Location: Children's Nutrition Research CenterTitle: Carboxylate metabolism changes induced by Fe deficiency in barley, a Strategy II plant species Author
Submitted to: Journal of Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/2/2012
Publication Date: 6/1/2012
Citation: Lopez-Millan, A.-F., Grusak, M.A., Abadia, J. 2012. Carboxylate metabolism changes induced by Fe deficiency in barley, a Strategy II plant species. Journal of Plant Physiology. 169(11):1121-1124. Interpretive Summary: Iron is a mineral required by all plants. Plants get their iron from the soil, but some soil types have less iron available for the plant to take up through its root system. This poor level of iron availability creates a problem for many crop plants, including barley, a cereal crop. When barley plants are challenged with limited amounts of iron, they grow poorly and their seed yield is diminished. In order to help barley plants absorb more iron, and thus achieve optimal yields, we examined what happens in barley roots when the plants are presented with limited amounts of iron. We used two barley cultivars that exhibited stronger or weaker abilities to cope with limited iron. In roots, we measured the activities of various enzymes that are responsible for the production of specific carbohydrate molecules. The carbohydrates we studied have been shown to help non-cereal crops cope with the stress of limited iron. The enzyme activities had not been previously studied in a cereal species like barley, at least not in relation to limited iron availability. We found the enzyme activities to be elevated in the roots of both barley cultivars when the plants were iron limited, but overall the activities were much lower than those found in iron-challenged non-cereal crops like tomato or soybean. And because we found no major differences in enzyme activities between the two barley cultivars, the activities of these enzymes, or the amounts of carbohydrates produced, could not explain the differences in iron-limited coping abilities between these cultivars. Nonetheless, our results show that cereal and non-cereal crop plants can activate some of the same root processes when challenged with limited amounts of iron. Future studies are needed to help us understand how to elevate these root processes even further in barley cultivars, in order to help them grow better in low-iron soils.
Technical Abstract: The effects of iron (Fe) deficiency on carboxylate metabolism were investigated in barley (Hordeum vulgare L.) using two cultivars, Steptoe and Morex, that differ in their Fe efficiency response. In both cultivars, root extracts of plants grown in Fe-deficient conditions showed higher activities of enzymes related to organic acid metabolism, including citrate synthase, malate dehydrogenase, and phosphoenolpyruvate carboxylase, compared to activities measured in root extracts of Fe-sufficient plants. Accordingly, the concentration of total carboxylates was higher in Fe-deficient roots of both cultivars, with citrate concentration showing the greatest increase. In xylem sap, the concentration of total carboxylates was also higher with Fe deficiency in both cultivars, with citrate and malate being the major organic acids. Leaf extracts of Fe-deficient plants also showed increases in citric acid concentration and in the activities of glucose-6-phosphate dehydrogenase and fumarase activities, and decreases in aconitase activity. Our results indicate that changes in root carboxylate metabolism previously reported in Strategy I species also occur in barley, a Strategy II plant species, supporting the existence of anaplerotic carbon fixation via increases in the root activities of these enzymes, with citrate playing a major role. However, these changes occur less intensively than in Strategy I plants. Activities of the anaerobic metabolism enzymes pyruvate decarboxylase and lactate dehydrogenase did not change in barley roots with Fe deficiency, in contrast to what occurs in Strategy I plants, suggesting that these changes may be Strategy I-specific. No significant differences were observed in overall carboxylate metabolism between cultivars, for plants challenged with high or low Fe treatments, suggesting that carboxylate metabolism changes are not behind the Fe-efficiency differences between these cultivars. Citrate synthase was the only measured enzyme with constitutively higher activity in Steptoe relative to Morex leaf extracts.