ACC Deaminase-Positive Halophilic Bacterial Isolates With Multiple Plant Growth-Promoting Traits Improve the Growth and Yield of Chickpea (Cicer arietinum L.) Under Salinity Stress

In this study, a total of 50 halophilic bacterial isolates were screened for 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, of these six with the highest ACC deaminase activity were selected for an increase in chickpea yield under salinity. The ACC deaminase activity among the isolates was ranged between 0.12 and 3.56 mM α-KB mg−1 min−1. These six isolates and one reference strain from the Agricultural College, Raichur, were used in the microcosm experiment during the rabi season of 2018. After 60 days of sowing, decreased rhizosphere pH and electrical conductivity (EC) from 8.4 to 7.6 and 4.3 to 3.4 dS m−1, respectively, were reported in chickpea. Among the treatments, Bacillus safensis (B. safensis)-inoculated plants showed a higher number of flowers (71 flowers/plant), pods (49.3 pods/plant), branches (33.3 branches/plant), and enhanced fresh weight (17.2 g/plant) and dry weight (8.1 g/plant). They were corroborated by improved nitrogen and phosphorus absorption of 71.5 and 43.5%, respectively, in B. safensis-treated plants. Based on the microcosm experimental findings, three cultures improving biometric and yield attributes were chosen for the field investigation. The field study was carried out at the Agricultural Research Station, Ganagavathi, during Kharif 2019. The chickpea plants treated with the consortium [B. safensis, Pseudomonas stutzeri, and Staphylococcus xylosus] increased the superoxide dismutase and catalase activity of plants by 258 and 196%, respectively. In addition, an increase in ascorbate peroxidase activity (0.41 μmol of ascorbate oxidized s−1 g−1 fresh weight) in the leaves and proline content was also recorded. The consortium (B. safensis, P. stutzeri, and S. xylosus) significantly increased nutrient uptake (N and P), the number of flowers, number of pods, and yield by 63.26, 39.03, 110, 59.96, and 17.56%, respectively, in chickpeas. Finally, inoculation with a mixture of three isolates is an effective method for increasing chickpea production under osmotic stress.

Resveratrol improves the iron deficiency adaptation of Malus baccata seedlings by regulating iron absorption

Background Resveratrol (Res), a phytoalexin, has been widely reported to participate in plant resistance to fungal infections. However, little information is available on its role in abiotic stress, especially in iron deficiency stress. Malus baccata is widely used as apple rootstock in China, but it is sensitive to iron deficiency. Results In this study, we investigated the role of exogenous Res in M. baccata seedings under iron deficiency stress. Results showed that applying 100 μM exogenous Res could alleviate iron deficiency stress. The seedlings treated with Res had a lower etiolation rate and higher chlorophyll content and photosynthetic rate compared with the apple seedlings without Res treatment. Exogenous Res increased the iron content in the roots and leaves by inducing the expression of MbAHA genes and improving the H+-ATPase activity. As a result, the rhizosphere pH decreased, iron solubility increased, the expression of MbFRO2 and MbIRT1 was induced, and the ferric-chelated reductase activity was enhanced to absorb large amounts of Fe2+ into the root cells under iron deficiency conditions. Moreover, exogenous Res application increased the contents of IAA, ABA, and GA3 and decreased the contents of DHZR and BL for responding to iron deficiency stress indirectly. In addition, Res functioned as an antioxidant that strengthened the activities of antioxidant enzymes and thus eliminated reactive oxygen species production induced by iron deficiency stress. Conclusion Resveratrol improves the iron deficiency adaptation of M. baccata seedlings mainly by regulating iron absorption.

Influence of Aluminum at Low pH on the Rhizosphere Processes of Masson Pine (Pinus Massoniana Lamb)

Trees in general are very tolerant of aluminum (Al, mainly Al3+ at pH ≦ 5.0), and the small effects seen in the contaminated soils may mislead people that the contamination is unimportant. We believe that the assessments with Al-sensitive Masson pine could have revealed a bigger difference. The key point of this study was to characterize the Al toxicity for Masson Pine. The objectives were to discover the specific eco-physiological relationship between pine roots and rhizosphere Al, and to investigate the Al effects on several parameters, measured in the rhizosphere of Masson pine. Masson pine seedlings were cultivated on a hydroponic setup. Through comprehensive laboratory dose-gradient experiments, Al-triggered composition of the root-released compounds and several rhizospheric parameters were determined by chromatography or spectroscopy. This study gives an important evidence of the Al-toxicity effects on the composition of root-released compounds and the root growth of Masson pine. Results showed that higher rhizospheric Al at pH 4.5 might contribute to increased release of sugars, and also could stimulate the release of oxalic acid and malic acid. The total of secreted amino acids were correlated with the rhizosphere Al. Zero additional Al induced no rhizosphere pH elevation, but Al-induced rhizosphere acidification (pH from 4.50 to 4.22) was observed at Al 100 µM. Greater additions of Al (>300 µM) suppressed the rhizosphere acidification at pH 3.92. Added Al had a negative effect on the dry weight of pine roots, but an opposite effect on Al accumulated in the roots was observed. The four endogenous hormones were also determined in the pine roots. Gibberellic acid (GA3) decreased, whereas abscisic acid (ABA) increased simultaneously with the addition of Al. Their inflexional concentrations were most frequently observed at 100 µM, which might be the threshold of Al toxicity for Masson pine. The secondary metabolites assayed have been studied in relation to the rhizospheric Al. The rhizosphere Al species at low pH can trigger pine roots to release the sugars (glucose, fructose + aldose), organic acids (oxalic acid, and malic acid), amino acids, secondary metabolites, and endogenous hormones during their growth. Meanwhile it also affected the growth of pine roots. This is an extensive study, which can help understanding the toxicity of Al to this important pioneer species of acid forest soils in south China.

Mycorrhization of Quercus Mongolica Seedlings by Tuber Melanosporum Alters Root Carbon Exudation and Rhizosphere Bacterial Communities

Aims To study how ectomycorrhizas (ECMs) mediate plant performance and rhizosphere soil bacterial communities via altered physiological characteristics and root carbon exudation. Methods Tuber melanosporum-colonized and uncolonized Quercus mongolica seedlings were grown on a substrate consisting of 41 % peat, 41 % pumice, 9 % pine bark and 9 % lime. Gas exchange fluorescence system, inductively coupled plasma atomic-emission spectrometer, high-performance liquid chromatography, gas chromatography and mass spectrometry, and 16S rRNA sequencing were used to analyze photosynthetic and nutritional characteristics, and rhizosphere carbon exudates, and bacterial communities. Results Tuber melanosporum mycorrhization increased leaf photosynthetic rate (by 69 %) and phosphorus concentration (94 %); increased rhizosphere pH (0.4 units), total organic carbon (TOC, 76 %) and acid phosphatase activity (33 %); but decreased leaf potassium concentration (26 %) and rhizosphere organic anions (50 %). Additionally, sugars like galactose were present in rhizosphere extract of colonized, but not uncolonized seedlings. Mycorrhization altered rhizosphere bacterial communities, with only ~10 % operational taxonomic units (OTUs) shared by both colonized and uncolonized seedlings; T. melanosporum enriched the phylum actinobacteria and the OTU of amb-16S-1323, IMCC26256 and PLTA13, but reduced SWB02. The abundances of different OTUs were differently affected by T. melanosporum colonization, and they were correlated with different physiological and/or rhizosphere factors. Conclusion Our results demonstrate that T. melanosporum ECM colonization can regulate carbon economy and rhizosphere bacterial communities of Q. mongolica seedlings grown in a previously sterilized peat-based substrate, to promote plant growth and nutrient cycling.

Resveratrol Improves the Iron Deficiency Adaptation of Malus Baccata Seedlings by Regulating Iron Absorption, Phytohormone Content, and ROS Migration

Resveratrol (Res), a phytoalexin, has been widely reported to participate in plant resistance to fungal infections. However, little information is available on its role in abiotic stress, especially in iron deficiency stress. Malus baccata is widely used as apple rootstock in China, but it is sensitive to iron deficiency. In this study, we investigated the role of exogenous Res in M. baccata seedings under iron deficiency stress. Results showed that applying 100 µmol exogenous Res could alleviate iron deficiency stress. The seedlings treated with Res had a lower etiolation rate and higher chlorophyll content and photosynthetic rate compared with the apple seedlings without Res treatment. Exogenous Res increased the iron content in the roots and leaves by inducing the expression of MbAHA genes and improving the H+-ATPase activity. As a result, the rhizosphere pH decreased, iron solubility increased, the expression of MbFRO2 and MbIRT1 was induced, and the ferric-chelated reductase activity was enhanced to absorb large amounts of Fe2+ into the root cells under iron deficiency conditions. Moreover, exogenous Res application increased the contents of IAA, ABA, and GA3 and decreased the contents of DHZR and BL for responding to iron deficiency stress indirectly. In addition, Res functioned as an antioxidant that strengthened the activities of antioxidant enzymes and thus eliminated reactive oxygen species production induced by iron deficiency stress. These findings are expected to enhance the application and examination of the physiological role of Res under iron deficiency stress in apples.

Linking Tree Health, Rhizosphere Physicochemical Properties, and Microbiome in Acute Oak Decline

Forest decline diseases are complex processes driven by biotic and abiotic factors. Although information about host–microbiome–environment interactions in agricultural systems is emerging rapidly, similar studies on tree health are still in their infancy. We used acute oak decline (AOD) as a model system to understand whether the rhizosphere physicochemical properties and microbiome are linked to tree health by studying these two factors in healthy and diseased trees located in three sites in different AOD stages—low, mid and severe. We found significant changes in the rhizosphere properties and microbiome composition across the different AOD sites and between the tree health conditions. Rhizosphere pH correlated with microbiome composition, with the microbial assemblages changing in more acidic soils. At the severe AOD site, the oak trees exhibited the lowest rhizosphere pH and distinct microbiome, regardless of their health condition, whereas, at the low and mid-stage AOD sites, only diseased trees showed lower pH and the microbial composition differed significantly from healthy trees. On these two sites, less extreme soil conditions and a high presence of host-beneficial microbiota were observed in the healthy oak trees. For the first time, this study gathers evidence of associations among tree health conditions, rhizosphere properties and microbiome as well as links aboveground tree decline symptoms to the belowground environment. This provides a baseline of rhizosphere community profiling of UK oak trees and paves the way for these associations to be investigated in other tree species suffering decline disease events.

Winter wheat adapts to environmental pH by changing H+ net flux in roots at the seedling stage

Changes in rhizosphere pH play an important role in wheat growth. To investigate the relationship between changes in rhizosphere pH and the growth of winter wheat roots and to explore the regulatory mechanism of acid and alkali resistance in winter wheat roots, the semi-winter wheat varieties Aikang 58 (AK58) and Bainong 4199 (BN4199) were used as materials for hydroponic experiments. Three pH levels (4.0, 6.5, and 9.0, with 6.5 as control) were applied during the wheat seedling stage. The results showed that the shoot and root biomass of the plants significantly decreased compared with the control under acid-base stress, with a more significant decrease with acid stress than alkali stress. Compared with the control, the root/shoot ratio increased under alkali stress and decreased under acid stress. The wheat root system showed H+ net efflux at pH 6.5 and 9.0, and the H+ net efflux rate at pH 4.0 was significantly lower than the control. The root activity of wheat was higher than the control at pH 9.0 and lower at pH4.0. The change of root pH was showed pH 4.0 < pH 6.5 < pH 9.0. Correlation analysis showed that changes in H+ net flux were significantly positively correlated to root activity and root pH. The H+ efflux rate and root activity of BN4199 were highe r than AK58 under acid and alkali stress, and the root/shoot ratio was relatively high, indicating strong acid and alkali resistance. We conclude that wheat could adapt to poor acid-base environments by adjusting root H+ net flux, and in practice, the root/shoot ratio could be used as index for the rapid determination of acid-base tolerance in wheat at the seedling stage.