Ratcliff William C, Kadam Supriya V, Denison Robert Ford
Ecology, Evolution and Behavior, University of Minnesota, Minneapolis, MN 55108, USA.
FEMS Microbiol Ecol. 2008 Sep;65(3):391-9. doi: 10.1111/j.1574-6941.2008.00544.x. Epub 2008 Jul 8.
The carbon that rhizobia in root nodules receive from their host powers both N(2) fixation, which mainly benefits the host, and rhizobium reproduction. Rhizobia also store energy in the lipid poly-3-hydroxybutyrate (PHB), which may enhance rhizobium survival when they are carbon limited, either in nodules or in the soil between hosts. There can be a conflict of interest between rhizobia and legumes over the rate of PHB accumulation, due to a metabolic tradeoff between N(2) fixation and PHB accumulation. To quantify the benefits of PHB to carbon-limited rhizobia, populations of genetically uniform rhizobia with high vs. low PHB (confirmed by flow cytometry) were generated by fractionating Sinorhizobium meliloti via density gradient centrifugation, and also by harvesting cells at early vs. late stationary phase. These rhizobia were starved for 165 days. PHB use during starvation was highly predictive of both initial reproduction and long-term population maintenance. Cultured S. meliloti accumulated enough PHB to triple their initial population size when starved, and to persist for c. 150 days before the population fell below its initial value. During the first 21 days of nodule growth, undifferentiated S. meliloti within alfalfa nodules accumulated enough PHB to support significant increases in reproduction and survival during starvation.
根瘤中的根瘤菌从宿主那里获得的碳，既为主要造福宿主的固氮作用提供能量，也为根瘤菌的繁殖提供能量。根瘤菌还将能量储存在脂质聚-3-羟基丁酸酯（PHB）中，当它们在根瘤中或宿主间的土壤中面临碳限制时，PHB可能会提高根瘤菌的存活率。由于在固氮作用和PHB积累之间存在代谢权衡，根瘤菌和豆科植物在PHB积累速率上可能存在利益冲突。为了量化PHB对碳限制条件下根瘤菌的益处，通过密度梯度离心法对苜蓿中华根瘤菌进行分级分离，并在对数生长期早期和晚期收获细胞，从而获得了具有高PHB和低PHB（通过流式细胞术确认）的基因一致的根瘤菌群体。这些根瘤菌饥饿培养165天。饥饿期间PHB的利用情况对初始繁殖和长期种群维持都具有高度预测性。培养的苜蓿中华根瘤菌在饥饿时积累了足够的PHB，使其初始种群数量增加两倍，并在种群数量降至初始值以下之前持续存活约150天。在根瘤生长的前21天，苜蓿根瘤内未分化的苜蓿中华根瘤菌积累了足够的PHB，以支持饥饿期间繁殖和存活的显著增加。
