Metabolic acclimation supports higher aluminium-induced secretion of citrate and malate in an aluminium-tolerant hybrid clone of Eucalyptus

Abstract

Background Eucalyptus is the main timber species, most of which are hybrid clones, and usually grow in aluminized acid soil in China. The exudation of organic acids from roots may contribute to detoxification of Al and lead to the Al-tolerance in Eucalyptus genotypes. To further understand the organic acid response in Al tolerance in Eucalyptus, the Al-tolerant Eucalyptus grandis × E. urophylla clone GL-9 (marked as “G9”) and the Al-sensitive Eucalyptus urophylla clone GL-4 (marked as “W4”) were used to investigate the secretion and metabolism of citrate and malate in roots. Results Eucalyptus seedlings in hydroponics were exposed to the presence or absence of 4.4 mM Al at pH 4.0 for 24 hours. The protein synthesis inhibitor cycloheximide (CHM) and the anion channel blocker phenylglyoxal (PG) were applied to explore possible pathways involved in organic acid secretion. The Al treatments caused higher Al accumulation in roots of both clones. The secretion of malate and citrate was greater in G9 than in W4, corresponding to the relatively higher tolerance in G9 to Al. The peak Al concentration occurred after 1 h in G9 roots and declined afterward, indicating the activation of detoxification to alleviate Al accumulation. After 6 h of Al exposure, the efflux of citrate dramatically increased in G9 after a substantial lag phase, while both peak Al accumulation in roots and peak malate secretion occurred and there was no induction of citrate secretion in W4. Enhanced activity for citrate synthase and phosphoenolpyruvate carboxylase, and reduced activity for NADP-isocitrate dehydrogenase, aconitase and NADP-malic enzyme were closely associated with the greater secretion of citrate in G9. Both PG and CHM were effective inhibitors of citrate and malate secretion in both Eucalyptus clones, except the malate secretion in W4 was not affected by CHM. Conclusions In two different Al-tolerant Eucalyptus clones, both secretion and internal accumulation of citrate and malate in roots were involved in Al detoxification. An anion channel on the plasma membrane could be an important mode of organic acid secretion. Citrate and relevant metabolizing enzymes led more important role in the response to Al in E. grandis × E. urophylla.