Wang Ruiting, Liu Jinxin, Jiang Wanyi, Ji Pingsheng, Li Yonggang
Agricultural College, Northeast Agricultural University, Harbin, China.
Department of Plant Pathology, University of Georgia, Tifton, GA, United States.
Front Microbiol. 2022 Apr 21;13:833968. doi: 10.3389/fmicb.2022.833968. eCollection 2022.
Alfalfa long-term continuous cropping (CC) can pose a serious threat to alfalfa production. However, the mechanism of alfalfa CC obstacle is unclear as of today. Our preliminary study showed that the main factors of CC obstacle were not the lack of nutrients or water in alfalfa rhizosphere soils. Further, we evaluated physic-chemical property, microbial population structure, and metabolite differences of alfalfa rhizosphere soils with CC for 1, 7, and 14 years based on analysis of metabolomics and microbiomics. Four phenolic acid metabolites, including p-coumaric acid, ferulic acid, vanillic acid, and p-hydroxybenzoic acid, were found to have significant differences among different CC years, which may be the key factors of CC obstacle. Among them, p-coumaric acid and ferulic acid could significantly decrease the germination rate of alfalfa seeds by 21.11 and 16.67% at the concentration of 100 μg/mL and the height (root length) of alfalfa seedlings by 21% (32.9%) and 13.72% (16.45%). Moreover, these metabolites could effectively promote the growth of some pathogenic fungi, causing alfalfa root rot. Among them, p-coumaric acid obviously and significantly aggravated the occurrence of alfalfa root rot. With the increase of CC years, soil microbial community changed from fungi to bacteria; fungi decreased by 10.83%, fungi increased by 8.08%, and beneficial microorganisms decreased with the increase of CC years. Field analysis and experimental verification showed that the above results were consistent with that of CC obstacle in the field. Among the key metabolites, the autotoxicity of p-coumaric acid was the strongest. This study fully proved that the continuous accumulation of autotoxic substances in alfalfa rhizosphere was the key factor causing alfalfa CC obstacles.
苜蓿长期连作会对苜蓿生产构成严重威胁。然而，截至目前，苜蓿连作障碍的机制尚不清楚。我们的初步研究表明，连作障碍的主要因素并非苜蓿根际土壤中缺乏养分或水分。此外，我们基于代谢组学和微生物组学分析，评估了连作1年、7年和14年的苜蓿根际土壤的理化性质、微生物种群结构和代谢物差异。发现对香豆酸、阿魏酸、香草酸和对羟基苯甲酸这四种酚酸代谢物在不同连作年限间存在显著差异，这可能是连作障碍的关键因素。其中，对香豆酸和阿魏酸在浓度为100μg/mL时可使苜蓿种子发芽率分别显著降低21.11%和16.67%，使苜蓿幼苗高度（根长）分别降低21%（32.9%）和13.72%（16.45%）。此外，这些代谢物可有效促进一些致病真菌的生长，导致苜蓿根腐病。其中，对香豆酸明显且显著加剧了苜蓿根腐病的发生。随着连作年限的增加，土壤微生物群落从真菌向细菌转变；真菌减少了10.83%，细菌增加了8.08%，有益微生物随着连作年限的增加而减少。田间分析和实验验证表明，上述结果与田间连作障碍情况一致。在关键代谢物中，对香豆酸的自毒作用最强。本研究充分证明，苜蓿根际自毒物质的持续积累是导致苜蓿连作障碍的关键因素。
