Dose-Dependent Effects of Nodule-Associated Kosakonia cowanii on Nodulation, Nitrogen Status and Yield Components of Common Bean (Phaseolus vulgaris L.) Under Greenhouse Conditions in Northern Ecuador

Abstract

This study evaluated the dose-dependent effects of a nodule-associated bacterial isolate subsequently identified by whole-genome sequencing as Kosakonia cowanii on nodulation, nitrogen status, growth, phenology, and yield components of common bean (Phaseolus vulgaris L. cv. Centenario) under greenhouse conditions in northern Ecuador. A randomized complete block design with five treatments and three replicates was used, including three inoculation doses (6 × 109, 6 × 104.5 and 6 × 102.25 CFU mL−1), a non-inoculated control and a nitrogen-fertilized control. Significant differences were detected for plant height, SPAD index, phenological development, nodulation and 100 seed weight. The lowest inoculation dose (T3) produced the strongest biological response, reaching 227.67 nodules plant−1, approximately 6.5 times more than the non-fertilized control, and the highest SPAD value at 60 days after sowing (41.16), exceeding both the non-fertilized control (26.84) and the nitrogen-fertilized treatment (33.14). T3 also achieved the greatest plant height (80.31 cm) and accelerated flowering and pod formation by up to two days compared with higher inoculation doses. Although grain yield did not differ significantly among treatments because of high experimental variability and limited statistical power, inoculated plants maintained yields comparable to the nitrogen-fertilized control while improving seed filling. Whole genome sequencing confirmed the identity of the dominant genome as K. cowanii (ANI = 96.73%; completeness = 99.03%; contamination = 0.63%), supporting the interpretation that this bacterium acts primarily as a plant growth-promoting and nodule-associated bacterium rather than a classical nitrogen-fixing symbiont. These findings highlight the potential of native K. cowanii as a component of sustainable biofertilization strategies for Andean bean production systems.