Auxin and Essential Metals

Among the essential micronutrients, transition metals such as copper, iron, and manganese (Cu, Fe, Mn) are important cofactors in metalloproteins, for example in peroxidases, oxidases, and superoxide dismutases. Free Cu2+ or Fe3+ catalyse the formation of hydroxyl radicals (OH*) via the Fenton reaction. OH is highly reactive and leads to unspecific oxidative degradation. Therefore, the intracellular levels of free Cu or Fe have to be kept low to avoid triggering the Fenton reaction inside the cell, which would cause unspecific injury (Polle and Schutzendubel 2003). Exposure to excess metals leads to over-accumulation and toxic effects when tissue concentrations exceed the optimal demand (Ducic and Polle 2005; Yruela 2005).

Different tissues exhibit different sensitivity to metal stress. For example, in poplar stem radial growth declined already when the plants were grown in the presence of 1 pM Cu in hydroponic nutrient solutions, whereas elongation growth and photosynthesis declined only when the plants were exposed to 50-fold higher Cu concentrations (Elobeid and Polle 2010). It was suspected that sub-toxic concentrations might already affect the phytohormone balance and thereby influence growth. In Helianthus annuus elongation growth of roots was more strongly reduced in the presence of excess Cu (80 pM) than that of shoots (Ouzounidou and Ilias 2005). Application of auxin diminished the negative influence of excess Cu and improved the water use efficiency of the plants. Thus, auxin application alleviated metabolic and physiological disturbances of Cu stress. The negative influence of excess Cu on the auxin physiology of roots was also documented in GUS reporter lines of Arabidopsis thaliana (Lequeux et al. 2010). Under these conditions, lignification was stimulated and growth of the main root inhibited (Lequeux et al. 2010). Overall, the results of these studies suggest that excess Cu activates stress pathways involving auxin degradation, perhaps in a similar way as Cd.

The involvement of Mn with auxin physiology is known for a long time. Already in the 1960s in vitro studies showed that Mn stimulated auxin degradation involving auxin oxidase activities (e.g., Stonier et al. 1968). An early investigation of Morgan and others (1966) with Gossypium hirsutum revealed that excess Mn caused toxicity symptoms, which resembled those of auxin deficiency. Their study showed that Mn stimulated IAA-oxidase activity and suggested that the enzyme can function in vivo to regulate auxin levels and down-stream processes. Similar hydroponic experiments with Mn-stressed G. hirsutum demonstrated that growth, IAA oxidase, leaf abscission, internode length, and similar symptoms were consistently manipulated by raising or lowering Mn levels in the plant culture medium (Morgan et al. 1976). It was concluded that there is a causal relationship between IAA oxidase and the responses that involve destruction of auxin (Morgan et al. 1976). Beffa et al. (1990) showed that auxin oxidase could be separated from peroxidase activities and that the product of the oxidation was indo-3yl-methanol. They suggested the existence of a specific enzymatic system, which catalyzes the oxidative degradation of IAA. Therefore, IAA oxidase may have an important role in regard to the fine regulation of the cellular level of auxin and, thus, growth. To investigate the influence of Mn stress on auxin physiology in poplar, GH3::GUS reporter lines have been employed (Elobeid 2008). Using this technique it was shown that the strong auxin signal in the vascular strands of the elongation zone of the stem was faded under the influence of Mn stress (Fig. 1a, b). Similarly the auxin signal in the lower parts of the stem, where radial growth occurs (Fig. 1c), and in roots, where side root emerge (Fig. 1e), disappeared under the influence of Mn (Fig. 1d, f). Since these effects occur before irreversible leaf injury and biomass loss, they support that Mn interferes with auxin physiology. Links between auxin and Mn have also been inferred from the analysis of an Arabidopsis thaliana mutant (ilr2) that had lost its responsiveness to application of conjugated auxin (auxin-isoleucine), but showed normal behavior to auxin (Magidin et al. 2003). This mutant was apparently unable to activate the required amido hydrolase and at the same time was insensitive to excess Mn suggesting modulation of Mn transport in the root by auxin (Magidin et al. 2003). When a vacuolar Ca2+/H+ antiporter (CAX4) was suppressed, application of Cd2+, Mn2+ and auxin altered root growth (Mei et al. 2009). DR5::GUS auxin reporter detected decreased auxin responses in the cax4 lines. Mei et al. (2009) concluded that the cation/H+ antiporter CAX4 plays an important function in root growth under heavy metal stress conditions.

Type of tissue

Control

Mn stress

Stem elongation zone

*

b

Stem secondary growth zone

c ^ L

i

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