Eucalyptus is the main plantation wood species, mostly grown in aluminized acid soils. To understand the response of Eucalyptus clones to aluminum (Al) toxicity, the Al-tolerant Eucalyptus grandis × E. urophylla clone GL-9 (designated “G9”) and the Al-sensitive E. urophylla clone GL-4 (designated “W4”) were employed to investigate the production and secretion of citrate and malate by roots. Eucalyptus seedlings in hydroponics were exposed to the presence or absence of 4.4 mM Al at pH 4.0 for 24 h. The protein synthesis inhibitor cycloheximide (CHM) and anion channel blocker phenylglyoxal (PG) were applied to explore possible pathways involved in organic acid secretion. The secretion of malate and citrate was earlier and greater in G9 than in W4, corresponding to less Al accumulation in G9. The concentration of Al in G9 roots peaked after 1 h and decreased afterwards, corresponding with a rapid induction of malate secretion. A time-lag of about 6 h in citrate efflux in G9 was followed by robust secretion to support continuous Al-detoxification. Malate secretion alone may alleviate Al toxicity because the peaks of Al accumulation and malate secretion were simultaneous in W4, which did not secrete appreciable citrate. Enhanced activities of citrate synthase (CS) and phosphoenolpyruvate carboxylase (PEPC), and reduced activities of isocitrate dehydrogenase (IDH), aconitase (ACO) and malic enzyme (ME) were closely associated with the greater secretion of citrate in G9. PG effectively inhibited citrate and malate secretion in both Eucalyptus clones. CHM also inhibited malate and citrate secretion in G9, and citrate secretion in W4, but notably did not affect malate secretion in W4. G9 immediately secrete malate from roots, which had an initial effect on Al-detoxification, followed by time-delayed citrate secretion. Pre-existing anion channel protein first contributed to malate secretion, while synthesis of carrier protein appeared to be needed for citrate excretion. The changes of organic acid concentrations in response to Al can be achieved by enhanced CS and PEPC activities, but was supported by changes in the activities of other enzymes involved in organic acid metabolism. The above information may help to further explore genes related to Al-tolerance in Eucalyptus.

中文翻译：

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在桉树的耐铝杂种克隆中，代谢驯化支持更高的铝诱导的柠檬酸盐和苹果酸盐的分泌

桉树是主要的人工林木种，主要生长在含铝酸性土壤中。为了了解桉树克隆对铝 (Al) 毒性的反应，耐铝的大桉 × 尾叶桉克隆 GL-9（指定为“G9”）和铝敏感的尾叶桉克隆 GL-4（指定为“W4”） ”）被用来研究根的柠檬酸盐和苹果酸盐的产生和分泌。将水培中的桉树幼苗暴露于存在或不存在 4.4 mM Al、pH 4.0 的环境中 24 小时。应用蛋白质合成抑制剂放线菌酮 (CHM) 和阴离子通道阻滞剂苯乙二醛 (PG) 来探索参与有机酸分泌的可能途径。苹果酸和柠檬酸盐的分泌在 G9 中比 W4 早且更多，对应于 G9 中较少的 Al 积累。G9 根中铝的浓度在 1 小时后达到峰值，然后下降，对应于苹果酸分泌的快速诱导。G9 中柠檬酸盐流出约 6 小时的时间滞后之后是强劲的分泌，以支持持续的铝解毒。单独的苹果酸分泌可以减轻铝毒性，因为铝积累和苹果酸分泌的峰值在 W4 中同时出现，W4 没有分泌可观的柠檬酸盐。柠檬酸合酶 (CS) 和磷酸烯醇式丙酮酸羧化酶 (PEPC) 活性增强，异柠檬酸脱氢酶 (IDH)、乌头酸酶 (ACO) 和苹果酸酶 (ME) 活性降低与 G9 中柠檬酸的更多分泌密切相关。PG 有效抑制两种桉树克隆中柠檬酸盐和苹果酸盐的分泌。CHM 还抑制 G9 中的苹果酸和柠檬酸盐分泌，以及 W4 中的柠檬酸盐分泌，但明显不影响 W4 中的苹果酸分泌。G9 立即从根部分泌苹果酸，这对铝解毒有初步影响，随后是延时的柠檬酸盐分泌。预先存在的阴离子通道蛋白首先有助于苹果酸分泌，而柠檬酸盐排泄似乎需要载体蛋白的合成。有机酸浓度响应铝的变化可以通过增强的 CS 和 PEPC 活性来实现，但得到了参与有机酸代谢的其他酶活性变化的支持。上述信息可能有助于进一步探索桉树耐铝相关基因。预先存在的阴离子通道蛋白首先有助于苹果酸分泌，而柠檬酸盐排泄似乎需要载体蛋白的合成。有机酸浓度响应铝的变化可以通过增强的 CS 和 PEPC 活性来实现，但得到了参与有机酸代谢的其他酶活性变化的支持。上述信息可能有助于进一步探索桉树耐铝相关基因。预先存在的阴离子通道蛋白首先有助于苹果酸分泌，而柠檬酸盐排泄似乎需要载体蛋白的合成。有机酸浓度响应铝的变化可以通过增强的 CS 和 PEPC 活性来实现，但得到了参与有机酸代谢的其他酶活性变化的支持。上述信息可能有助于进一步探索桉树耐铝相关基因。