In Vitro Assessment of Endophytic Bacteria Originating from Cycas rumphii and Dioon spinulosum as Stress-Tolerant Plant Growth-Promoting Bacteria

<p>The ability of cycads to grow in harsh environments is thought to be associated with endophytic bacteria in their roots. This study aimed to characterize endophytic bacteria from <em>Cycas rumphii</em> and <em>Dioon spinulosum</em> as stress-tolerant plant growth-promoting bacteria (PGPB). Fine roots of <em>C. rumphii</em> and <em>D. spinulosum</em> were isolated using serial dilution plating and characterized through colony morphological observation, hypersensitivity, antagonistic activity, PGP activity, and abiotic stress tolerance assays. Fourteen isolates were obtained (eight from <em>C. rumphii</em> and six from <em>D. spinulosum</em>), with higher abundance and diversity in <em>D. spinulosum</em> (2.51 × 10¹⁰ CFU/g; H′ = 2.016) than <em>C. rumphii</em> (4.64 × 10⁹ CFU/g; H′ = 1.752). All isolates were non-pathogenic; 92% fixed nitrogen, 92% solubilized phosphate, and one (DS1A) produced IAA (0.200 ppm). Four isolates showed antagonism against <em>Xanthomonas campestris</em> (0.40–0.67 cm inhibition) and tolerance to drought, salinity (5–10%), and moderate heat, with growth at pH 5–6. Molecular identification by 16S rRNA sequencing revealed that CR1A, CR1B, and DS1B belong to <em>Stenotrophomonas maltophilia</em>, whereas DS1A belongs to <em>Bacillus </em>sp. These findings highlight cycads as a source of endophytic PGPB candidates for supporting plant health, particularly when grown in the environmental stress condition areas.</p>