Living within plant tissues means that endophytic bacteria are adapted to an environment providing a constant and steady source of nutrients. They are also less exposed to (a)biotic stresses prevailing in the rhizosphere. In return, endophytic bacteria have developed strategies enabling them to overcome, modulate or evade defence responses deployed by the host plant against penetration of microorganisms, thereby being recognized as 'non hostile: Our studies have also shown that colonization and establishment of strain PICF7 in olive roots causes local and systemic transcriptomic changes, many of them related to defensive responses. There is an increasing interest on endophytic bacteria due to numerous applications they may have in agricultura! biotechnology. Among the beneficia! properties associated with endophytic lifestyle, promotion of plant growth and control of diseases have a special significance. Promotion of growth can be due to direct or indirect mechanisms. Direct plant growth promotion can be due to mobilization of (micro )nutrients (biofertilization) or synthesis of phytohormones (phytostimulation). Indirect plant growth promotion is usually a consequence of suppressing the effects caused by deleterious microorganisms. In this case, mechanisms involved can be different: antagonism or direct antibiosis against the pathogen, competition for nutrients and/ or space, or stimulation of defensive mechanisms of the plant host against pathogens attack. Endophytic bacteria can also stimulate plant growth by aiding the host to cope with presence of pollutants or heavy metals (rhizoremediation) or water and saline stresses. Many questions on how, when and why a bacteria! consortium is set as endophytic in any given host plant remain to be elucidated. They can basically be summarized in: (i) What are the driving forces operating to build a certain endophytic community? and (ii) what does the endophytic microbiome do for the host plant? To know the answers to these questions will be helpful to take full advantage of the potential of endophytic bacteria as sustainable and environmentfriendly biotechnological tools in agriculture and forestry.

Endophytes (bacteria and fungi) can be detected at any moment within the tissues of healthy plants without producing any visible deleterious symptom. It is generally accepted that all plants harbour highly diverse endophytic microbiomes playing relevant roles, poorly understood though, for the growth, health, development and fitness of the host. These microbial communities also help the plant to cope with different (a)biotic stresses. Current powerful metagenomics and culture-independent approaches are revealing that endophytic communities are much more diverse and complex than information obtained by traditional culture-dependent methods. Endophytic bacteria penetrate into plant tissues primarily by the roots, although they can also do so through leaves, stems, flowers, or cotyledons. Sorne can move systemically and be localized at distant tissues from the original penetration place. They can enter passively through cracks, wounds or points of emergence oflateral roots. Active penetration appears to take place preferably by the root differentiation and elongation regions, using the intercellular spaces of the root epidermis. Our studies have shown that root hairs play an important role in the endophytic colonization of olive (Oleae europaea L.) roots by strains of Pseudomonas spp. However, the knowledge on bacteria! traits involved in both inner colonization and persistence of plant tissues is scarce. The type IV 'pili: lipopolysaccharide and exopolysaccharides have been confirmed as crucial components. On the contrary, we have demonstrated that 'swimming' motility and production of the siderophore pyoverdine are not involved in the endophytic colonization ability of olive roots by strain Pseudomonas fluorescens PICF7. Comparative analysis of endophytic bacteria genomes is currently offering valuable information to identify bacteria! traits involved in endophytic lifestyle, although data are is still scant and controversia!.

Trabajo presentado en el International Congress Soil and food, resources for a healthy life (The application of beneficial microorganisms in sustainable agriculture), celebrado en Lasi (Rumanía) del 22 al 24 de octubre de 2015.

Research supported by grants P07-CVI02624 and P12-AGR667 (Junta de Andalucía, Spain), co-financed by ERDF from the EU.

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