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Purpose Numerous Bacillus spp. in soils show strong plant growth-promotion effects, but the molecular mechanism of most of these bacteria interacting with plants remains unknown. Uncovering the interaction pattern is of considerable importance for potential applications in agriculture. Methods In the present study, we isolated the bacterium Bacillus megaterium strain WW1211 from the rhizosphere of Arabidopsis and investigated the mechanism of WW1211 on plant growth promotion at the cellular and molecular levels. Results Our results showed that inoculation of WW1211 substantially promoted shoot biomass and lateral root initiation but inhibited primary root growth. The promotion effect of WW1211 was counteracted by the addition of auxin biosynthesis or transport inhibitors, indicating an auxin-dependent pattern. The expression of auxin response and biosynthesis genes in Arabidopsis seedlings and the concentration of indole-3-acetic acid in Arabidopsis roots were enhanced by WW1211 inoculation. Furthermore, the meristematic zone length and the cell division in primary roots were inhibited by WW1211 inoculation. Auxin was enriched in the quiescent center, and the expression of auxin efflux and influx genes in the root tips was suppressed, indicating that WW1211 inhibited the growth of primary roots by modification of the auxin cycle and redistribution in the root meristem. Conclusion Our results suggest that B. megaterium strain WW1211 induces auxin biosynthesis and simultaneously regulates auxin redistribution in plant shoots and roots, thus stimulating plant growth and lateral root initiation.

Key message Changes in glucose-6-phosphate dehydrogenase (G6PD) isoforms activities and expression were investigated in soybean roots under drought, suggesting that cytosolic G6PD plays a main role by regulating H 2 O 2 signal and redox homeostasis. G6PD acts a vital role in plant growth, development and stress adaptation. Drought (PEG6000 treatment) could markedly increase the enzymatic activities of cytosolic G6PD ( Cyt-G6PD ) and compartmented G6PD (mainly plastidic P2-G6PD) in soybean roots. Application of G6PD inhibitor upon drought condition dramatically decreased the intracellular NADPH and reduced glutathione levels in soybean roots. Nitric oxide (NO) and hydrogen peroxide (H 2 O 2 ) participated in the regulation of Cyt-G6PD and P2-G6PD enzymatic activities under drought stress. Diphenylene iodonium (DPI), an inhibitor of NADPH oxidase, abolished the drought-induced accumulation of H 2 O 2 . The exogenous application of H 2 O 2 and its production inhibitor (DPI) could stimulate and inhibit the NO accumulation, respectively, but not vice versa. qRT-PCR analysis confirmed that NO, as the downstream signal of H 2 O 2 , positively regulated the transcription of genes encoding Cyt-G6PD ( GPD5 , G6PD6 , G6PD7 ) under drought stress in soybean roots. Comparatively, NO and H 2 O 2 signals negatively regulated the gene expression of compartmented G6PD ( GPD1 , G6PD2 , G6PD4 ), indicating that a post-transcriptional mechanism was involved in compartmented G6PD regulation. Taken together, the high Cyt-G6PD activity is essential for maintaining redox homeostasis upon drought condition in soybean roots, and the H 2 O 2 -dependent NO cascade signal is differently involved in Cyt-G6PD and compartmented G6PD regulation.

Background Drought severely limits crop growth and yield. Highland barley ( Hordeum vulgare L.), a naturally stress-tolerant crop, serves as an ideal model for investigating the molecular and physiological mechanisms underlying plant drought adaptation. Results In this study, two highland barley varieties, Xi-La 22 (XL22) and Zang-Qing 17 (ZQ17), with different degrees of drought tolerance, were used to investigate the mechanism of drought tolerance in highland barley. Compared with XL22, ZQ17 exhibited significant reductions in the biomass and root/shoot ratio, and the increase in ion leakage and malondialdehyde content under drought stress. Natural drought experiment further confirmed that ZQ17 had a higher fatality rate and water loss rate than XL22, indicating that XL22 had higher drought tolerance. Drought induced significant increase in H 2 O 2 and O 2 . − levels in both barley varieties, especially in ZQ17. Moreover, compared with XL22, more distribution of H 2 O 2 in chloroplasts of ZQ17 might lead to greater degradation of photosynthetic protein complexes (PSI, PSII, LHCII trimers), thereby reducing photosynthetic capacity. The activities of glutathione reductase and glutathione peroxidase and content of reduced glutathione and ascorbate acid were markedly higher in XL22 compared with ZQ17 under drought stress. RNA-seq results showed many genes related to reactive oxygen species (ROS) scavenging, osmotic adjustment (LEA, HSP, aquaporins), hormone signaling and transcription factors (TFs) were specifically up-regulated in XL22. Weighted gene co-expression network analysis (WGCNA) further identified key modules and clarified core hub genes in XL22, mainly including genes in bZIP, AP2/ERF, bHLH transcription factor (TF) families and ABA signaling pathway, which help maintain high ROS scavenging capacity, root/shoot ratio and photosynthetic performance. Conclusion This study reveals that drought-tolerant highland barley maintains antioxidant activity, photosynthetic complex integrity, hormone signaling, and drought-responsive TF activation, providing insights for barley germplasm screening.

Glucose-6-phosphate dehydrogenase (G6PDH or G6PD) is the key regulatory enzyme in the oxidative pentose phosphate pathway (OPPP). The cytosolic isoforms including G6PD5 and G6PD6 account for the major part of the G6PD total activity in plant cells. Here, we characterized the single null mutant and and double mutant . Compared to wild type, the mutant seeds showed a reduced germination rate and root elongation under salt stress. The seeds and seedlings lacking and accumulate more reactive oxygen species (ROS) than the wild type under salt stress. Cytosolic G6PD (cy-G6PD) affected the expression of NADPH oxidases and the G6PD enzymatic activities in the mutant , in which the NADPH oxidases genes are disrupted by T-DNA insertion and generation of ROS is inhibited, were lower than that in the wild type. The NADPH level in mutants was decreased under salt stress. In addition, we found that G6PD5 and G6PD6 affected the activities and transcript levels of various antioxidant enzymes in response to salt stress, especially the ascorbate peroxidase and glutathione reductase. Exogenous application of ascorbate acid and glutathione rescued the seed and root phenotype of under salt stress. Interestingly, the cytosolic G6PD negatively modulated the NaCl-blocked primary root growth under salt stress in the root meristem and elongation zone.

Mitogen-activated protein kinase kinase 9 (MKK9) is an upstream activator of mitogen-activated protein kinase 3 (MAPK3) and MAPK6 in planta. To investigate MKK9 roles in mitochondrial respiration in Arabidopsis, MKK9DD, the active allele with mutations of Thr-201 and Ser-205 to Asp, and MKK9KR, the allele lacking MKK9 activity with a mutation of Lys-76 to Arg, were used. Results showed that the total respiratory rate (Vt), alternative pathway capacity (Valt) and cytochrome pathway capacity (Vcyt) increased under 0–100 mM NaCl treatments but decreased under 150–300 mM NaCl treatments in Col-0 callus. However, the activation of MKK9 by dexamethasone (DEX) increased Vt, Valt and Vcyt under 200 mM NaCl treatment; moreover, Valt showed more increase than Vcyt. The activation of MKK9 in MKK9DD callus sharply increased AOX protein expression under normal and NaCl conditions, but the increase was not observed in MKK9KR callus. Further results indicated that MAPK3 and MAPK6 were involved in the MKK9-induced increase of AOX protein levels. qRT-PCR results showed that MKK9-MAPK3/MAPK6 was involved in the NaCl-induced AOX1b and AOX1d expression, but only MKK9-MAPK3 was necessary for AOX2 expression; in addition, MAPK3 regulated the AOX1a transcription in an MKK9-independent manner. MKK9 positively regulated SOD and CAT activities by affecting MAPK3 and MAPK6 and negatively regulated APX and POD activities by affecting MAPK3. Moreover, MKK9 functions as a positive factor in H2O2 accumulation under salt stress. The regulation of ethylene on alternative respiration was also associated with MKK9 under salt stress. Taken together, the MKK9-MAPK3/MAPK6 pathway plays a pivotal role in increasing alternative respiration in the salt-treated Arabidopsis callus.

Nitrogen (N) availability is a critical factor for plant development and crop yield, and it closely correlates to carbon (C) metabolism. Uncoupling protein (UCP) and alternative oxidase (AOX) exhibit a strong correlation with N and C metabolism. Here, we investigated the functions of UCP1 and AOX1a using their mutants and complementation lines in Arabidopsis adaptation to low N. Low N markedly increased AOX1a and UCP1 expression, alternative pathway capacity and UCP activity. Eight-day-old aox1a/ucp1 seedlings were more sensitive to low N than Col-0 and single mutants, exhibiting lower primary root length and higher anthocyanin accumulation. The net photosynthetic rate, electron transport rate, PSII actual photochemical efficiency, stomatal conductance and carboxylation efficiency were markedly decreased in ucp1 and aox1a / ucp1 compared to those in Col-0 and aox1a under low N stress; comparatively, chlorophyll content and non-photochemical quenching coefficient were the lowest and highest in aox1a / ucp1 , respectively. Nitrate acquisition rate was accelerated in aox1a/ucp1 , but its transport activity was decreased, which resulted in low nitrate content and nitrate reductase activity under low N condition. The C/N ratio in seeds, but not in leaves, is higher in aox1a / ucp1 than that in Col-0, aox1a and ucp1 under low N condition. RNA-seq analysis revealed that many genes involved in photosynthesis and C/N metabolism were markedly down-regulated in aox1a/ucp1 under low N stress. These results highlight the key roles of UCP1 and AOX1a in modulating photosynthetic capacity, C/N assimilation and distribution under low N stress.

Main Conclusion Brassinosteroid is necessary for sugar promotion of Arabidopsis hypocotyl elongation in darkness, and sugar positively regulates BRASSINA-ZOLE RESISTANT1 (BZR1) at both transcription and protein levels. Sugar has the ability to induce Arabidopsis hypocotyl elongation in the dark, but the detailed mechanisms remain not well understood. Here, we report that the steroidal phytohormone brassinosteroid (BR) is involved in sugar promotion of hypocotyl elongation in the dark. Sugar-induced hypocotyl elongation was significantly repressed in the BR-deficient mutant det2-1, BR-insensitive mutant bri1-5, and wild-type plants (Col-0), but not in the BR-hypersensitive mutants bzr1-1D and bes1-D treated with the BR biosynthetic inhibitor brassinazole (BRZ). Sugar also up-regulated the expression of genes that are related to cell elongation in a BR-dependent manner, and this effect was more remarkable in bzr1-1D and bes1-D than in their corresponding wild types in the presence of BRZ, suggesting an important role of BZR1 and bri1-ems-suppressor 1 (BES1) in this process. Sugar treatment seems to have little effect on BR biosynthesis, but enhances the expression of BZR1 and BES1, two transcription factors in BR signaling, in the dark. Furthermore, sugar treatment maintains higher BZR1 protein levels in plants grown in the dark. Collectively, our results indicate that BR is required for sugar promotion of hypocotyl elongation in darkness in Arabidopsis.

In the present study, the role of ethylene in nitric oxide (NO)-mediated protection by modulating ion homeostasis in Arabidopsis callus under salt stress was investigated. Results showed that the ethylene-insensitive mutant etr1-3 was more sensitive to salt stress than the wild type (WT). Under 100 mM NaCl, etr1-3 callus displayed a greater electrolyte leakage and Na⁺/K⁺ ratio but a lower plasma membrane (PM) H⁺-ATPase activity compared to WT callus. Application of exogenous 1-aminocyclopropane-1-carboxylic acid (ACC, an ethylene precursor) or sodium nitroprusside (SNP, a NO donor) alleviated NaCl-induced injury by maintaining a lower Na⁺/K⁺ ratio and an increased PM H⁺-ATPase activity in WT callus but not in etr1-3 callus. The SNP actions in NaCl stress were attenuated by a specific NO scavenger or an ethylene biosynthesis inhibitor in WT callus. Under 100 mM NaCl, the NO accumulation and ethylene emission appeared at early time, and NO production greatly stimulated ethylene emission in WT callus. In addition, ethylene induced the expression of PM H⁺-ATPase genes under salt stress. The recovery experiment showed that NaCl-induced injury was reversible, as signaled by the similar recovery of Na⁺/K⁺ ratio and PM H⁺-ATPase activity in WT callus. Taken together, the results indicate that ethylene and NO cooperate in stimulating PM H⁺-ATPase activity to modulate ion homeostasis for salt tolerance, and ethylene may be a part of the downstream signal molecular in NO action.

Background Glucose-6-phosphate dehydrogenase (G6PDH or G6PD) functions in supply of NADPH, which is required for plant defense responses to stresses. However, whether G6PD functions in the abscisic acid (ABA) signaling pathway remains to be elucidated. In this study, we investigated the involvement of the cytosolic G6PD5 in the ABA signaling pathway in Arabidopsis . Results We characterized the Arabidopsis single null mutant g6pd5. Phenotypic analysis showed that the mutant is more sensitive to ABA during seed germination and root growth, whereas G6PD5 -overexpressing plants are less sensitive to ABA compared to wild type (WT). Furthermore, ABA induces excessive accumulation of reactive oxygen species (ROS) in mutant seeds and seedlings. G6PD5 participates in the reduction of H 2 O 2 to H 2 O in the ascorbate-glutathione cycle. In addition, we found that G6PD5 suppressed the expression of Abscisic Acid Insensitive 5 ( ABI5 ), the major ABA signaling component in dormancy control. When G6PD5 was overexpressed, the ABA signaling pathway was inactivated. Consistently, G6PD5 negatively modulates ABA-blocked primary root growth in the meristem and elongation zones. Of note, the suppression of root elongation by ABA is triggered by the cell cycle B-type cyclin CYCB1 . Conclusions This study showed that G6PD5 is involved in the ABA-mediated seed germination and root growth by suppressing ABI5 .

The roles of ethylene, hydrogen peroxide (H2O2), and calcium in inducing the capacity of the alternative respiratory pathway (AP) under chilling temperature in Arabidopsis thaliana calli were investigated. Exposure of wild-type (WT) calli, but not the calli of ethylene-insensitive mutants, etr1-3 and ein2-1, to chilling led to a marked increase of the AP capacity and triggered a rapid ethylene emission and H2O2 generation. Increasing ethylene emission by applying 1-aminocyclopropane-1-carboxylic (an ethylene precursor) markedly enhanced the AP capacity in WT calli, but not in etr1-3 and ein2-1 calli, whereas suppressing ethylene emission by applying aminooxyacetic acid (an ethylene biosynthesis inhibitor) abolished the chilling-induced AP capacity in WT calli. Furthermore, exogenous H2O2 treatment increased the AP capacity in WT calli, but not in etr1-3 and ein2-1 calli, while both catalase (H2O2 scavenger) and diphenylene iodonium (DPI, an inhibitor of NADPH oxidase) completely inhibited the chilling-induced H2O2 generation and largely inhibited the chilling-induced AP capacity. Interestingly, the chilling-induced AP capacity was completely inhibited by DPI and EGTA (calcium chelator). Further investigation demonstrated that H2O2 and calcium induced ethylene emission under chilling stress. Ethylene modulated the chilling-induced increase of pyruvate content and the expression of alternative oxidase genes (AOX1a and AOX1c). Taken together, these results indicate that H2O2-, calcium- and ethylene-dependent pathways are required for chilling-induced increase in AP capacity. However, only ethylene is indispensable for the activation of the AP capacity.