Mimosine facilitates metallic cation uptake by plants through formation of mimosine-cation complexes.

KEY MESSAGE: Iron deficiency conditions as well as iron supplied as a Fe(III)-mimosine complex induced a number of strategy I and strategy II genes for iron uptake in leucaena. Leucaena leucocephala (leucaena) is a tree-legume that can grow in alkaline soils, where metal-cofactors like Fe(III) are sparingly available. Mimosine, a known chelator of Fe(III), may facilitate Fe(III) uptake in leucaena by serving as a phytosiderophore. To test if mimosine can serve as a phytosiderophore, three sets of experiments were carried out. First, the binding properties and solubility of metal-mimosine complexes were assessed through spectrophotometry. Second, to study mimosine uptake in plants, pole bean, common bean, and tomato plants were supplied with mimosine alone and metal-mimosine complexes. Third, the expression of strategy I (S1) and strategy II (S2) genes for iron uptake from the soil was studied in leucaena plants exposed to different Fe(III) complexes. The results of this study show that (i) mimosine has high binding affinity for metallic cations at alkaline pH, Fe(III)-mimosine complexes are water soluble at alkaline pH, and that mimosine can bind soil iron under alkaline pH; (ii) pole bean, common bean, and tomato plants can uptake mimosine and transport it throughout the plant; and (iii) a number of S1 and S2 genes were upregulated in leucaena under iron-deficiency condition or when Fe(III) was supplied as a Fe(III)-mimosine complex. These findings suggest that leucaena may utilize both S1 and S2 strategies for iron uptake; and mimosine may play an important role in both strategies.

Mimosine accumulation in Leucaena leucocephala in response to stress signaling molecules and acute UV exposure.

Mimosine is a non-protein amino acid of Fabaceae, such as Leucaena spp. and Mimosa spp. Several relevant biological activities have been described for this molecule, including cell cycle blocker, anticancer, antifungal, antimicrobial, herbivore deterrent and allelopathic activities, raising increased economic interest in its production. In addition, information on mimosine dynamics in planta remains limited. In order to address this topic and propose strategies to increase mimosine production aiming at economic uses, the effects of several stress-related elicitors of secondary metabolism and UV acute exposure were examined on mimosine accumulation in growth room-cultivated seedlings of Leucaena leucocephala spp. glabrata. Mimosine concentration was not significantly affected by 10 ppm salicylic acid (SA) treatment, but increased in roots and shoots of seedlings treated with 84 ppm jasmonic acid (JA) and 10 ppm Ethephon (an ethylene-releasing compound), and in shoots treated with UV-C radiation. Quantification of mimosine amidohydrolase (mimosinase) gene expression showed that ethephon yielded variable effect over time, whereas JA and UV-C did not show significant impact. Considering the strong induction of mimosine accumulation by acute UV-C exposure, additional in situ ROS localization, as well as in vitro antioxidant assays were performed, suggesting that, akin to several secondary metabolites, mimosine may be involved in general oxidative stress modulation, acting as a hydrogen peroxide and superoxide anion quencher.