Guangxi Key Laboratory of Forest Ecology and Conservation, College of Forestry, Guangxi University, 100 East University Road, Nanning, 530004, Guangxi, People's Republic of China.
School of Biological Sciences, University of Western Australia, Perth, 6009, Australia.
BMC Plant Biol. 2021 Jan 6;21(1):14. doi: 10.1186/s12870-020-02788-4.
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.
桉树是主要的人工林种植树种，主要种植在铝酸化土壤中。为了了解桉树无性系对铝（Al）毒性的反应，选用耐铝桉树杂种无性系 GL-9（命名为“G9”）和铝敏感桉树无性系 GL-4（命名为“W4”）来研究根系对柠檬酸和苹果酸的产生和分泌。
在 pH 值为 4.0 时，将水培中的桉树幼苗暴露于有或没有 4.4 mM Al 的环境中 24 小时。应用蛋白合成抑制剂环己酰亚胺（CHM）和阴离子通道阻滞剂苯甲酰甘氨酸（PG）来探索有机酸分泌可能涉及的途径。G9 中苹果酸和柠檬酸的分泌比 W4 更早、更大，相应地，G9 中 Al 的积累较少。G9 根中的 Al 浓度在 1 小时后达到峰值，随后下降，与苹果酸的快速诱导分泌相对应。G9 中柠檬酸流出的时间滞后约 6 小时，随后迅速分泌以支持持续的 Al 解毒。单独的苹果酸分泌可能会减轻 Al 毒性，因为在 W4 中 Al 积累和苹果酸分泌的峰值是同时发生的，而 W4 没有分泌可观的柠檬酸。柠檬酸合酶（CS）和磷酸烯醇丙酮酸羧激酶（PEPC）的活性增强，异柠檬酸脱氢酶（IDH）、乌头酸酶（ACO）和苹果酸酶（ME）的活性降低，与 G9 中柠檬酸的大量分泌密切相关。PG 有效抑制了桉树无性系中柠檬酸和苹果酸的分泌。CHM 也抑制了 G9 中的苹果酸和柠檬酸分泌，以及 W4 中的柠檬酸分泌，但显著不影响 W4 中的苹果酸分泌。
G9 立即从根部分泌苹果酸，这对 Al 解毒有初步作用，随后是柠檬酸的延迟分泌。预先存在的阴离子通道蛋白首先有助于苹果酸的分泌，而载体蛋白的合成似乎是柠檬酸排泄所必需的。有机酸浓度对 Al 的响应可以通过增强 CS 和 PEPC 活性来实现，但需要涉及有机酸代谢的其他酶活性的变化来支持。上述信息可能有助于进一步探索桉树中与 Al 耐性相关的基因。
