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Soil drying enhances root ABA accumulation and rhizosheath formation, but whether ABA mediates rhizosheath formation is unclear. Here, we used the ABA-deficient mutant Az34 to investigate molecular and morphological changes by which ABA could affect rhizosheath formation. Mild soil drying with intermittent watering

Moderate soil drying (MSD) is a promising agricultural technique that can reduce water consumption and enhance rhizosheath formation promoting drought resistance in plants. The endophytic fungus Piriformospora indica (P. indica) with high auxin production may be beneficial for rhizosheath formation. However, the integrated role of P. indica with native soil microbiome in rhizosheath formation is unclear. Here, we investigated the roles of P. indica and native bacteria on rice rhizosheath formation under MSD using high-throughput sequencing and rice mutants. Under MSD, rice rhizosheath formation was significantly increased by around 30% with P. indica inoculation. Auxins in rice roots and P. indica were responsible for the rhizosheath formation under MSD. Next, the abundance of the genus Bacillus, known as plant growth-promoting rhizobacteria, was enriched in the rice rhizosheath and root endosphere with P. indica inoculation under MSD. Moreover, the abundance of Bacillus cereus (B. cereus) with high auxin production was further increased by P. indica inoculation. After inoculation with both P. indica and B. cereus, rhizosheath formation in wild-type or auxin efflux carrier OsPIN2 complemented line rice was higher than that of the ospin2 mutant. Together, our results suggest that the interaction of the endophytic fungus P. indica with the native soil bacterium B. cereus favors rice rhizosheath formation by auxins modulation in rice and microbes under MSD. This finding reveals a cooperative contribution of P. indica and native microbiota in rice rhizosheath formation under moderate soil drying, which is important for improving water use in agriculture.

Summary Cluster roots of white lupin are induced by low phosphorus (LP) to efficiently access unavailable P, but how soilborne microbes are associated with cluster root formation (CRF) is unclear. We investigated the roles of soilborne bacteria in CRF response to LP by high‐throughput sequencing and root–bacteria interactions. Cluster root number was significantly decreased in plants grown in sterilized soil compared with nonsterilized soil. Proteobacteria was enriched in CR, as shown by microbiome analysis of soil (bulk, rhizosphere, and rhizosheath) and roots (main, lateral, and CR). Large‐scale gene expression level implicated ethylene mediation in CRF. Klebsiella pneumoniae (P7), a soilborne bacterium belonging to Proteobacteria, was isolated from CR. Among 11 isolated strains, P7 exhibited the highest 1‐aminocyclopropane‐1‐carboxylate deaminase (ACCD) activity; this enzyme inhibits the biosynthesis of ethylene in plants by the cleavage of the ethylene precursor 1‐aminocyclopropane‐1‐carboxylic acid and promotes CRF under LP. We constructed an ACCD‐deficit mutant accd in the P7 genetic background. The loss‐of‐function mutation failed to promote CRF under LP conditions. Also, auxin responses may be involved in K. pneumoniae‐ethylene‐mediated CRF. Overall, we propose that the soilborne bacterium K. pneumoniae promotes CRF of white lupin in response to LP by ethylene mediation.

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