|RADMER, LORIEN - University Of Minnesota|
|Samac, Deborah - Debby|
Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/22/2011
Publication Date: 1/15/2012
Citation: Radmer, L., Tesfaye, M., Somers, D.A., Temple, S.J., Vance, C.P., Samac, D.A. 2012. Aluminum resistance mechanisms in oat (Avena sativa L.). Plant and Soil. 351(1&2):121-134.
Interpretive Summary: Aluminum is widespread in agricultural soils, and aluminum toxicity is a major limiting factor in crop production throughout the world. In soils with a pH of 5.5 and below, aluminum becomes soluble and is rapidly taken up by plant roots resulting in root stunting and poor plant growth. Certain organic acids such as malic acid can prevent aluminum uptake by sequestering aluminum outside of the root. We tested whether expression of an alfalfa gene involved in organic acid synthesis would increase the amount of malate in oat plants and increase resistance to aluminum. Oat plants expressing the alfalfa malate dehydrogenase gene were identified and found to produce the alfalfa enzyme. One line with multiple copies of the gene, which had the highest amount of the enzyme, showed increased aluminum resistance but also had overall reduced plant growth compared to other lines. Plants expressing the alfalfa enzyme did not secrete more malic acid from roots. Addition of other genes may be necessary for increasing malic acid secretion. To gain a better understanding of aluminum resistance in oat, seedlings were tested for aluminum resistance in a hydroponic assay. The results of the assay showed that oat seedlings are highly resistant to aluminum and secrete malic acid in response to aluminum stress. The seedlings took up little aluminum, suggesting that the aluminum was sequestered outside of roots. Most aluminum was associated with mucilage and root cells sloughed off from the root tip. Of the 15 cultivars tested, most were found to be highly resistant in the hydroponic assay. The hydroponic assay was found to be more reliable for identifying aluminum resistant plants and cultivars than other published methods based on staining for the presence of aluminum in roots. This research indicates that oat has effective aluminum tolerance mechanisms to exclude aluminum from roots. 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: Enhanced aluminum (Al) resistance has been observed in several dicots over-expressing enzymes involved in organic acid synthesis; however, this method for improving Al resistance has not been investigated in cereal crops such as oat (Avena sativa L.). Oat is considered among the most Al resistant cereal crops but little research has been done to understand the basis of resistance. Oat seedlings were found to be highly resistant to Al in a simple hydroponic assay; a concentration of 325 microM AlK(SO4)2 was needed to cause a 50% decrease in root growth compared to root growth in the Al-free treatment. A malate dehydrogenase gene, neMDH, from alfalfa (Medicago sativa L.) was used to transform oat using the Sugarcane bacilliform badnavirus (ScBV) promoter. In a transgenic line with multiple copies of the transgene and the highest accumulation of neMDH protein, Al resistance was enhanced relative to untransformed control lines. However, overall root growth of this transgenic line was reduced and root epidermal cells accumulated more Al than control lines. Notably, expression of neMDH in transgenic oat did not enhance malate secretion. Control and transgenic lines were found to release large amounts of malate with exposure to Al. Hematoxylin and morin staining patterns of Al treated root tips of control plants and transgenic oat showed that aluminum accumulated in the root epidermis and did not penetrate into the cortex. This suggests malate plays a role in oat aluminum resistance. Screening 15 oat cultivars confirmed that most oat cultivars are naturally resistant to high Al concentrations and effectively exclude Al from roots.