Tobacco belongs to the family Solanaceae, which easily forms continuous cropping obstacles. Continuous cropping exacerbates the accumulation of autotoxins in tobacco rhizospheric soil, affects the normal metabolism and growth of plants, changes soil microecology, and severely reduces the yield and quality of tobacco. In this study, the types and composition of tobacco autotoxins under continuous cropping systems are summarized, and a model is proposed, suggesting that autotoxins can cause toxicity to tobacco plants at the cell level, plant-growth level, and physiological process level, negatively affecting soil microbial life activities, population number, and community structure and disrupting soil microecology. A combined strategy for managing tobacco autotoxicity is proposed based on the breeding of superior varieties, and this approach can be combined with adjustments to cropping systems, the induction of plant immunity, and the optimization of cultivation and biological control measures. Additionally, future research directions are suggested and challenges associated with autotoxicity are provided. This study aims to serve as a reference and provide inspirations needed to develop green and sustainable strategies and alleviate the continuous cropping obstacles of tobacco. It also acts as a reference for resolving continuous cropping challenges in other crops.

The effects of rhizosphere microorganisms on plant growth and the associated mechanisms are a focus of current research, but the effects of exogenous combined inoculation with arbuscular mycorrhizal fungi (AMF) and plant growth-promoting rhizobacteria (PGPR) on seedling growth and the associated rhizosphere microecological mechanisms have been little reported. In this study, a greenhouse pot experiment was used to study the effects of single or double inoculation with AM fungi (Funneliformis mosseae) and two PGPR (Bacillus sp., Pseudomonas sp.) on the growth of tobacco seedlings, together with high-throughput sequencing technology to reveal associated rhizosphere microecological mechanisms. All inoculation treatments significantly increased the aboveground dry weight; root dry weight; seedling nitrogen, phosphorus, and potassium uptake; plant height; stem thickness; maximum leaf area; chlorophyll content; total root length, surface area, and volume; and average root diameter. The highest values for these indices were observed in the combined treatment of F. mosseae and Pseudomonas sp. SG29 (A_SG29). Furthermore, the A_SG29 treatment yielded the highest diversity indexes and largest percentages of significantly enriched bacterial taxa, and significantly promoted the colonization of AMF in tobacco roots and Pseudomonas in rhizosphere soil. Differential metabolic-pathway predictions using PICRUSt2 showed that the A_SG29 treatment significantly increased the metabolic pathway richness of tobacco rhizosphere microorganisms, and significantly up-regulated some metabolic pathways that may benefit plant growth. Co-inoculation with F. mosseae and Pseudomonas sp. SG29 promoted tobacco-seedling growth by significantly improving rhizosphere microbial communities' structure and function. In summary, the combined inoculation of AMF and SG29 promotes tobacco seedling growth, optimizes the rhizosphere microbial community's structure and function, and serves as a sustainable microbial co-cultivation method for tobacco seedling production.

Pullulan is an extracellular water-soluble polysaccharide with wide applications. In this study, we screened strains that could selectively produce high molecular weight pullulan for application in industrial pullulan production. A new fungus strain A4 was isolated from soil and identified as Aureobasidium melanogenum based on colony characteristics, morphology, and internally transcribed spacer analysis. Thin-layer chromatography, Fourier-transform infrared spectroscopy, and nuclear magnetic resonance analysis suggested that the dominant exopolysaccharide produced by this strain, which presented a molecular weight of 1.384 × 10⁶ Dalton in in-gel permeation chromatography, was pullulan. The culture conditions for A. melanogenum A4 were optimized at 30 °C and 180 rpm: carbon source, 50 g/L maltose; initial pH 7; and 8 g/L Tween 80. Subsequently, batch fermentation was performed under the optimized conditions in a 5-L stirred-tank fermentor with a working volume of 3 L. The fermentation broth contained 303 g/L maltose, which produced 122.34 g/L pullulan with an average productivity of 1.0195 g/L/h and 82.32 g/L dry biomass within 120 h. The conversion efficiency of maltose to pullulan (Y%) and specific production rate (g/h/g dry cells) (Qs) reached 40.3% and 0.0251 g/L/g dry cells, respectively. The results showed strain A4 could be a good candidate for industrial production.

Plants utilize carbohydrates as the main energy source, but much focus has been on the impact of N and K on plant growth. Less is known about the combined impact of NH₄⁺ and K⁺ nutrition on photoassimilate distribution among plant organs, and the resultant effect of such distribution on growth of tobacco seedlings, hence this study. Here, we investigated the synergetic effect of NH₄⁺ and K⁺ nutrition on photoassimilate distribution, and their resultant effect on growth of tobacco seedlings. Soluble sugar and starch content peaks under moderate NH₄⁺ and moderate K⁺ (2-2 mM), leading to improved plant growth, as evidenced by the increase in tobacco weight and root activity. Whereas, a drastic reduction in the above indicators was observed in plants under high NH₄⁺ and low K⁺ (20-0.2 mM), due to low carbohydrate synthesis and poor photoassimilate distribution. A strong positive linear relationship also exists between carbohydrate (soluble sugar and starch) and the activities of these enzymes but not for invertase. Our findings demonstrated that NH₄⁺ and K⁺-induced ion imbalance influences plant growth and is critical for photoassimilate distribution among organs of tobacco seedlings.