Ligaba Ayalew, Shen Hong, Shibata Koichi, Yamamoto Yoko, Tanakamaru Shigemi, Matsumoto Hideaki
Research Institute for Bioresources, Okayama University, Chuo 2-20-1, Kurashiki 710-0046, Japan.
Physiol Plant. 2004 Apr;120(4):575-584. doi: 10.1111/j.0031-9317.2004.0290.x.
Exudation of organic anions is believed to be a common tolerance mechanism for both aluminium toxicity and phosphorus deficiency. Nevertheless, which of these stresses that actually elicit the exudation of organic anions from rape (Brassica napus L) remains unknown, and the combined effects of Al toxicity and P deficiency on rape have not been reported before. Therefore, in the current study, Brassica napus var. Natane nourin plants grown with or without 0.25 mM P were exposed to 0 or 50 micro M AlCl(3) and several parameters related to the exudation of organic anions from the roots were investigated. Eight days of P deficiency resulted in a significant growth reduction, but P deficiency alone did not induce exudation of organic anions. In contrast, Al strongly induced organic acid exudation, while simultaneously inhibiting root growth. Increased in-vitro activity of citrate synthase (CS, EC 4.1.3.7), malate dehydrogenase (MDH, EC 1.1.1.37) and phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31), together with reduced root respiration, indicated that the Al-induced accumulation and subsequent exudation of citrate and malate were associated with both increased biosynthesis and reduced metabolism of citric and malic acid. Phosphorus-sufficient plants showed more pronounced aluminium-induced accumulation and exudation of organic anions than P-deficient plants. A divided root chamber experiment showed the necessity of direct contact between Al and roots to elicit exudation of organic anions. Prolonged exposure (10 days) to Al resulted in a decrease in the net exudation of citrate and malate, and the rate of decrease was much more rapid in P-deficient plants than in P-sufficient plants. It is concluded that P nutrition affects the level of Al-induced synthesis and exudation of organic anions. However, the mechanism needs further investigation.
有机阴离子的渗出被认为是植物耐受铝毒和缺磷的一种常见机制。然而，究竟是哪种胁迫真正引发了油菜（Brassica napus L）根系有机阴离子的渗出尚不清楚，而且铝毒和缺磷对油菜的综合影响此前未见报道。因此，在本研究中，将施与或未施0.25 mM磷的甘蓝型油菜品种“丰羽”植株暴露于0或50 μM的AlCl₃中，并对与根系有机阴离子渗出相关的几个参数进行了研究。缺磷8天导致显著的生长受抑，但单独缺磷并未诱导有机阴离子的渗出。相反，铝强烈诱导有机酸渗出，同时抑制根系生长。柠檬酸合酶（CS，EC 4.1.3.7）、苹果酸脱氢酶（MDH，EC 1.1.1.37）和磷酸烯醇式丙酮酸羧化酶（PEPC，EC 4.1.1.31）的体外活性增加，同时根系呼吸降低，这表明铝诱导的柠檬酸和苹果酸积累及随后的渗出与柠檬酸和苹果酸生物合成增加及代谢减少有关。磷充足的植株比缺磷植株表现出更明显的铝诱导有机阴离子积累和渗出。分根室实验表明铝与根系直接接触对于引发有机阴离子渗出是必要的。长时间暴露于铝（10天）导致柠檬酸和苹果酸的净渗出减少，且缺磷植株的减少速率比磷充足植株快得多。得出的结论是，磷营养影响铝诱导的有机阴离子合成和渗出水平。然而，其机制需要进一步研究。
