toxic effects of Al to cellular structures and processes including cell wall stiffening, oxidative stress, membrane lipid peroxidation, disruption of calcium homeostasis, and cell division inhibition. More specifically, a greater number of genes associated with cell wall loosening, reactive oxygen species generation and scavenging, and Ca2+ homeostasis were up-regulated while cell cycle and histone genes were down-regulated in response to Al stress. Transcript profiling also identified three novel putative genes (ABC transporter, multidrug and toxin efflux protein, and a metal-binding isoprenylated protein) with potential roles in binding and/or sequestration of Al. Finally, interfering RNA-induced silencing of two Al-induced genes, pectin acetylesterase (PAE) and annexin, in A17 hairy roots conferred a degree of tolerance to Al in root growth assays suggesting that these genes may play a role in Al toxicity.