Morphogenetic modifications induced by Pseudomonas fluorescens A6RI and Glomus mosseae BEG12 in the root system of tomato differ according to plant growth conditions

The ability of fluorescent pseudomonads and glomalean fungi to promote plant growth has been reported many times. However, little is known of their effects on root morphogenesis. Growth and root morphogenesis were compared in tomato (Lycopersicon esculentum) plants inoculated or not with a model strain of Pseudomonas fluorescens (A6RI) or with the arbuscular mycorrhizal fungus Glomus mosseae (BEG12). Plants were cultivated in a sandy-loam soil mixed with sand at two different ratios (2 : 1 and 1 : 2), in gnotobiotic conditions. Plant growth was evaluated by measuring root and shoot fresh weight, and various morphometric parameters were compared. Growth of control plants was less in the 1 : 2 than in the 2 : 1 soil : sand mixture, and root morphogenesis was also significantly altered by soil:sand mixture. P. fluorescens A6RI promoted plant growth and increased total root length, surface area and volume in the more fertile soil:sand mixture (2 : 1) whereas G. mosseae BEG12 did the same in the less fertile soil : sand mixture (1 : 2). The relevance of differences between the two types of organisms in relation to the environmental conditions favourable for the expression of their beneficial effects is discussed in the context of microbial inoculation.