THE ROLE OF MIMOSINE, A NON-PROTEIN AMINO ACID IN LEUCAENA LEUCOCEPHALA SUBSP. GLABRATA (GIANT LEUCAENA)

Abstract

Leucaena leucocephala subsp. glabrata (giant leucaena) is a fast-growing tree-legume which is widely grown for its foliage for use as a nutritious fodder in the tropical and subtropical regions of the world. It is highly tolerant to many environmental stresses, including drought, low soil fertility, salinity, acidic and alkaline soils, and disease and pest infestation. The entire plant of giant leucaena contains a non-protein amino acid, called mimosine, which is also toxic to animals. High amounts of mimosine is found in the foliage, especially in the young shoots, where its concentrations may be higher than 5% of the dry weight. It is not known why giant leucaena produces so much mimosine. The overall goal of this research was to determine the biochemical and physiological roles of mimosine in giant leucaena. It was hypothesized that mimosine facilitates iron uptake and provides stress tolerance in giant leucaena. The specific objectives of this research were to (i) determine the mimosine content of giant leucaena at various life stages and in various plant parts, (ii) study the possible role of mimosine as a phytosiderophore, and (iii) study the possible role of mimosine as stress-response molecule in giant leucaena. Mimosine was known to bind metallic cations such as Fe(III), Fe(II), Zn(II), Cu(II), Co(II), Mn(II), Ni(II) and Cd(II). It was established in this research that mimosine helps to solubilize iron from insoluble ferric hydroxides. Results of this study showed that mimosine is secreted into the rhizosphere by the roots and of giant leucaena. Some amounts of mimosine is also found to be secreted from the green foliage, and also leached out from the decomposing leaves. Mimosine released to the soil binds and solubilizes iron and facilitates iron uptake by the plant. Thus it was established that mimosine serves as a phytosiderophore in giant leucaena. It was also shown in this study that mimosine accumulates in giant leucaena during favorable growth conditions when water and nutrients are easily available. Thus mimosine serves a storage molecule, which is utilized during drought when nutrient uptake is severely inhibited. Mimosine also has antioxidant properties that alleviate oxidative damage to the tissues during environmental stresses such high light intensity, UV radiation, and heavy metal toxicity. Thus, mimosine was shown to be a stress-response molecule in giant leucaena.