AUTHOR=Kwak Gi-Young , Choi Okhee , Goo Eunhye , Kang Yongsung , Kim Jinwoo , Hwang Ingyu 

TITLE=Quorum Sensing-Independent Cellulase-Sensitive Pellicle Formation Is Critical for Colonization of Burkholderia glumae in Rice Plants

JOURNAL=Frontiers in Microbiology

VOLUME=10

YEAR=2020

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2019.03090

DOI=10.3389/fmicb.2019.03090

ISSN=1664-302X

ABSTRACT=<p>Bacteria form biofilms as a means to adapt to environmental changes for survival. Pellicle is a floating biofilm formed at the air–liquid interface in static culture conditions; however, its functional roles have received relatively little attention compared to solid surface-associated biofilms in gram-negative bacteria. Here we show that the rice pathogen <italic>Burkholderia glumae</italic> BGR1 forms cellulase-sensitive pellicles in a bis-(3′-5′)-cyclic dimeric guanosine monophosphate (c-di-GMP)- and flagellum-dependent, but quorum sensing (QS)-independent, manner. Pellicle formation was more favorable at 28°C than at the optimum growth temperature (37°C), and was facilitated by constitutive expression of <italic>pelI</italic>, a diguanylate cyclase gene from <italic>B. glumae</italic>, or <italic>pleD</italic>, the GGDEF response regulator from <italic>Agrobacterium tumefaciens</italic>. Constitutive expression of <italic>pelI</italic> or <italic>pleD</italic> raised the levels of c-di-GMP, facilitated pellicle formation, and suppressed swarming motility in <italic>B. glumae</italic>. QS-defective mutants of <italic>B. glumae</italic> formed pellicles, while flagellum-defective mutants did not. Pellicles of <italic>B. glumae</italic> were sensitive to cellulase but not to proteinase K or DNase I. A gene cluster containing seven genes involved in bacterial cellulose biosynthesis, <italic>bcsD</italic>, <italic>bcsR</italic>, <italic>bcsQ</italic>, <italic>bcsA</italic>, <italic>bcsB</italic>, <italic>bcsZ</italic>, and <italic>bcsC</italic>, homologous to known genes involved in cellulose biosynthesis in other bacteria, was identified in <italic>B. glumae</italic>. Mutations in each gene abolished pellicle formation. These results revealed a positive correlation between cellulase-sensitive pellicles and putative cellulose biosynthetic genes. Pellicle-defective mutants did not colonize as successfully as the wild-type strain BGR1 in rice plants, which resulted in a significant reduction in virulence. Our findings show that cellulase-sensitive pellicles produced in a QS-independent manner play important roles in the interactions between rice plants and <italic>B. glumae</italic>.</p>