J Basic Microbiol. 2019;59:1031–1039. www.jbm-journal.com © 2019 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim | 1031 Received: 7 April 2019 | Revised: 20 June 2019 | Accepted: 16 July 2019 DOI: 10.1002/jobm.201900202 RESEARCH PAPER Occurrence of diversified N‐acyl homoserine lactone mediated biofilm‐forming bacteria in rice rhizoplane Viveka Balasundararajan | Balachandar Dananjeyan Department of Agricultural Microbiology, Tamil Nadu Agricultural University, Coimbatore, India Correspondence Dananjeyan Balachandar, Department of Agricultural Microbiology, Tamil Nadu Agricultural University, Coimbatore 641003, India. Email: dbalu@tnau.ac.in Funding information Ministry of Human Resource Development, New Delhi, India, Grant/ Award Number: F. No. 5‐5/2014 – TS VII Abstract Quorum sensing (QS)‐mediated biofilm‐forming rhizobacteria are indispensa- ble due to their competitiveness in the crop rhizosphere. In the present work, we have reported on the occurrence of diversified bacterial species capable of producing N‐acyl homoserine lactone (AHL) as the QS signal in the roots of a rice plant grown under field conditions. The AHL‐producing bacteria were directly isolated from the rice root by the biosensor reporter (Chromobacterium violaceum CV026) overlay method and characterized for biofilm production by the microtiter plate method. A total of 48 QS‐positive bacterial isolates were purified from different aged (7, 20, 24, 26, and 36 days) rice seedlings. The in vitro biofilm production and genetic diversity as revealed by BOX‐PCR fingerprinting showed high variability among the isolates. Most of the best biofilm‐forming isolates produced a N‐butyryl DL‐homoserine lactone (a C4‐ AHL type) signal in the medium. The 16S ribosomal RNA (rRNA) gene sequence of these putative elite isolates identified that they were close to Aeromonas hydrophila (QS7‐4; QS36‐2), A. enteropelongenes (QS20‐8), A. veronii (QS36‐3), Enterobacter sp. (QS20‐11), Klebsiella pneumoniae (QS24‐6), Kosako- nia cowanii (QS24‐21), Providentia rettigeri (QS24‐2), Sphingomonas aquatilis (QS24‐17), and Pseudomonas sihuiensis (QS24‐20). These strains profusely colonized the rice root upon inoculation and formed biofilms on the surface of the root under gnotobiotic conditions. Developing inoculants from these strains would ensure competitive colonization on the rhizoplane of the crop through their biofilm and thereby improve plant growth and health. KEYWORDS biofilm, N‐acyl homoserine lactone, plant‐associated bacteria, quorum sensing, rice 1 | INTRODUCTION The soil microbiome and its role in soil and plant health are well known [1]. However, root ‐associated microorganisms, recently designated as rhizo‐ micro- biome, have not yet fully exploited for the improve- ment of nutrient supply and for protection of crops against biotic and abiotic stresses [2]. The soil microorganisms, which are attracted by the root chemicals migrate and proliferate in the soil around the root regions or on the surface of the root, and this process is known as rhizosphere colonization. This event is the primary step for the soil micro- organisms either to cause disease or to benefit the plant they colonize [3]. When the beneficial bacteria colonize the rhizosphere on the plant root, they normally promote plant growth through several mechanisms and also suppress or eliminate the