Abstract. Psam 1 is a single-copy gene which is activated during early plant-fungal interaction in wild- type pea inoculated with Glomus mosseae and which codes for PSAM 1, a putative protein of 108 amino acids. A synthetic peptide was designed in an antigenic region of this protein to produce a polyclonal antibody against PSAM 1 and to investigate its cellular localiza- tion. Western blot analysis revealed that a polypeptide of about 14.5 kDa accumulated more in mycorrhizal than non-mycorrhizal pea roots. The PSAM 1 antigen was immunolocated in planta in arbuscule-containing cells of mycorrhizal roots and especially in the cyto- plasm surrounding young arbuscules in cortical cells, which suggests that its accumulation is somehow related to the symbiotic state of these cells. Key words: Glomus ± Pisum ± Symbiosis-related protein ± Mycorrhiza Soil-borne fungi belonging to the order Glomales (Zygomycetes) are obligate symbionts that are able to colonize the roots of most higher plants (Newman and Reddell 1987) to form arbuscular mycorrhizas (AMs) which are generally mutually bene®cial associations. The AM fungi obtain carbohydrate from the host whilst contributing to the transfer of mineral nutrients, pre- dominantly phosphate, from the soil to the plant (Koide and Schreiner 1992). The establishment of AM symbi- osis involves a morphologically well-de®ned series of events. This starts with appressorium formation on the root surface from which a fungal hypha penetrates and develops into the root tissues, and culminates in the formation of arbuscules which is accompanied by major cellular and metabolic changes in both plant and fungal cells (Gianinazzi-Pearson 1996). The integration of both partners leading to a functional symbiosis is considered to result from the production and perception of signals which spatially and temporally regulate the expression of symbiosis-related (SR) genes (Gianinazzi et al. 1995). The isolation of isogenic pea mutants unable to form an AM has provided proof that plant SR genes do control AM development (Duc et al. 1989). Although the colonization process of roots by AM fungi has been clearly described, relatively little is known of the molecular mechanisms and the nature of signalling between the two partners required for the formation and the functioning of the symbiosis. The search for plant genes whose expression is modi®ed in response to AM colonization is one of the most challenging ®elds of mycorrhizal research today. Reports on changes in plant gene expression following AM colonization have mainly been concerned with weak or transient increases in plant defense response (Gianinazzi-Pearson et al. 1996). More-recent investiga- tions have revealed that novel plant genes are also dierentially expressed during AM development (Tahiri- Alaoui and Antoniw 1996; Murphy et al. 1997; Burleigh and Harrison 1997). Analysis of RNA accumulation in the AM of pea has led to the isolation of plant cDNAs corresponding to genes which are up- or down-regulated during the early stages of symbiosis development (Mar- tin-Laurent et al. 1996). One of these cDNA fragments represents a single-copy gene, psam 1, the expression of which is enhanced during early plant-fungal interactions in wild-type pea inoculated with G. mosseae (Martin- Laurent et al. 1997). Comparison of psam 1 expression during AM interactions in wild-type pea (Myc + Nod + ) and in the non-nodulating mutant (Myc + Nod ) ) showed that gene induction is speci®c to early my- corrhiza development, and not linked to rhizobial infection (Martin-Laurent et al. 1997). Psam 1 encodes for PSAM 1 a protein of 108 amino acids with a theoretical molecular weight of 12.08 kDa and an Abbreviations: AM  arbuscular mycorrhiza; SR  symbiosis- related Correspondence to: V. Gianinazzi-Pearson; E-mail: gianina@epoisses.inra.fr; Fax: 33(3)80633263 Planta (1998) 207: 153±157 Rapid communication Cellular localization of a plant protein PSAM 1 in arbuscular mycorrhizas of Pisum sativum Fabrice Martin-Laurent 1 , Christine Arnould 1 , Odile Chatagnier 1 , Diederik van Tuinen 1 , Philipp Franken 2 , Silvio Gianinazzi 1 , Vivienne Gianinazzi-Pearson 1 1 Laboratoire de Phytoparasitologie, INRA/CNRS, CMSE-INRA, BV 1540, F-21034 Dijon Cedex, France 2 Max-Planck-Institut fuÈr Terrestrische Mikrobiologie, Abteilung Biochemie und Laboratorium fuÈr Mikrobiologie, Philipps-UniversitaÈt, Karl-von-Frisch-Strasse, D-35043 Marburg, Germany Received: 27 June 1998 / Accepted: 27 July 1998