Effect of sustainable fertilizations on PGP microbial communities in rice paddy fields: a focus on phosphate

Phosphorous (P) is an essential macroelement for plant development but it is often present in forms that are unavailable for plant uptake. Growing attention is therefore directed towards P-solubilising microbes able to supply P to plants. Phosphorous content in soils is influenced by agronomic practices, but despite its importance in plant nutrition, the knowledge associated with it is scarce. This work aims at exploring the rhizosphere microbiota of rice cultivated in soils that underwent different agronomic practices; the goals are to detect rhizobacteria that show plant growth-promoting activities (PGPA), constitute a rice-specific strain collection, and overall investigate the microbial factors that influence P nutrition. Samples were collected from two paddies, one treated with phosphate and the other with a cover crop. Bacteria were isolated from rhizosphere soil, rhizoplane and endosphere of both rice and cover crop. Overall, 558 isolates from rice and 82 from the cover crop underwent de-replication at the strain level through BOX-PCR fingerprint. Up to 5 candidates per each BOX-profile-variant were further screened for PGPA (hydrolysation of P forms, production of indole-3-acetic acid (IAA) and siderophores, fixation of atmospheric nitrogen, ACC-deaminase activity). Of the 231 rice-associated isolates tested, 25.1% showed tricalcium phosphate (TCP)-solubilising abilities. Moreover, 9 rhizosphere isolates produced IAA. Regarding the cover crop-associated strains, 10.6% solubilised TCP. The promising isolates will be tested in vivo for assessing rice seed germination promotion. Finally, the microbial communities of different soil compartments will be explored through metabarcoding of the 16S rRNA gene for bacteria and archaea and ITS for fungi. The outputs of this project will help to understand the role of P-solubilizing bacteria in rice nutrition under different agronomic settlements and develop biofertilizers able to enhance the uptake of soil endogenous phosphate. This research is funded by Regione Lombardia (project P-RICE, n. 23, d.d.s. n. 4403 28/03/2018).