*Article NB-LRRs* **Not Responding Consecutively to** *Fusarium oxysporum* **Proliferation Caused Replant Disease Formation of** *Rehmannia glutinosa*

**Aiguo Chen 1,2, Li Gu <sup>1</sup> , Na Xu <sup>2</sup> , Fajie Feng <sup>1</sup> , Dexin Chen <sup>2</sup> , Chuyun Yang <sup>1</sup> , Bao Zhang <sup>1</sup> , Mingjie Li 1,\* and Zhongyi Zhang 1,\***


Received: 4 June 2019; Accepted: 25 June 2019; Published: 29 June 2019

**Abstract:** Consecutive monoculture practice facilitates enrichment of rhizosphere pathogenic microorganisms and eventually leads to the emergence of replant disease. However, little is known about the interaction relationship among pathogens enriched in rhizosphere soils, Nucleotide binding-leucine-rich repeats (NB-LRR) receptors that specifically recognize pathogens in effector-triggered immunity (ETI) and physiological indicators under replant disease stress in *Rehmannia glutinosa*. In this study, a controlled experiment was performed using different kinds of soils from sites never planted *R. glutinosa* (NP), replanted *R. glutinosa* (TP) and mixed by different ration of TP soils (1/3TP and 2/3TP), respectively. As a result, different levels of TP significantly promoted the proliferation of *Fusarium oxysporum* f.sp. *R. glutinosa* (FO). Simultaneously, a comparison between FO numbers and *NB-LRR* expressions indicated that *NB-LRRs* were not consecutively responsive to the FO proliferation at transcriptional levels. Further analysis found that *NB-LRRs* responded to FO invasion with a typical phenomenon of "promotion in low concentration and suppression in high concentration", and 6 *NB-LRRs* were identified as candidates for responding *R. glutinosa* replant disease. Furthermore, four critical hormones of salicylic acid (SA), jasmonic acid (JA), ethylene (ET) and abscisic acid (ABA) had higher levels in 1/3TP, 2/3TP and TP than those in NP. Additionally, increasing extents of SA contents have significantly negative trends with FO changes, which implied that SA might be inhibited by FO in replanted *R. glutinosa*. Concomitantly, the physiological indexes reacted alters of cellular process regulated by NB-LRR were affected by complex replant disease stresses and exhibited strong fluctuations, leading to the death of *R. glutinosa*. These findings provide important insights and clues into further revealing the mechanism of *R. glutinosa* replant disease.

**Keywords:** *Rehmannia glutinosa* L.; replant disease; rhizosphere microbes; *NB-LRR*; plant hormone
