Plantibacter flavus, Curtobacterium herbarum, Paenibacillus taichungensis, and Rhizobium selenitireducens Endophytes Provide Host-Specific Growth Promotion of Arabidopsis thaliana, Basil, Lettuce, and Bok Choy Plants

Abstract

A collection of bacterial endophytes isolated from stem tissues of plants growing in soils highly contaminated with petroleum hydrocarbons were screened for plant growth-promoting capabilities. Twenty-seven endophytic isolates significantly improved the growth of Arabidopsis thaliana plants in comparison to that of uninoculated control plants. The five most beneficial isolates, one strain each of Curtobacterium herbarum, Paenibacillus taichungensis, and Rhizobium selenitireducens and two strains of Plantibacter flavus were further examined for growth promotion in Arabidopsis, lettuce, basil, and bok choy plants. Host-specific plant growth promotion was observed when plants were inoculated with the five bacterial strains. P. flavus strain M251 increased the total biomass and total root length of Arabidopsis plants by 4.7 and 5.8 times, respectively, over that of control plants and improved lettuce and basil root growth, while P. flavus strain M259 promoted Arabidopsis shoot and root growth, lettuce and basil root growth, and bok choy shoot growth. A genome comparison between P. flavus strains M251 and M259 showed that both genomes contain up to 70 actinobacterial putative plant-associated genes and genes involved in known plant-beneficial pathways, such as those for auxin and cytokinin biosynthesis and 1-aminocyclopropane-1-carboxylate deaminase production. This study provides evidence of direct plant growth promotion by Plantibacter flavusIMPORTANCE The discovery of new plant growth-promoting bacteria is necessary for the continued development of biofertilizers, which are environmentally friendly and cost-efficient alternatives to conventional chemical fertilizers. Biofertilizer effects on plant growth can be inconsistent due to the complexity of plant-microbe interactions, as the same bacteria can be beneficial to the growth of some plant species and neutral or detrimental to others. We examined a set of bacterial endophytes isolated from plants growing in a unique petroleum-contaminated environment to discover plant growth-promoting bacteria. We show that strains of Plantibacter flavus exhibit strain-specific plant growth-promoting effects on four different plant species.

Keywords: Arabidopsis; Plantibacter flavus; basil; bok choy; endophyte; host specificity; lettuce; plant growth promotion; plant microbiology.

FIG 1

Effects of the top five endophytes on total biomass of Arabidopsis plants at 21 days after inoculation. Filled circles represent means of results for each treatment. Horizontal dashed lines represent means of results for control plants. Statistical significance in comparison to control plants is noted with one (P < 0.05) or three (P < 0.001) asterisks.

FIG 2

Effects of the top five endophytes on total root length of Arabidopsis plants at 21 days after inoculation. Filled circles represent means of results for each treatment. Horizontal dashed lines represent means of results for control plants. Statistical significance in comparison to control plants is noted with one (P < 0.05), two (P < 0.01), or three (P < 0.001) asterisks.

FIG 3

Effects of the top five endophytes on root tip abundance of Arabidopsis plants. Filled circles represent means of results for each treatment. Horizontal dashed lines represent means of results for control plants. Statistical significance in comparison to control plants is noted with one (P < 0.05), two (P < 0.01), or three (P < 0.001) asterisks.

FIG 4

Effects of the top five endophytes on root length of lettuce, basil, and bok choy plants. Filled circles represent means of results for each treatment. Horizontal dashed lines represent means of results for control plants. Statistical significance in comparison to control plants is noted with one (P < 0.05), two (P < 0.01), or three (P < 0.001) asterisks or with one dot (P < 0.1).

FIG 5

Effects of the top five endophytes on aerial biomass of lettuce, basil, and bok choy plants. Filled circles represent means of results for each treatment. Horizontal dashed lines represent means of results for control plants. Statistical significance in comparison to control plants is noted with one (P < 0.05) or two (P < 0.01) asterisks.

FIG 6

Full genomes of Plantibacter flavus strains M251 and M259, showing general metabolic features (subsystems and number of annotated genes).

FIG 7

Mauve alignment of Plantibacter flavus strain M251 and M259 genomes showing areas of missing genes in one relative to the other (gene islands). The sequence numbers of islands are given in Table S2 in the supplemental material. Gene family names in bold have functions equivalent to items in the putative plant association gene list of Levy et al. (34).

FIG 8

Image of a microtiter plate used for a rapid screening test. Each microtiter plate contained 96 Arabidopsis plants consisting of 12 uninoculated control plants and 84 endophyte-treated plants. The plates were equipped with lid spacers to allow for taller growth.