Functions of plant hormones involved in the formation of nodules on the roots of legumes

采用Tn5诱变方法分离了11个产低吲哚-3-乙酸（IAA）的elkanii缓生根瘤菌突变体。各突变体产生的IAA量为野生型的2.2 ~ 13.6%。静息细胞反应也表明，将吲哚-3-丙酮酸转化为吲哚-3-乙醛的生物合成步骤在所有突变体中都被阻断。这表明吲哚-3-丙酮酸脱羧酶（IPDC）是elkanii酵母IAA生物合成的关键酶。在豆科植物siratro上施用一些低IAA产生的突变体，其根上的无瘤数比施用野生型减少了10% ~ 60%，说明elkanii产生的IAA对根瘤的形成起重要作用。另一方面，研究表明寄主植物根系分泌的黄酮类化合物可诱导结瘤菌中结瘤基因的表达。我们推测结瘤因子通过茉莉酸生物合成和寄主植物结瘤的早期事件促进了类黄酮的生物合成。为了验证这一假设，我们建立了一个生物测定系统，利用悬浮培养的甘氨酸max细胞来检测茉莉酸或nod因子对苯丙氨酸解氨酶（PAL）活性的影响，PAL是类黄酮生物合成的关键酶。

We have isolated the 11 low-indole-3-acetic acid (IAA)-producing mutants of Bradyrhizobium elkanii by Tn5 mutagenesis. The amounts of IAA produced by the respective mutants were 2.2 to 13.6 % of that in the wild type. It was also shown by resting cell reactions that biosynthetic step to convert indole-3-pyrubic acid to indole-3-acetaldehyde was blocked in all the mutants. This suggests that indole-3-pyruvate decarboxylase (IPDC) functions as a key enzyme in the IAA biosynthesis of B. elkanii. When some of the low-IAA-producing mutants were applied to the leguminous plant siratro, the number of noduration on the roots reduced 10 to 60% of that when the wild type was applied, indicating that IAA produced by B. elkanii plays impotant roles on the formation of root nodules. On the other hand, it has been shown that flavonoids secreted from the roots of host plants induced the expression of nod genes in the nodule bacteria. We speculated that nod factors causes promotion of flavonoid biosynthesis via jasmonate biosynthesisas as well as the early events of nodulation in the host plants. To investigate this hypothesis, we developed a bioassay system using suspension-cultured cells of Glycine max to examine the effect of jasmonic acid or nod factors on phenylalanine ammonia-lyase (PAL) activity, which is a key enzyme in the flavonoid biosynthesis.