Muñoz J A, Coronado C, Pérez-Hormaeche J, Kondorosi A, Ratet P, Palomares A J
Departamento de Microbiología y Parasitología. Facultad de Farmacia, Universidad de Sevilla, 41012 Sevilla, Spain.
Proc Natl Acad Sci U S A. 1998 Aug 4;95(16):9687-92. doi: 10.1073/pnas.95.16.9687.
Polygalacturonase (PG) is one of the most important enzymes associated with plant cell wall degradation. It has been proposed to participate in the early steps of the Rhizobium-legume interaction. We have identified two classes of cDNA fragments corresponding to two classes of PG genes in the Medicago genome. One of this class, represented by E2 in M. truncatula and Pl1 in M. sativa, seems to be related to previously characterized plant PG genes expressed in pollen. We have isolated the genomic clone containing the entire gene corresponding to the second class (E3). We showed that MsPG3 is a single gene in the Medicago genome coding for PG. By reverse transcription-PCR, MsPG3 expression was detected in roots 1 day after Rhizobium inoculation. The early induction of the MsPG3, as also seen by in situ hybridization experiments, supports its involvement in the early stages of the Rhizobium-legume infection process. In addition, by analyzing the expression of a MsPG3 promoter-gus construct in Vicia hirsuta-transgenic root nodules, we showed that MsPG3 was expressed in all cells of nodule primordia and in the cells of the invasion zone. By Northern blot, MsPG3 transcripts are not detected in various Medicago tissues, indicating that the function of this gene is related closely to symbiosis. Thus, our results strongly suggest the involvement of MsPG3 gene during meristem formation and/or in the infection process, probably by facilitating cell wall rearrangement, penetration of the bacteria through the root hair wall, or infection thread formation and release of bacteria in plant cells. MsPG3 represents a class of PG genes, distinct from the pollen-specific genes, and it is the first pectic encoded enzyme demonstrated to be involved in Rhizobium-legume symbiosis.
多聚半乳糖醛酸酶（PG）是与植物细胞壁降解相关的最重要的酶之一。有人提出它参与根瘤菌与豆科植物相互作用的早期步骤。我们在苜蓿基因组中鉴定出了两类与两类PG基因相对应的cDNA片段。其中一类，以蒺藜苜蓿中的E2和紫花苜蓿中的Pl1为代表，似乎与先前在花粉中表达的已鉴定的植物PG基因有关。我们分离出了包含对应于第二类（E3）的完整基因的基因组克隆。我们表明MsPG3是苜蓿基因组中编码PG的单个基因。通过逆转录PCR，在接种根瘤菌1天后在根中检测到MsPG3的表达。原位杂交实验也表明，MsPG3的早期诱导支持其参与根瘤菌 - 豆科植物感染过程的早期阶段。此外，通过分析MsPG3启动子 - gus构建体在硬毛野豌豆转基因根瘤中的表达，我们表明MsPG3在根瘤原基的所有细胞和侵入区的细胞中表达。通过Northern印迹分析，在各种苜蓿组织中未检测到MsPG3转录本，表明该基因的功能与共生密切相关。因此，我们的结果强烈表明MsPG3基因参与分生组织形成和/或感染过程，可能是通过促进细胞壁重排、细菌穿过根毛壁的穿透、感染丝的形成以及细菌在植物细胞中的释放。MsPG3代表一类与花粉特异性基因不同的PG基因，并且它是第一个被证明参与根瘤菌 - 豆科植物共生的果胶编码酶。
