Max-Planck-Institut für terrestrische Mikrobiologie, D-35043 Marburg, Germany.
Proc Natl Acad Sci U S A. 2013 Aug 20;110(34):13965-70. doi: 10.1073/pnas.1301653110. Epub 2013 Aug 5.
The mechanisms underpinning broad compatibility in root symbiosis are largely unexplored. The generalist root endophyte Piriformospora indica establishes long-lasting interactions with morphologically and biochemically different hosts, stimulating their growth, alleviating salt stress, and inducing local and systemic resistance to pathogens. Cytological studies and global investigations of fungal transcriptional responses to colonization of barley and Arabidopsis at different symbiotic stages identified host-dependent colonization strategies and host-specifically induced effector candidates. Here, we show that in Arabidopsis, P. indica establishes and maintains biotrophic nutrition within living epidermal cells, whereas in barley the symbiont undergoes a nutritional switch to saprotrophy that is associated with the production of secondary thinner hyphae in dead cortex cells. Consistent with a diversified trophic behavior and with the occurrence of nitrogen deficiency at the onset of saprotrophy in barley, fungal genes encoding hydrolytic enzymes and nutrient transporters were highly induced in this host but not in Arabidopsis. Silencing of the high-affinity ammonium transporter PiAMT1 gene, whose transcripts are accumulating during nitrogen starvation and in barley, resulted in enhanced colonization of this host, whereas it had no effect on the colonization of Arabidopsis. Increased levels of free amino acids and reduced enzymatic activity for the cell-death marker VPE (vacuolar-processing enzyme) in colonized barley roots coincided with an extended biotrophic lifestyle of P. indica upon silencing of PiAMT1. This suggests that PiAmt1 functions as a nitrogen sensor mediating the signal that triggers the in planta activation of the saprotrophic program. Thus, host-related metabolic cues affect the expression of P. indica's alternative lifestyles.
根共生广泛兼容性的基础机制在很大程度上仍未得到探索。广谱根系内生真菌梨形侧耳与形态和生物化学上不同的宿主建立持久的相互作用，刺激它们的生长，缓解盐胁迫，并诱导对病原体的局部和系统抗性。细胞学研究和真菌对大麦和拟南芥不同共生阶段定植的转录反应的全球调查确定了宿主依赖的定植策略和宿主特异性诱导的效应子候选物。在这里，我们表明在拟南芥中，梨形侧耳在活表皮细胞内建立并维持生物营养，而在大麦中，共生体经历营养转变为腐生，这与死皮层细胞中次生较薄菌丝的产生有关。与多样化的营养行为一致，并且在大麦腐生开始时出现氮缺乏，真菌编码水解酶和养分转运蛋白的基因在这种宿主中高度诱导，但在拟南芥中没有诱导。高亲和力铵转运蛋白 PiAMT1 基因的沉默，其转录物在氮饥饿期间和大麦中积累，导致对该宿主的定植增强，而对拟南芥的定植没有影响。在被梨形侧耳定殖的大麦根中，游离氨基酸水平升高，细胞死亡标记物 VPE（液泡加工酶）的酶活性降低，这与 PiAMT1 沉默后梨形侧耳延长生物营养生活方式相一致。这表明 PiAmt1 作为氮传感器发挥作用，介导触发腐生程序在植物体内激活的信号。因此，宿主相关的代谢线索影响梨形侧耳替代生活方式的表达。
