The Arabidopsis-related halophyte Thellungiella halophila: boron tolerance via boron complexation with metabolites?

ABSTRACT Tolerance to boron (B) is still not completely understood. We tested here the hypothesis that Thellungiella halophila, an Arabidopsis thaliana-related 'extremophile' plant, with abundance of B in its natural environment, is tolerant to B, and examined the potential mechanisms of this tolerance. With 1-10 m m B applied ([B]ext) to Thellungiella and Arabidopsis grown in hydroponics, the steady-state accumulated B concentration ([B]int) in the root was below [B]ext, and was similar in both, suggesting both extrude B actively. Whether grown in soil or hydroponically, the shoot [B]int was higher in Arabidopsis than in Thellungiella, suggesting more effective net B exclusion by Thellungiella root. Arabidopsis exhibited toxicity symptoms including reduced shoot fresh weight (FW), but Thellungiella was not affected, even at similar levels of shoot-accumulated [B]int (about 10 to 40 m m B in 'shoot water'), suggesting additional B tolerance mechanism in Thellungiella shoot. At [B]ext = 5 m m, the summed shoot concentration of the potentially B-binding polyhydroxyl metabolites (malic acid, fructose, glucose, sucrose and citric acid) in Arabidopsis was below [B]int, but in Thellungiella it was over twofold higher than [B]int, and therefore likely to allow appreciable 1:2 boron-metabolite complexation in the shoot. This, we suggest, is an important component of Thellungiella B tolerance mechanism. [ABSTRACT FROM AUTHOR]