Silvester Warwick B, Parsons Richard, Watt Peter W
Department of Biological Sciences, University of Dundee, Dundee DD1 4HN, Scotland, UK.
Department of Anatomy and Physiology, University of Dundee, Dundee DD1 4HN, Scotland, UK.
New Phytol. 1996 Apr;132(4):617-625. doi: 10.1111/j.1469-8137.1996.tb01880.x.
In Gunnera, Nostoc cells invade secretory tissue forming well defined symbiotic areas within the stems and are termed internal nodules (Silvester, 1976). Excised, but intact, internal stem Nostoc nodules taken from Gunnera magellanica show light-stimulated nitrogenase activity and release a small, but measurable, proportion of their current N fixation as NH into the external solution. When nodules are disrupted and Nostoc extracted anaerobically, 90% or more of the estimated N fixation is released from the Nostoc cells as NH into the surrounding medium. Use of N confirmed that only 12% of N fixed is retained within the cells of Nostoc. The remaining 88% was identified as NH released outside the cells. Within the intact nodule system, N uptake showed that 2-5% of recently fixed N remains within the Nostoc cells and up to 30% of extracellular N is in asparagine after 1 h. Evidence is presented that stimulation of nitrogenase by light in the intact Gunnera/Nostoc system produces more NH than can be assimilated by the host cells, resulting in significant NH accumulation.
在大叶草属植物中，念珠藻细胞侵入分泌组织，在茎内形成界限分明的共生区域，这些区域被称为内部根瘤（西尔维斯特，1976年）。从大叶草属的麦哲伦大叶草中切下但保持完整的茎内念珠藻根瘤，表现出光刺激的固氮酶活性，并将其当前固定氮的一小部分（但可测量）以铵的形式释放到外部溶液中。当根瘤被破坏且念珠藻在厌氧条件下被提取出来时，估计固定氮的90%或更多以铵的形式从念珠藻细胞释放到周围介质中。使用¹⁵N证实，仅12%的固定氮保留在念珠藻细胞内。其余88%被鉴定为细胞外释放的铵。在完整的根瘤系统中，¹⁵N吸收表明，最近固定氮的2 - 5%保留在念珠藻细胞内，1小时后细胞外氮的30%以天冬酰胺的形式存在。有证据表明，在完整的大叶草属/念珠藻系统中，光对固氮酶的刺激产生的铵超过了宿主细胞能够同化的量，导致铵大量积累。
