Zhang Juan, Subramanian Senthil, Stacey Gary, Yu Oliver
Donald Danforth Plant Science Center, 975 N. Warson Road, Saint Louis, MO 63132, USA.
Plant J. 2009 Jan;57(1):171-83. doi: 10.1111/j.1365-313X.2008.03676.x. Epub 2008 Sep 30.
Flavonoids play critical roles in legume-rhizobium symbiosis. However, the role of individual flavonoid compounds in this process has not yet been clearly established. We silenced different flavonoid-biosynthesis enzymes to generate transgenic Medicago truncatula roots with different flavonoid profiles. Silencing of chalcone synthase, the key entry-point enzyme for flavonoid biosynthesis led to flavonoid-deficient roots. Silencing of isoflavone synthase and flavone synthase led to roots deficient for a subset of flavonoids, isoflavonoids (formononetin and biochanin A) and flavones (7,4'-dihydroxyflavone), respectively. When tested for nodulation by Sinorhizobium meliloti, flavonoid-deficient roots had a near complete loss of nodulation, whereas flavone-deficient roots had reduced nodulation. Isoflavone-deficient roots nodulated normally, suggesting that isoflavones might not play a critical role in M. truncatula nodulation, even though they are the most abundant root flavonoids. Supplementation of flavone-deficient roots with 7, 4'-dihydroxyflavone, a major inducer of S. meliloti nod genes, completely restored nodulation. However, the same treatment did not restore nodulation in flavonoid-deficient roots, suggesting that other non-nod gene-inducing flavonoid compounds are also critical to nodulation. Supplementation of roots with the flavonol kaempferol (an inhibitor of auxin transport), in combination with the use of flavone pre-treated S. meliloti cells, completely restored nodulation in flavonoid-deficient roots. In addition, S. meliloti cells constitutively producing Nod factors were able to nodulate flavone-deficient roots, but not flavonoid-deficient roots. These observations indicated that flavones might act as internal inducers of rhizobial nod genes, and that flavonols might act as auxin transport regulators during nodulation. Both these roles of flavonoids appear critical for symbiosis in M. truncatula.
类黄酮在豆科植物 - 根瘤菌共生中发挥着关键作用。然而，单个类黄酮化合物在此过程中的作用尚未明确确立。我们使不同的类黄酮生物合成酶沉默，以产生具有不同类黄酮谱的转基因蒺藜苜蓿根。查尔酮合酶是类黄酮生物合成的关键起始酶，其沉默导致根中类黄酮缺乏。异黄酮合酶和黄酮合酶的沉默分别导致根中一类黄酮亚组缺乏，即异黄酮（芒柄花黄素和鹰嘴豆芽素A）和黄酮（7,4'-二羟基黄酮）。在用苜蓿中华根瘤菌测试结瘤情况时，类黄酮缺乏的根结瘤几乎完全丧失，而黄酮缺乏的根结瘤减少。异黄酮缺乏的根正常结瘤，这表明异黄酮可能在蒺藜苜蓿结瘤中不发挥关键作用，尽管它们是根中最丰富的类黄酮。用苜蓿中华根瘤菌结瘤基因的主要诱导剂7,4'-二羟基黄酮补充黄酮缺乏的根，可完全恢复结瘤。然而，相同处理并未恢复类黄酮缺乏的根的结瘤，这表明其他非结瘤基因诱导的类黄酮化合物对结瘤也至关重要。用黄酮醇山奈酚（一种生长素运输抑制剂）补充根，并结合使用经黄酮预处理的苜蓿中华根瘤菌细胞，可完全恢复类黄酮缺乏的根的结瘤。此外，组成型产生结瘤因子的苜蓿中华根瘤菌细胞能够使黄酮缺乏的根结瘤，但不能使类黄酮缺乏的根结瘤。这些观察结果表明，黄酮可能作为根瘤菌结瘤基因的内源性诱导剂，并且黄酮醇可能在结瘤过程中作为生长素运输调节剂。类黄酮的这两种作用似乎对蒺藜苜蓿的共生至关重要。
