Possible involvement of nitric oxide in promoting the initial growth of rice seedlings at low temperature by inoculation of <i>Bacillus pumilus</i> strain TUAT1 spores

<i>Bacillus pumilus</i> strain TUAT1, a type of plant growth – promoting rhizobacteria (PGPR), is used as an ingredient in biofertilizer. Although we confirmed that <i>B. pumilus</i> TUAT1 promotes the growth of rice seedlings at 25°C, this effect has not been shown at lower temperatures. In this study, we confirmed that inoculation of rice seedlings with spores of <i>B. pumilus</i> TUAT1 promoted seedling emergence from soil and subsequent growth at 15°C. Except for the effect on root growth, these effects disappeared when seeds were treated with 2–4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, a nitric oxide (NO) scavenger, simultaneously with <i>B. pumilus</i> TUAT1 inoculation. Increased NO accumulation was detected in seed embryos 3 h after inoculation, suggesting that NO plays a role in the effects of <i>B. pumilus</i> TUAT1 inoculation. Transcriptome analysis using RNA-seq suggested the involvement of immune responses and cytochrome respiratory pathways in increasing NO levels in embryos after inoculation with <i>B. pumilus</i> TUAT1. Transcriptome analysis also indicated that transcription of genes involved in cold tolerance was accelerated in embryos following the increase in NO resulting from inoculation with <i>B. pumilus</i> TUAT1. These findings suggest that a novel NO signaling mechanism is involved in PGPR-induced growth promotion in plants. These results also indicate that inoculation with <i>B. pumilus</i> TUAT1 spores may help overcome the initial growth failure of seedlings in direct sowing culture of rice in cold regions.短小芽孢杆菌（Bacillus pumilus）菌株TUAT1是一类植物促生根际细菌（plant growth-promoting rhizobacteria, PGPR），常被用作生物肥料的组分之一。尽管我们已证实短小芽孢杆菌TUAT1可在25℃下促进水稻幼苗生长，但低温下的促生效应尚未被明确验证。本研究证实，在15℃条件下，接种短小芽孢杆菌TUAT1孢子可促进水稻幼苗破土出苗及后续生长。除对根系生长的促生效应外，当在接种该菌株的同时，使用2-(4-羧基苯基)-4,4,5,5-四甲基咪唑啉-1-氧自由基-3-氧化物（一种一氧化氮（NO）清除剂）处理种子时，其余促生效应均会消失。接种后3小时，可在种子胚中检测到一氧化氮积累量显著升高，这表明一氧化氮在短小芽孢杆菌TUAT1的促生效应中发挥了关键作用。采用RNA-seq开展的转录组分析显示，接种短小芽孢杆菌TUAT1后，种子胚中一氧化氮水平升高与免疫应答及细胞色素呼吸通路的调控密切相关。转录组分析还发现，在该菌株接种引发的一氧化氮水平升高后，种子胚中与耐寒性相关的基因转录过程被显著加速。上述研究结果表明，一种新型的一氧化氮信号通路参与了根际促生菌诱导的植物生长促生效应。本研究结果同时证实，在寒冷地区水稻直播栽培中，接种短小芽孢杆菌TUAT1孢子或可有效克服幼苗初期生长受阻的问题。