Rock Weathering, Plant Growth-promoting Bacteria from Desert Plants Allow the Growth of Cactus Seedling in Rocks

ABSTRACT Light, fluorescent, and field emission scanning electron microscopy, combined with microbial counts on culture media, revealed the presence of numerous microorganisms (bacteria, fungi, and actimomycetes) living as rhizoplane populations of roots of at least three species of cactus and a wild fig tree growing in rocks without soil. These plants are responsible for weathering of volcanic rock in rocky areas of Baja California Sur, Mexico. The dominant bacterial groups colonizing the rhizoplane were fluorescent pseudomonads and bacilli. N2-fixation, solubilization of insoluble phosphate, and rock disintegration to a powder by some of the root-colonizing microorganisms was significant. These bacteria were thermotolerant and halotolerant and capable of significantly depleting the useful minerals, such as P, K, Mg, Mn, Fe, Cu, and Zn in the rocks. Cacti inoculated with these bacteria and growing in crushed volcanic rocks grew normally without N and P fertilization for at least 12 months. Uninoculated cacti barely grew and many died. The microbial community survived in the rhizoplane of cacti during the 10-month annual dry season. This study may show the possible involvement of rhizoplane bacteria, possibly plant growth-promoting bacteria, in rock weathering by cacti growing in hot deserts.