Mitra Raka Mustaphi, Long Sharon Rugel
Department of Biological Sciences, Stanford University, Stanford, California 94305, USA.
Plant Physiol. 2004 Feb;134(2):595-604. doi: 10.1104/pp.103.031518. Epub 2004 Jan 22.
In the Medicago truncatula/Sinorhizobium meliloti symbiosis, the plant undergoes a series of developmental changes simultaneously, creating a root nodule and allowing bacterial entry and differentiation. Our studies of plant genes reveal novel transcriptional regulation during the establishment of the symbiosis and identify molecular markers that distinguish classes of plant and bacterial symbiotic mutants. We have identified three symbiotically regulated plant genes encoding a beta,1-3 endoglucanase (MtBGLU1), a lectin (MtLEC4), and a cysteine-containing protein (MtN31). MtBGLU1 is down-regulated in the plant 24 h after exposure to the bacterial signal, Nod factor. The non-nodulating plant mutant dmi1 is defective in the ability to down-regulate MtBGLU1. MtLEC4 and MtN31 are induced 1 and 2 weeks after bacterial inoculation, respectively. We examined the regulation of these two genes and three previously identified genes (MtCAM1, ENOD2, and MtLB1) in plant symbiotic mutants and wild-type plants inoculated with bacterial symbiotic mutants. Plant (bit1, rit1, and Mtsym1) and bacterial (exoA and exoH) mutants with defects in the initial stages of invasion are unable to induce MtLEC4, MtN31, MtCAM1, ENOD2, and MtLB1. Bacterial mutants (fixJ and nifD) and a subset of plant mutants (dnf2, dnf3, dnf4, dnf6, and dnf7) defective for nitrogen fixation induce the above genes. The bacA bacterial mutant, which senesces upon deposition into plant cells, and two plant mutants with defects in nitrogen fixation (dnf1 and dnf5) induce MtLEC4 and ENOD2 but not MtN31, MtCAM1, or MtLB1. These data suggest the presence of at least three transcriptionally distinct developmental stages during invasion of M. truncatula by S. meliloti.
在蒺藜苜蓿/苜蓿中华根瘤菌共生体系中，植物会同时经历一系列发育变化，形成根瘤并允许细菌侵入和分化。我们对植物基因的研究揭示了共生建立过程中全新的转录调控机制，并鉴定出了区分植物和细菌共生突变体类别的分子标记。我们已经鉴定出三个受共生调控的植物基因，分别编码一种β-1,3-内切葡聚糖酶（MtBGLU1）、一种凝集素（MtLEC4）和一种含半胱氨酸的蛋白质（MtN31）。在植物暴露于细菌信号结瘤因子24小时后，MtBGLU1表达下调。非结瘤植物突变体dmi1在下调MtBGLU1的能力上存在缺陷。MtLEC4和MtN31分别在细菌接种后1周和2周被诱导表达。我们研究了这两个基因以及之前鉴定出的三个基因（MtCAM1、ENOD2和MtLB1）在接种细菌共生突变体的植物共生突变体和野生型植物中的调控情况。在侵染初始阶段存在缺陷的植物（bit1、rit1和Mtsym1）和细菌（exoA和exoH）突变体无法诱导MtLEC4、MtN31、MtCAM1、ENOD2和MtLB1。固氮存在缺陷的细菌突变体（fixJ和nifD）以及一部分植物突变体（dnf2、dnf3、dnf4、dnf6和dnf7）能够诱导上述基因表达。在沉积到植物细胞后就衰老的bacA细菌突变体以及两个固氮存在缺陷的植物突变体（dnf1和dnf5）能够诱导MtLEC4和ENOD2，但不能诱导MtN31、MtCAM1或MtLB1。这些数据表明，在苜蓿中华根瘤菌侵染蒺藜苜蓿的过程中，至少存在三个转录上不同的发育阶段。
