The ability to colonize the rhizosphere is an important basics requirement for field application of plant growth-promoting rhizobacteria (PGPR) strains. There are complex signal exchanges and mutual recognition between microbes and plants. In this study, phosphate-solubilizing Pseudomonas sp. P34, a PGPR strain with affinity to wheat, was isolated from the wheat rhizosphere by wheat germ agglutinin (WGA). The plasmid pTR102 harboring the luciferase luxAB gene was transferred into P34 to create P34-L. The labeled strain was used to track the temporal and spatial characteristics of colonization in wheat rhizosphere and its effects on wheat development. The transcript level of phosphate transporter gene TaPT4, a phosphorus deficiency indicator gene, in wheat roots was monitored by quantitative reverse-transcription PCR. The experimental results indicated that there was a high density of stain P34-L within the top 8-cm depth of the wheat rhizosphere on day 36 of wheat growth. The strain could survive in the wheat rhizosphere for a long time, and colonize new spaces in wheat rhizosphere following the extension of wheat roots. Compared with uninoculated wheat plants, those inoculated with P34-L showed significantly increased phosphorus accumulation in leaves, seedling fresh and dry weight, root fresh and dry weight, total root length, and number of root tips, forks, crossings, which showed a great value of application of the strain on wheat production by promoting the root growth and dry matter accumulation. Strain P34-L down-regulated the transcript level of TaPT4 in wheat roots, which means a well phosphorus supplementation environment was established by P34-L. 3 ImportanceMany PGPR strains often failed to achieve the desired effects when applied in the field. One major reason for the failure is lack of a special affinity between a certain strain and the target host plant, so those strains have low competitive ability with the indigenous microorganism, and unable to survive constantly in rhizosphere. In this work, a new technique to isolate wheat-specific phosphate-solubilizing PGPR strain by WGA was established. The isolate P34 was confirmed can colonize the wheat rhizosphere, and have significantly ability in promoting phosphorus absorption and wheat growth by luminescence labeling techniques. Furthermore, the phosphate-solubilizing ability of this affinity PGPR strain was verified in gene level by quantitative reverse-transcription PCR. These results lay a firm foundation for further research on the relationships between PGPR and their host plants. Meanwhile, this work supplied a potential ideal biofertilizer producing strain for sustainable agriculture.

21The ability to colonize the rhizosphere is an important basics requirement for field 22 application of plant growth-promoting rhizobacteria (PGPR) strains. There are complex signal 23 exchanges and mutual recognition between microbes and plants. In this study, 24 phosphate-solubilizing Pseudomonas sp. P34, a PGPR strain with affinity to wheat, was isolated 25 from the wheat rhizosphere by wheat germ agglutinin (WGA). The plasmid pTR102 harboring 26 the luciferase luxAB gene was transferred into P34 to create P34-L. The labeled strain was used 27 to track the temporal and spatial characteristics of colonization in wheat rhizosphere and its 28 effects on wheat development. The transcript level of phosphate transporter gene TaPT4, a 29 phosphorus deficiency indicator gene, in wheat roots was monitored by quantitative 30 reverse-transcription PCR. The experimental results indicated that there was a high density of 31 stain P34-L within the top 8-cm depth of the wheat rhizosphere on day 36 of wheat growth. The 32 strain could survive in the wheat rhizosphere for a long time, and colonize new spaces in wheat 33 rhizosphere following the extension of wheat roots. Compared with uninoculated wheat plants, 34 those inoculated with P34-L showed significantly increased phosphorus accumulation in leaves, 35 seedling fresh and dry weight, root fresh and dry weight, total root length, and number of root 36 tips, forks, crossings, which showed a great value of application of the strain on wheat 37 production by promoting the root growth and dry matter accumulation. Strain P34-L 38 down-regulated the transcript level of TaPT4 in wheat roots, which means a well phosphorus 39 supplementation environment was established by P34-L. 40 3 41 Importance 42Many PGPR strains often failed to achieve the desired effects when applied in the field. One 43 major reason for the failure is lack of a special affinity between a certain strain and the target 44 host plant, so those strains have low competitive ability with the indigenous microorganism, and 45 unable to survive constantly in rhizosphere. In this work, a new technique to isolate 46 wheat-specific phosphate-solubilizing PGPR strain by WGA was established. The isolate P34 47 was confirmed can colonize the wheat rhizosphere, and have significantly ability in promoting 48 phosphorus absorption and wheat growth by luminescence labeling techniques. Furthermore, the 49 phosphate-solubilizing ability of this affinity PGPR strain was verified in gene level by 50 quantitative reverse-transcription PCR. These results lay a firm foundation for further research 51 on the relationships between PGPR and their host plants. Meanwhile, this work supplied a 52 potential ideal biofertilizer producing strain for sustainable agriculture. 53 54 4

No abstract

Rhizosphere colonization is a pre-requisite for the favorable application of plant growth-promoting rhizobacteria (PGPR). Exchange and mutual recognition of signaling molecules occur frequently between plants and microbes. Here, the luciferase luxAB gene was electrotransformed into the phosphate-solubilizing strain Pseudomonas sp. WS32, a type of plant growth-promoting rhizobacterium with specific affinity for wheat. A labeled WS32 strain (WS32-L) was applied to determine the temporal and spatial traits of colonization within the wheat rhizosphere using rhizoboxes experimentation under natural condition. The effects of colonization on wheat root development and seedling growth were evaluated, and RNA sequencing (RNA-seq) was performed to explore the transcriptional changes that occur in wheat roots under WS32 colonization. The results showed that WS32-L could survive in the wheat rhizosphere for long periods and could expand into new zones following wheat root extension. Significant increases in seedling fresh and dry weight, root fresh and dry weight, root surface area, number of root tips, and phosphorus accumulation in the wheat leaves occurred in response to WS32 rhizosphere colonization. RNA-seq analysis showed that a total of 1485 genes in wheat roots were differentially expressed between the inoculated conditions and the uninoculated conditions. Most of the transcriptional changes occurred for genes annotated to the following functional categories: “phosphorus and other nutrient transport,” “hormone metabolism and organic acid secretion,” “flavonoid signal recognition,” “membrane transport,” and “transcription factor regulation.” These results are therefore valuable to future studies focused on the molecular mechanisms underlying the growth-promoting activities of PGPR on their host plants.