MSPG3, A MEDICAGO-SATIVA POLYGALACTURONASE GENE EXPRESSED DURING THE ALFALFA RHIZOBIUM-MELILOTI INTERACTION

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 ident ified two classes of cDNA fragments corresponding to two classes of PG genes in the Medicago genome. One of this class, represented by E-2 i n M. truncatula and Pl(1) in M, sativa, seems to be related to previou sly characterized plant PG genes expressed in pollen, We have isolated the genomic clone containing the entire gene corresponding to the sec ond class (E-3) We showed that MsPG3 is a single gene in the Medicago genome coding for PG, By reverse transcription-PCR, MsPG3 expression w as detected in roots 1 day after Rhizobium inoculation. The early indu ction of the MsPG3, as also seen by ill situ hybridization experiments , supports its involvement in the early stages of the Rhizobium-legume infection process, In addition, by analyzing the expression of a MsPG 3 promoter-gus construct in Vicia hirsuta-transgenic root nodules, we showed that MsPG3 was expressed in all cells of nodule primordia and i n the cells of the invasion zone. By Northern blot, MsPG3 transcripts are not detected in various Medicago tissues, indicating that the func tion of this gene is related closely to symbiosis, Thus, our results s trongly suggest the involvement of MsPG3 gene during meristem formatio n 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-specif ic genes, and it is the first pectic encoded enzyme demonstrated to be involved in Rhizobium-legume symbiosis.