Wirén Nicolaus VON, Römheld Volker, Shioiri Takayuki, Marschner Horst
Institute for Plant Nutrition (330), University of Hohenheim, 70593 Stuttgart, Germany.
Pharmaceutical Chemistry, Nagoya City University, Tanabe-dori, Mizuho-ku, Nagoya 467, Japan.
New Phytol. 1995 Aug;130(4):511-521. doi: 10.1111/j.1469-8137.1995.tb04328.x.
Graminaceous plant species respond to iron (Fe)-deficiency stress by enhancing the release of phytosiderophores from the roots and the uptake of Fe-phytosiderophores. For studying the mobilization and uptake of apoplasmic root Fe by barley (inherently high phytosiderophore release) and sorghum (inherently low phytosiderophore release) in axenic and nonaxenic (inoculated) nutrient solution, Fe pools in the root apoplasm were loaded during plant preculture with 10 M Fe(III)-EDTA. After 27 d growth in Fe-deficient nutrient solution, inoculated barley plants developed moderate Fe-deficiency chlorosis compared with the less chlorotic axenic plants. In inoculated plants, recovery of phytosiderophores and mobilization of apoplasmic root Fe tended to be slightly lower than in axenic plants, and in both treatments apoplasmic root Fe was completely depleted at harvest. As determined by the nonsoluble Fe fraction (> 0·2 μm) in the nutrient solution and at the rhizoplane, the microbial uptake and immobilization of apoplasmic root Fe was estimated at about 3% of the total amount of apoplasmic root Fe after preculture and at less than 10% of plant Fe uptake. Under axenic conditions, Fe-deficient sorghum also depleted apoplasmic root Fe and developed moderate Fe-deficiency chlorosis, although phytosiderophore recovery was 5-10-fold lower than in barley. By contrast, in inoculated sorghum plants, phytosiderophore recovery and Fe mobilization were extremely low. At harvest, in inoculated sorghum plants apoplasmic Fe pools were still considerably loaded and plant Fe uptake was c. 60% lower than that of axenic plants, resulting in severe Fe-deficiency chlorosis. Thus, in Fe-deficient sorghum plants, the lower rate of phytosiderophore release and its degradation restricted an efficient mobilization of apoplasmic root Fe in the presence of micro-organisms. In barley, however, the higher rate of phytosiderophore release allowed a complete mobilization of apoplasmic root Fe even in inoculated nutrient solution. Furthermore, the results show that the dominating effect of micro-organisms in their competition with barley and sorghum for apoplasmic root Fe is the degradation of phytosiderophores rather than the immobilization or uptake of Fe.
禾本科植物物种通过增强根系对植物铁载体的释放以及对铁 - 植物铁载体的吸收来应对缺铁胁迫。为了研究无菌和非无菌（接种）营养液中，大麦（固有植物铁载体释放量高）和高粱（固有植物铁载体释放量低）对质外体根中铁的转运和吸收情况，在植物预培养期间，用10 M Fe(III)-EDTA加载根质外体中的铁库。在缺铁营养液中生长27天后，与黄化程度较轻的无菌植物相比，接种的大麦植株出现了中度缺铁黄化症状。在接种的植株中，植物铁载体的回收和质外体根中铁的转运往往略低于无菌植物，并且在两种处理中，收获时质外体根中的铁完全耗尽。根据营养液和根际平面中不可溶性铁部分（> 0·2 μm）确定，预培养后微生物对质外体根中铁的吸收和固定估计约为质外体根中铁总量的3%，且低于植物铁吸收量的10%。在无菌条件下，缺铁的高粱也耗尽了质外体根中的铁，并出现了中度缺铁黄化症状，尽管植物铁载体的回收量比大麦低5 - 10倍。相比之下，在接种的高粱植株中，植物铁载体的回收和铁的转运极低。收获时，接种的高粱植株质外体铁库仍大量存在，且植物铁吸收量比无菌植株低约60%，导致严重缺铁黄化。因此，在缺铁的高粱植株中，植物铁载体释放率较低及其降解限制了在有微生物存在的情况下质外体根中铁的有效转运。然而，在大麦中，即使在接种的营养液中，较高的植物铁载体释放率也能使质外体根中的铁完全转运。此外，结果表明，微生物在与大麦和高粱竞争质外体根中铁时的主要作用是植物铁载体的降解，而非铁的固定或吸收。
