Oxalato, the Ion that Hides: Intracellular Accumulation in an Aluminum Tolerant Rice Cultivar

Abstract

Organic acid exudation is one of the main mechanisms of aluminum (Al) tolerance in plants. The contribution of intracellular accumulation of these compounds, nevertheless, remains much less explored, especially in rice. This study examined data from experiments carried out with two upland rice cultivars that differ in their response to Al: Comum branco (tolerant) and Caiapó (sensitive). The data come from the experiment described in Zonta et al . The new finding is that oxalate had a ratio of 2.6, while citrate and malate are mainly exuded. Principal component analysis (PCA) also shows that oxalate is functionally independent from the other organic acids. None of this was reported in the original study. The work used chromatographic profiles and multivariate statistics to investigate the distribution pattern of oxalate between root cellular content and exudate when plants were exposed to Al, and to contrast this pattern with those of citrate and malate. In the tolerant cultivar, oxalate concentration in the cellular content increased strongly under Al stress, with rises of 128% at 4 days and 113% at 15 days (p < 0.01). No change was detected in the exudate for oxalate, while for citrate and malate, the opposite happened as their concentrations in the exudate increased sharply under Al, with increases up to 10-fold larger in the tolerant cultivar than in the sensitive one. Chromatographic profiles taken 15 days confirmed these patterns. They showed large peaks of citrate and malate in the exudate of Comum branco, while oxalate was the dominant peak in the cellular content of the same cultivar. Principal component analysis revealed that oxalate is functionally independent from citrate and malate, as its vector stood perpendicular to the vectors of the two acids. The interaction between cultivar and detection site was significant only for oxalate. In the sensitive cultivar Caiapó, oxalate concentrations did not change in response to Al, regardless of the detection site. Together, these results demonstrate that the distribution profile of oxalate differs clearly from those of citrate and malate, and that this difference is expressed in distinct ways between the two cultivars. This progressive intracellular accumulation of oxalate, occurring alongside larger absolute amounts of citrate and malate, points to an energy-saving internal detoxification strategy in the tolerant cultivar.