Multiomic approaches reveal hormonal modulation and nitrogen uptake and assimilation on the initial growth of maize inoculated with Herbaspirillum seropedicae

Herbaspirillum seropedicae is an endophytic bacterium that can fix nitrogen and promote a hormonal imbalance that leads to a plant growth-promoting effect when used as a microbial inoculant. Studies focused on mechanisms of action are crucial for a better understanding of the bacteria-plant interaction and optimization of plant growth-promoting response. The work aims to understand the underlined mechanisms responsible for the early stimulatory growth effects of the H. seropedicae inoculation in maize. To perform it, we combined transcriptomic and proteomic approaches with physiological analysis. The results obtained with the inoculation showed increased root biomass (233 and 253%) and shoot biomass (249 and 264%), respectively, for the fresh and dry mass of maize seedlings and increased green content and development. Omics data analysis for the positive biostimulation phenotype revealed that inoculation increases N-uptake and N-assimilation machinery through differential expressed nitrate transporters and amino acids pathway, as well carbon/nitrogen metabolism integration by the tricarboxylic acid cycle and the polyamines pathway. Additionally, phytohormone levels of root and shoot tissues increased in bacterium-inoculated-maize plants leading to feedback regulation by the ubiquitin-proteasome system. The early biostimulatory effect of H. seropedicae partially results from hormonal imbalance coupled with efficient nutrient uptake-assimilation and a boost in primary anabolic metabolism of carbon-nitrogen integrative pathways.

草螺菌（Herbaspirillum seropedicae）是一类可固氮的内生细菌，作为微生物接种剂（microbial inoculant）施用时，可通过诱导植物激素失衡产生促植物生长效应。聚焦其作用机制的研究，对于深入解析细菌-植物互作关系、优化促植物生长响应至关重要。本研究旨在阐明草螺菌接种玉米后，介导早期促生长效应的潜在机制。为此，本研究联合转录组学（transcriptomic）、蛋白质组学（proteomic）技术与生理分析开展实验。接种实验结果显示，玉米幼苗的根生物量鲜重、干重分别提升233%与253%，地上部生物量鲜重、干重分别提升249%与264%，同时植株青绿含量与生长发育水平均得到改善。针对该阳性生物刺激表型的组学数据分析表明，草螺菌接种可通过差异表达的硝酸盐转运蛋白与氨基酸代谢通路，增强植物的氮吸收与氮同化系统，并通过三羧酸循环与多胺通路实现碳/氮代谢的整合。此外，接种细菌的玉米植株根、地上部组织的植物激素水平均出现上调，进而通过泛素-蛋白酶体系统（ubiquitin-proteasome system）介导反馈调控。草螺菌的早期生物刺激效应，部分源于激素失衡与高效的养分吸收-同化作用，以及碳氮整合通路的初级合成代谢增强。