Pseudostellariae heterophylla, an important medicinal plant, has been shown to suffer from serious replanting disease that causes significant declines in both yield and quality. The objective of this study was to isolate the soil-born pathogen and antagonistic microorganism for investigation of the relationship between these microorganism and consecutive monoculture problems of P. heterophylla. In this study, we isolated the bacteria with different morphology from P. heterophylla rhizosphere soil, detected the pathogenicity of these bacteria to P. heterophylla plantlet and antagonistic ability to Fusarium of P. heterophylla biotype, identified their species and analyzed their abundance with qRT-PCR in rhizosphere soil of different planting years. We isolated Enterobacter sp1 and Burkholderia sp1 from P. heterophylla rhizosphere soil, and found Enterobacter sp1 possessed the pathogenicity to P. heterophylla plantlet and Burkholderia sp1 was antagonistic for Fusarium of P. heterophylla biotype. However, Burkholderia sp1 was not the antagonistic microorganism to Enterobacter sp1. The growth of Enterobacter sp1 was stimulated by phenolic acids at a certain concentration nearly closed to that in P. heterophylla rhizosphere soil, which increased as planting years of P. heterophylla increased. On the contrary, the phenolic acids had no promoting effect on the growth of Burkholderia sp1. And with the increase of planting years, the abundance of Burkholderia sp1 decreased and abundance of Enterobacter sp1 increased in the P. heterophylla rhizosphere soil. The results indicated that harmful microorganisms increase and beneficial microorganisms decrease, which were mediated by the accumulated autotoxins in rhizosphere soil of consecutively monocultured P. heterophylla, and hence leads to the imbalance of microbial community structure and the degradation of soil ecological function. So several abatement methods, such as crop rotation, specific microbial fertilizer and organic matter, could be used to improve the structure and functional diversity of microbial community, and overcome the obstacles of continuous cropping.

Three experiments were conducted to study the allelopathic stimulation and its underlying molecular mechanism of achyranthes medicinal plants in continuously monoculture system. The stimulators in the rhizospheric soil of continuously monocultured achyranthes plants were extracted by water and organic solvents. Results of the bioassay showed that the rhizospheric soil extracts had a significant promotive effect on the growth of achyranthes in continuous monoculture system, implying that the extracts, especially the water extracts might contain plant activators to stimulate the growth of the medicinal plants. Subtractive hybridization suppression (SSH) was used to investigate gene expression profiles of achyranthes in response to the extract treatments. Ten up-regulated genes from SSH-cDNA library were sequenced and assigned. Results indicated that flavonoids and phytosterol might play an important role in the positively allelopathic stimulation on achyranthes plants in continuous monoculture system. Comparative proteomics were employed to further unveil the molecular mechanism of allelopathic stimulation induced by the extracts. Compared with protein expression profile in control, 25 differentially expressed proteins and their functions were detected and identified in the treated plants. The results suggested that the extracts from continuously monocultured rhizospheric soils under Chinese medicinal achyranthes activated the genes encoding the key enzymes involved in terpenes and flavonoids synthesis, which in turn led to increased de novo synthesis of the stimulators, and hence promoted growth of achyranthes in consecutively monoculture system.

Monocropping is one of cultivation systems for many crops in production, especially for Pseudostellariae heterophylla, one of Chinese herbal medicinal plants. Continuous cropping will cause a series of problem. In the paper, we designed a continuous cropping of Pseudostellariae heterophylla and rotaton of Pseudostellariae heterophylla and rice to explore effects on photosynthetic physiology and medicinal quality of P. heterophylla. The results showed that compared to the rotation, the yield of continuous cropping of P. heterophylla was significantly (P<0.01) declined, only corresponding to one third of the rotation's, polysaccharide content and ginseng saponins Rb1 of the tuberous root reduced by 88.08% and 44.33%, respectively. The dynamic measurement of FluorCam fluorescence indicated that F o was not declined significantly (P>0.05), and NPQ decreased significantly (P<0.05). The relative chlorophyll content of leaf (SPAD) (P<0.01) and P n (P<0.05) declined. The decreased cell density in leaf and root was observed with microscope. In conclusion, the photosynthetic system was damaged by continuous cropping, especially for the protection mechanism. The light energy absorbed by PSII antenna pigment cannot be used in photosynthesis or dissipated, but accumulated in the leaves, which causes disordered photosynthesis, declined photosynthetic rate and malformed organs. The pathologic, and finally affects the morphogenesis of root that may be an important faction in restricting the yield and quality of P. Heterophylla.