Physiology of iron deficiency chlorosis resistance in soybean

Authors: VASCONCELOS, MARTA - BAYLOR COLLEGE MED; LI, GLORIA - BAYLOR COLLEGE MED; LI, CHEE-MING - BAYLOR COLLEGE MED; Grusak, Michael

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 3/31/2008
Publication Date: 10/11/2008
Citation: Vasconcelos, M.W., Li, G.W., Li, C., Grusak, M.A. 2008. Physiology of iron deficiency chlorosis resistance in soybean [abstract]. XIV International Symposium on Iron Nutrition and Interactions in Plants, October 11-16, 2008, Beijing, China. p. 67.

Interpretive Summary:

Technical Abstract: Soybeans (Glycine max L.) are an important source of plant protein in the human diet and a leading source of vegetable oil in the world. It is also gaining importance in the production of biodiesel and in programs of phytoremediation. Notably, it is one of the most important agricultural legumes in the US. It is generally planted in the Midwest region of the country, which has a high incidence of soils that are calcareous and have low iron availability. Iron deficiency chlorosis (IDC) is a direct result of an impaired capacity of the plant to extract soluble iron from the soil, leading to severe leaf chlorosis, low photosynthetic rates, and yield reductions of several million metric tons each year. One of the most common strategies against IDC is to cultivate lines that are naturally less susceptible to iron deficiency. These lines, however, are not always the highest yielding, and the mechanisms whereby they are more tolerant to IDC are not well understood. In order to devise breeding and genetic transformation programs that aim at generating high-yielding, IDC-resistant soybean lines, it would be useful to better understand the physiological and molecular mechanisms that enable tolerant plants to survive under iron-limiting conditions. To this end, 15 IDC-tolerant and 15 IDC-susceptible soybean accessions were identified in the USDA soybean collection. Plants were grown in hydroponic conditions simulating iron-deficient, calcareous-soils, and several physiological and molecular IDC-related aspects were studied. These were: 1) unifoliate and first trifoliate leaf chlorosis scoring; 2) membrane-localized root reductase activity; 3) mineral concentration in different plant parts; 4) whole-plant mineral partitioning; 5) root length; 6) cloning and analysis of soybean iron reductase genes. No correlation was found between root reductase activity and chlorosis in the unifoliate or the first trifoliate leaves. Also, root length was not correlated with chlorosis scoring. A negative correlation was found between the dry weight of the first true leaves and chlorosis in the first trifoliate. Finally, four iron reductase genes were cloned from soybean roots (GmFRO1 to GmFRO4), and their expression levels were studied in Williams 82 plants grown under iron-deficient and iron-sufficient conditions. It was found that all reductase genes were expressed in roots and shoots, and two of the genes (GmFRO1 and GmFRO2) were up-regulated in iron deficiency and down-regulated with iron resupply. IDC resistance was not found to be correlated with root length, higher reductase activity, higher concentration of total heme proteins in the roots, seed iron concentration, or increased capacity to remobilize iron from the cotyledons. IDC resistance was, however, negatively correlated with the dry weight and iron concentration of the first unifoliate leaves, and positively correlated with the amount of iron in the first trifoliate leaves. In summary, IDC response in soybean is a complex mechanism that most likely involves a combination of factors that are genetically inherited but environmentally dependent.