To study root colonization by strain IR-94MF1 of Penicillium rugulosum a transformant w-T3, resistant to hygromicin B
was produced by Reyes et al. [25]. In a non-sterile soil microcosm assay, colonization of the rhizosphere of 5-week-old
maize plants by w-T3 was significantly affected by the source of P used. The lowest rate of colonization was observed in the
non-fertilized control or when ordinary superphosphate was added, while a tenfold increase in colonization was observed
when the Navay PR from Venezuela was used. Interestingly, plants inoculated with w-T3 and fertilized with Navay PR
contained significantly more P in their shoots, confirming the importance of a good root colonization by PSM, to properly