Identification of the nitrogen-fixing Pseudomonas stutzeri major flagellar gene regulator FleQ and its role in biofilm formation and root colonization

Flagellar biosynthesis and motility are subject to a four-tiered transcriptional regulatory circuit in Pseudomonas,and the master regulator FleQ appears to be the highest-level regulator in this hierarchical regulatory cascade.Pseudomonas stutzeri A1501 is motile by a polar flagellum;however,the motility and regulatory mechanisms involved in this process are unknown.Here,we searched the A1501 genome for flagella and motility genes and found that approximately 50 genes,which were distributed in three non-contiguous chromosomal regions,contribute to the formation,regulation and function of the flagella.The non-polar mutation of fleQ impaired flagellar biosynthesis,motility and root colonization but enhanced biofilm formation.FleQ positively regulates the expression of flagellar class Ⅱ-Ⅳ genes,suggesting a regulatory cascade that is coordinated similar to that of the well-known P.aeruginosa.Based on our results,we propose that flagellar genes in P.stutzeri A1501 are regulated in a cascade regulated by FleQ and that flagellum-driven motility properties may be necessary for competitive rhizosphere colonization.

Biofilm-overproducing Bacillus subtilis B12ΔYwcc decreases Cd uptake in Chinese cabbage through increasing Cd-immobilizing related gene abundance and root surface colonization

Biofilm-producing bacteria can decrease Cd uptake in vegetables, but mechanisms underlying this effect are poorly characterized. In this study, two mutant strains B12ΔYwcc and B12ΔSlr R were constructed from a biofilm-producing Bacillus subtilis strain B12. Then, the impacts of strain B12 and its high biofilm-producing mutant strain B12ΔYwcc and low biofilmproducing mutant strain B12ΔSlr R on Cd availability and uptake in Chinese cabbage and the related mechanisms were investigated in the Cd-polluted soil. Strain B12 and its mutants B12ΔYwcc and B12ΔSlr R increased the dry biomasses of edible tissues by 54%–130% compared with the controls. Strain B12 and its mutant B12ΔYwcc reduced the soil available Cd content by 36%–50% and root and edible tissue Cd contents by 23%–50% compared with the controls. Furthermore, the mutant strain B12ΔYwcc reduced the edible tissue Cd content by40% and increased the polysaccharide content by 23%, invertase activity by 139%, and gene copies of the cum A by 4.5-fold, eps A by 7.1-fold, and cad A by 4.3-fold, which were involved in Cd adsorption in the rhizosphere soils, respectively, compared with strain B12. The polysaccharide content and cum A, eps A, and cad A gene copy numbers showed significantly reverse correlations with the available Cd content. Notably, the mutant strain B12ΔYwcc showed better ability to colonize the vegetable root surface than strain B12. These findings demonstrated that the biofilm-overproducing mutant strain B12ΔYwcc increased the polysaccharide production and Cd-immobilizing related cum A, eps A, and cad A gene copies, resulting in lower Cd availability and accumulation in Chinese cabbage in the Cd-polluted soil.