Tamasloukht M'Barek, Séjalon-Delmas Nathalie, Kluever Astrid, Jauneau Alain, Roux Christophe, Bécard Guillaume, Franken Philipp
Max-Planck-Institut für Terrestrische Mikrobiologie and Laboratorium für Mikrobiologie, Philipps-Universität, Karl-von-Frisch-Strasse, 35043 Marburg, Germany.
Plant Physiol. 2003 Mar;131(3):1468-78. doi: 10.1104/pp.012898.
During spore germination, arbuscular mycorrhizal (AM) fungi show limited hyphal development in the absence of a host plant (asymbiotic). In the presence of root exudates, they switch to a new developmental stage (presymbiotic) characterized by extensive hyphal branching. Presymbiotic branching of the AM fungus Gigaspora rosea was induced in liquid medium by a semipurified exudate fraction from carrot (Daucus carota) root organ cultures. Changes in RNA accumulation patterns were monitored by differential display analysis. Differentially appearing cDNA fragments were cloned and further analyzed. Five cDNA fragments could be identified that show induced RNA accumulation 1 h after the addition of root exudate. Sequence similarities of two fragments to mammalian Nco4 and mitochondrial rRNA genes suggested that root exudates could influence fungal respiratory activity. To support this hypothesis, additional putative mitochondrial related-genes were shown to be induced by root exudates. These genes were identified after subtractive hybridization and putatively encode a pyruvate carboxylase and a mitochondrial ADP/ATP translocase. The gene GrosPyc1 for the pyruvate carboxylase was studied in more detail by cloning a cDNA and by quantifying its RNA accumulation. The hypothesis that respiratory activity of AM fungi is stimulated by root exudates was confirmed by physiological and cytological analyses in G. rosea and Glomus intraradices. Oxygen consumption and reducing activity of both fungi was induced after 3 and 2 h of exposition with the root factor, respectively, and the first respiration activation was detected in G. intraradices after approximately 90 min. In addition, changes in mitochondrial morphology, orientation, and overall biomass were detected in G. rosea after 4 h. In summary, the root-exuded factor rapidly induces the expression of certain fungal genes and, in turn, fungal respiratory activity before intense branching. This defines the developmental switch from asymbiosis to presymbiosis, first by gene activation (0.5-1 h), subsequently on the physiological level (1.5-3 h), and finally as a morphological response (after 5 h).
在孢子萌发过程中，丛枝菌根（AM）真菌在没有宿主植物的情况下（非共生状态）菌丝发育有限。在根分泌物存在时，它们会转变到一个新的发育阶段（共生前期），其特征是菌丝大量分支。AM真菌玫瑰巨孢囊霉的共生前期分支是由胡萝卜（野胡萝卜）根器官培养物的半纯化分泌物组分在液体培养基中诱导产生的。通过差异显示分析监测RNA积累模式的变化。对差异出现的cDNA片段进行克隆并进一步分析。可以鉴定出五个cDNA片段，它们在添加根分泌物后1小时显示出诱导的RNA积累。两个片段与哺乳动物Nco4和线粒体rRNA基因的序列相似性表明根分泌物可能影响真菌的呼吸活性。为支持这一假设，其他推定的线粒体相关基因也显示受根分泌物诱导。这些基因是在消减杂交后鉴定出来的，推定编码丙酮酸羧化酶和线粒体ADP/ATP转位酶。通过克隆cDNA并定量其RNA积累，对丙酮酸羧化酶的基因GrosPyc1进行了更详细的研究。AM真菌的呼吸活性受根分泌物刺激这一假设在玫瑰巨孢囊霉和根内球囊霉中通过生理学和细胞学分析得到了证实。分别在用根因子处理3小时和2小时后，两种真菌的氧气消耗和还原活性被诱导，在根内球囊霉中大约90分钟后检测到第一次呼吸激活。此外，4小时后在玫瑰巨孢囊霉中检测到线粒体形态、方向和总体生物量的变化。总之，根分泌的因子在强烈分支之前迅速诱导某些真菌基因的表达，进而诱导真菌的呼吸活性。这定义了从非共生到共生前期的发育转变，首先是通过基因激活（0.5 - 1小时），随后在生理水平上（1.5 - 3小时），最后是作为形态学反应（5小时后）。
