The ability of some endophytes to colonize the xylem provides the opportunity for their systemic spread throughout the rest of the plant, via the transpirational stream in the xylem lumen. However, not all endophytes are capable of colonizing the aerial parts of plants. This may reflect the inability of some to adapt and survive the different niches represented by aerial tissues and organs (Compant et al. 2010). In switchgrass, B. phytofirmans strain PsJN titers were higher in the root than in the leaves 7 days post­inoculation of the roots. However, by 14 days post-inoculation, titers were higher in leaves and sheaths than in the roots, indicating translocation to these tissues (Kim et al. 2012). Generally, bacterial endophyte titers in the aerial plant tissues are reported to be lower than in the root (Rosenbleuth and Martinez-Romero 2006; Compant et al. 2008). In addition, a fair amount of variation can be observed in these tissues. Compant et al. (2008) reported that PsJN could be found in only 10-60% of grape inflorescence stalks and grape berries following initial inoculation of roots. These were localized to xylem vessels, and only a single or few cells were observed. These results further indicated the importance of the xylem for systemic spread of endophytes, allowing them to reach as far as the reproductive tissues. However, this spread was very slow, taking 5 weeks to reach inflorescence tissues. The very low titers of PsJN that ended up in these tissues was attributed to competition with other co-localized endophytes, which can inhabit different tissues and organs, reflecting different niches of colonization (Compant et al. 2011). This report of endophytic bacteria being low or absent in flowers and fruits echoes other comments (Hallman 2001), suggesting low vertical transmission. Bacterial colonization, in general, varies from one cultivar to another and depends on many factors. For example, in soybean, plant genotype, tissue age, season of isolation, and herbicide application, all affected colonization (Kuklinsky-Sobral et al. 2004).