Soilborne bacterium Klebsiella pneumoniae promotes cluster root formation in white lupin through ethylene mediation.

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.

Effects of hydrogen peroxide and l-tryptophan on antioxidative potential, apoptosis, and mammalian target of rapamycin signaling in bovine intestinal epithelial cells.

Amino acids are primarily absorbed in the ruminant small intestine, and the small intestine is a target organ prone to oxidative stress, causing intestinal disfunction. Previous study suggested that l-Trp could benefit intestinal function and production performance. This study aimed to explore the effects of l-Trp on hydrogen peroxide (H2O2)-induced oxidative injury in bovine intestinal epithelial cells (BIEC) and the potential mechanism. The effects of l-Trp on cell apoptosis, antioxidative capacity, AA transporters, and the mammalian target of rapamycin (mTOR) signaling pathway were evaluated in BIEC treated with 0.8 mMl-Trp for 2 hours combined with or without H2O2 induction. In addition, to explore whether the effects of 0.8 mMl-Trp on oxidative stress were related to mTOR, an mTOR-specific inhibitor was used. The percentage of apoptosis was measured using flow cytometry. The relative gene abundance and protein expression in BIEC were determined using real-time PCR and Western blot assay, respectively. Results showed l-Trp at 0.4 and 0.8 mM enhanced the cell viability, and it was inhibited by l-Trp at 6.4 mM. l-Tryptophan at 0.4, 0.8, and 1.6 mM remarkably decreased the percentage of apoptosis and enhanced antioxidative capacity in H2O2-mediated BIEC. Moreover, l-Trp at 0.8 mM increased the relative gene abundance and protein expression of antioxidative enzymes and AA transporters, and the mTOR signaling pathway. The mTOR inhibitor lowered the protein expression of large neutral amino acid transporter 1, but the inhibition of mTOR did not alter the activities of catalase and superoxide dismutase or protein expression of alanine-serine-cysteine transporter 2 with or without H2O2 induction. l-Tryptophan increased catalase and superoxide dismutase activities in H2O2-mediated BIEC, although not with a present mTOR inhibitor. l-Tryptophan increased the protein expression of large neutral amino acid transporter 1 and alanine-serine-cysteine transporter 2 in H2O2-mediated BIEC with or without the presence of an mTOR inhibitor. The present work suggested that l-Trp supplementation could alleviate oxidative injury in BIEC by promoting antioxidative capacity and inhibiting apoptosis, and the mTOR signal played vital roles in the alleviation.

Root acid phosphatases and rhizobacteria synergistically enhance white lupin and rice phosphorus acquisition.

The rhizosheath is a belowground area that acts as a communication hub at the root-soil interface to promote water and nutrient acquisition. Certain crops, such as white lupin (Lupinus albus), acquire large amounts of phosphorus (P), owing partially to exudation of acid phosphatases (APases). Plant growth-promoting rhizobacteria also increase soil P availability. However, potential synergistic effects of root APases and rhizosheath-associated microbiota on P acquisition require further research. In this study, we investigated the roles of root purple APases (PAPs) and plant growth-promoting rhizobacteria in rhizosheath formation and P acquisition under conditions of soil drying (SD) and P treatment (+P: soil with P fertilizer; -P: soil without fertilizer). We expressed purple acid phosphatase12 (LaPAP12) in white lupin and rice (Oryza sativa) plants and analyzed the rhizosheath-associated microbiome. Increased or heterologous LaPAP12 expression promoted APase activity and rhizosheath formation, resulting in increased P acquisition mainly under SD-P conditions. It also increased the abundance of members of the genus Bacillus in the rhizosheath-associated microbial communities of white lupin and rice. We isolated a phosphate-solubilizing, auxin-producing Bacillus megaterium strain from the rhizosheath of white lupin and used this to inoculate white lupin and rice plants. Inoculation promoted rhizosheath formation and P acquisition, especially in plants with increased LaPAP12 expression and under SD-P conditions, suggesting a functional role of the bacteria in alleviating P deficit stress via rhizosheath formation. Together, our results suggest a synergistic enhancing effect of LaPAP12 and plant growth-promoting rhizobacteria on rhizosheath formation and P acquisition under SD-P conditions.

NRT1.1B improves selenium concentrations in rice grains by facilitating selenomethinone translocation.

Selenium (Se) is an essential trace element for humans and other animals, yet approximately one billion people worldwide suffer from Se deficiency. Rice is a staple food for over half of the world's population that is a major dietary source of Se. In paddy soils, rice roots mainly take up selenite. Se speciation analysis indicated that most of the selenite absorbed by rice is predominantly transformed into selenomethinone (SeMet) and retained in roots. However, the mechanism by which SeMet is transported in plants remains largely unknown. In this study, SeMet uptake was found to be an energy-dependent symport process involving H+ transport, with neutral amino acids strongly inhibiting SeMet uptake. We further revealed that NRT1.1B, a member of rice peptide transporter (PTR) family which plays an important role in nitrate uptake and transport in rice, displays SeMet transport activity in yeast and Xenopus oocyte. The uptake rate of SeMet in the roots and its accumulation rate in the shoots of nrt1.1b mutant were significantly repressed. Conversely, the overexpression of NRT1.1B in rice significantly promoted SeMet translocation from roots to shoots, resulting in increased Se concentrations in shoots and rice grains. With vascular-specific expression of NRT1.1B, the grain Se concentration was 1.83-fold higher than that of wild type. These results strongly demonstrate that NRT1.1B holds great potential for the improvement of Se concentrations in grains by facilitating SeMet translocation, and the findings provide novel insight into breeding of Se-enriched rice varieties.