39 Seed Bacterial Endophytes: Common Genera, Seed-to-Seed Variability and Their Possible Role in Plants M. Rosenblueth, A. López-López, J. Martínez, M.A. Rogel, I. Toledo and E. Martínez-Romero Centro de Ciencias Genómicas, Universidad Nacional Autónoma de México Cuernavaca, Mor. Mexico Keywords: corn symbiosis, Phaseolus vulgaris, beans, biodiversity, inoculants, plant growth Abstract Seeds contain endophytic bacteria that may be transmitted from generation to generation. Some of these bacteria can benefit plant growth and defense against abiotic and biotic stresses. Little is known however about the mechanisms of bacterial colonization of seeds and their transmission from generation to generation in host plants. In this study we have demonstrated that individual seeds of maize (Zea mays L.) taken from the same cob and bean seeds (Phaseolus vulgaris L.) from different pods and within individual pods differ in their bacterial content and population diversity. We suggest that this bacterial variability within seed population of individual plants may contribute to the species adaptation to diverse environments and be harnessed in the production of crop plants. INTRODUCTION Bacteria and fungi can be vertically transmitted in plants by seeds (Mundt and Hinkle, 1976; Baker and Smith, 1966). Thus one would expect natural selection to favor host plants that tightly control the kind and number of microbes that migrate into the developing seeds. Although diverse bacterial species have been found as endophytes colonizing different plant tissues, some genera are more frequently recovered as plant or seed endophytes than others (Cankar et al., 2005; Rosenblueth and Martinez-Romero, 2006; Compant et al., 2008; Hardoim et al., 2008; Mano and Morisaki, 2008; Ulrich et al., 2008; Mastretta et al., 2009; Puente et al., 2009). With a novel strategy of stable-isotope-probing of 13 C-DNA containing endo- phytes that were recovered from plants fed with 13 CO 2 Rasche et al. (2009) detected Acinetobacter. A new species of Cohnella was also obtained from Phaseolus coccineus as a nodule endophyte (Garcia-Fraile et al., 2008), and a new species of Paenibacillus was isolated from surface-sterilized seeds of peas Pisum sativum (Smerda et al., 2005). Similarly, we identified several novel lineages among bacteria in bean seeds corresponding to a number of genera (Bacillus, Enterococcus, Nocardioides, Knoellia, Acinetobacter, Rhizobium, Phyllobacterium, Paracoccus and Sphingomonas) within phyla Firmicutes, Actinobacteria, and alpha, gamma and beta Proteobacteria (López- López et al., 2010). A novel Rhizobium species, R. endophyticum, was also identified and found not to be symbiotic (López-López et al., 2010). The non-symbiotic rhizobia were also found in the seed interior and symbiotic R. etli on the seed coats (Perez-Ramírez et al., 1998). Symbiotic rhizobia were also found as natural maize endophytes (Gutiérrez- Zamora and Martínez-Romero, 2001). In this study we report endophytic bacteria of individual seeds extracted from maize (Zea mays) and bean (Phaseolus vulgaris L.) plants. Both bean and maize are suitable for studies on seed endophytes as their seeds are large and can easily be extracted from surface disinfected pods or cobs. MATERIALS AND METHODS Seeds from Phaseolus vulgaris cultivars DOR 364 and BAT 477 were surface disinfected, placed in 5 lt pots containing sterilized vermiculite, irrigated with free N Proc. XXVIII th IHC – IS on Envtl., Edaphic & Gen. Factors Affecting Plants, Seeds and Turfgrass Eds.: G.E. Welbaum et al. Acta Hort. 938, ISHS 2012