Endophytic and rhizospheric bacteria associated with the phytoremediators crown flower and candle bush, and their roles in phosphate– potassium solubilization

Crown flower and candle bush show strong tolerance to the multi-constraint conditions of the Putri Cempo landfill, including drought and heavy-metal stresses that are worsening with climate change and contributing to soil degradation. This resilience is likely supported by beneficial microorganisms, particularly phosphate- and potassium-solubilizing bacteria that improve nutrient availability under adverse conditions. This study compared the population density, colony morphotypes, and nutrient-solubilization capacity of endophytic and rhizospheric bacteria from both plants to identify promising isolates for biofertilizer and bio-phytoremediation. Bacteria were isolated from rhizosphere soil and surface-sterilized leaf and root tissues using Pikovskaya medium, and their PSI and KSI were assessed on Pikovskaya and Aleksandrov media. Plant growth traits and rhizosphere characteristics were also evaluated. Candle bush growing directly on the dump-site showed lower total bacterial and PSB densities than the crown flower located 200 m away. Despite higher nutrients, elevated heavy-metal levels likely suppressed bacterial abundance. Among 38 PSB isolates, 52.63% exhibited dual phosphate- and potassium-solubilizing activity. The highest PSI (5.90) and KSI (5.11) were produced by isolates KRhizS5 and KRe8 from candle bush, while isolates BRhizS7 and BRhizS3 yielded the highest values in crown flower. These isolates show strong potential as biofertilizers and partner agents in bio-phytoremediation.