Abstract: To characterize the growth and development of a symbiotic nitrogen-fixing mutant esn1 of Medicago truncatula, in this study we set up 4 different growth conditions： normal nitrogen, low nitrogen with rhizobia inoculation, low nitrogen without rhizobia inoculation, 5 mmol·L^-1 N without rhizobia inoculation. The results showed that under normal nitrogen growth conditions, all organs of the mutant esn1, including roots, stems, leaves, flowers, pods, seeds, flowering period, and number of branches were normal； but under low-nitrogen conditions, the growth rate of the mutant esn1 with rhizobia inoculation was almost identical to that of uninoculated, indicating that it could hardly fix nitrogen. Further analysis found that although the mutant had severe nitrogen fixation defect, it still retained a very weak （<1%） nitrogen fixation ability. Under low-nitrogen conditions, the development of the root system, leaves, and plant height of the mutant esn1 was not different from that of the wild-type plant. These results indicate that esn1 only has extremely severe defect in symbiotic nitrogen fixation, while other growth and development processes are not affected.

Key words: Medicago truncatula, Symbiotic nitrogen fixation, Mutants, Phenotypic characterization, Growth and development

摘要： 本研究设置了4个生长条件（正常、低氮2 mmol·L^-1 N+不接种根瘤菌、低氮+接种根瘤菌、5 mmol·L^-1 N+不接种根瘤菌）对一个蒺藜苜蓿（Medicago truncatula）共生固氮突变体esn1进行了生长发育表型鉴定,以期明确该材料的生长发育特征。结果显示,在正常生长条件下,突变体esn1的所有器官,包括根、茎、叶、花、果荚、种子以及花期、分枝数均正常；但在低氮条件下,其接菌的生长速度与未接菌的生长速度几乎一样,这表明该突变体几乎不能固氮。进一步分析发现,该突变体虽然存在严重的固氮缺陷,但其仍然保留非常微弱的（<1%）固氮能力。在低氮条件下,突变体esn1根系、叶片、株高等发育与野生型植株的也没有差异。这些结果表明,esn1仅在共生固氮方面存在极其严重的缺陷,而其他的生长发育过程均不受影响。

关键词: 蒺藜苜蓿, 共生固氮, 突变体, 表型鉴定, 生长发育