Following rhizoplane colonization, internalization of the bacteria and their development as endophytes can occur quite rapidly, within days of inoculation/rhizoplane contact (Compant et al. 2008; Prieto and Mercado — Blanco 2008; Zakria et al. 2008). In order to colonize the plant interior, bacteria must make their way past the root surface. This can happen through the presence of surface openings, such as cracks produced during lateral root emergence (James and Olivares 1998), or other wounds. Furthermore, other root areas, such as the elongation and differentiation regions may contain cells that are more fragile or less differentiated, and more susceptible to bacterial penetration (Reinhold-Hurek and Hurek

2011) . As with rhizosphere and rhizoplane colonization competence, a variety of bacterial traits are associated with competence for endophytic colonization. These include flagella, nod genes, type IV pili and twitching motility (Compant et al. 2008). Many of these traits are associated with bacterial adherence and movement, or bio-control of other surrounding microorganisms, providing a competitive advantage for the colonizing bacteria. In addition, bacterially-secreted, cell wall-degrading enzymes are important for bacterial penetration (Quadt-Hallmann et al. 1997) and internal colonization, including cellulolytic and pectinolytic enzymes (Quadt-Hallmann et al. 1997; Kovtunovych et al. 1999). The endophyte Burkholderia phytofirmans strain PsJN, known to colonize switchgrass (Kim et al. 2012), produces endogluconase and polygalacturonase (Compant et al. 2005b), to aid in cell wall degradation.

Following initial root penetration, bacterial colonization proceeds within the root cortex, and can extend into the central vascular cylinder xylem vessels (Compant et al. 2008; Priedo and Mercado-Blanco 2008; Zakria et al. 2008). However, not all bacterial endophytes colonize the xylem. For example, Priedo and Mercado-Blanco (2008) noted that Pseudomonas fluorescens PICF7 remained in the root cortex region and was never found in the xylem, with no subsequent translocation elsewhere. The inability of some endophytes to colonize the xylem and move past the root may be due to the presence of filters formed at branch root junctions (Shane et al. 2000), which may limit bacterial movement (Zakria et al. 2008). In addition, as endophytes are aided in their penetration through the root endodermis and pericycle by cell wall-degrading enzymes (James et al. 2002), it may be that some endophytes produce sub-optimal levels of enzymes to allow penetration into the vascular tissue.

Colonization within regions like the root cortex occurs within the intercellular spaces, outside of living cells (Reinhold-Hurek and Hurek 1998; Priedo and Mercado-Blanco 2008), which is not surprising as these are rich in minerals (potassium, calcium, sulfur, phosphorus, chlorine), sugars (Madore and Webb 1981) and non-carbohydrate metabolites, such as various amino acids and organic acids (Canny and McCully 1988; Canny and Huang 1993). Endophyte alterations of apoplastic pH can alter enzyme activities, sugar uptake of host cells, and sugar concentrations for the colonizing microbes (Bacon and Hinton 2002). Hence, this environment is supportive of endophyte growth, promoting compound cycling between the endophyte and the plant.