Panax ginseng C. A. Meyer (Ginseng, Araliaceae) in China and Korea is a kind of special pharmaceutical crops with great advantages and features, because the ginseng roots containing useful medicinal ingredients are highly valued, which are used world-widely to treat various diseases by herbal medicine practitioners (Jiao et al., 2014). However, a lot of factors such as deteriorated soil conditions (Dou et al., 1996; Huang et al., 1996), auto-toxicity (He et al., 2009), and plant diseases (soil sickness) would cause the continuous cropping obstacle of ginseng, which restricted the healthy growth of ginseng for long periods, and this also becomes a serious problem that restricts the development of ginseng industry. This study primarily focuses on plant diseases (Zhao, 1995).

Because of the long-period plantation in the same area, soil-borne pathogens including bacteria, fungi, and nematodes get the opportunity to infect the ginseng root, which makes the ginseng plant vulnerable and low quality (Kim et al., 2006; Ohh et al., 1986). Among them, fungi are the major pathogens to cause ginseng root diseases, for example, ginseng rust rot disease caused by the infection of Cylindrocarpon destructans which is one of most popular and well known fungi. C. destructans usually affects the surface of ginseng root (Chung, 1975; Lee et al., 2015; Yu, 1987). Ginseng rust rot disease is widespread in many regions and countries with an incidence of 25% - 44%, or in even serious area, 80% - 100% to badly influence the production and value of ginseng root.

It has been reported that the indirect activities of ecological effects of plant root exudates and imbalanced proportion of soil microbial population are considered to be the main factors to cause plant diseases. Some populations of microbes could make use of specific components of root exudates to achieve rapid growth by chemotactic response, thus inhibiting the growth of other beneficial microbial to change the composition and quantity of root exudates, which further provides more carbon and energy for the chemotactic and pathogenic microorganisms to create a vicious cycle resulting in stunting plant growth (Dixon et al., 1993; Zhou et al., 2010). Some report also said that chemotaxis may present a competitive advantage for certain detrimental microorganisms in early establishment on the root of many crop plants leading to reduce root vigor (Ling et al., 2011).

Chemotaxis of specific microorganisms toward root exudates in soil is related to attraction toward specific components detected in the exudate (Li et al., 2014). During the time of ginseng growth, the ginseng root usually secretes many kinds of secondary metabolites, which are regarded as the specific components in the root exudates, and as it happens, the most productive period of secretion of ginseng root exudates are coincidentally the sameas the period of ginseng diseases outbreak. There is no relevant report showing whether exudates affect the outbreak of main soil-borne diseases and chemotactic response of major pathogens of ginseng, or which kinds of pathogens would be induced by the response, or what the relevant factors are, or what the mechanism is, or how to control the happening of plant diseases by exudates.

Therefore, in this paper, three polarities (the aqueous phase, butanol phase and petroleum ether) of ginseng root exudates are used as experimental materials to research the chemotaxis response of C. destructans towards ginseng root exudates. This study will help us to understand the real process of energy exchange and information transfer between ginseng root and its surrounding soil environment, and to explore the important theoretical and practical significance to solve the problem of continuous cropping obstacle in ginseng.

The previous study has reported that organics produced by photosynthesis could be released into the rhizosphere as root exudates, some of which were all oleochemical and semio chemicals to provide carbon, energy and nutrition for microbial growth and multiplication. Meanwhile, some soil microorganisms with the help of chemotactic responses to the rhizosphere and root surface for colonization and reproduction. Different root exudates have certain influence in microbe species, microbial flora and microbial physiological characters, and the quantity of microbe is positively correlated with the root exudates accumulation (Badri and Vivanco, 2009; Bais et al., 2006). In the soil, pathogenic fungi are frequently affected by the root exudates which may attract or repel the nomadic pathogens, and may stimulate or inhibit the germination of resting vegetative forms. For example, some essential amino acids (arginine, lysine and histidine) secreted by mulberry sapling roots and flavonoids released by macadamias roots could promote conidia germination, mycelia growth and zoospores accumulation of some funguses (Liu et al., 2005; Zhenli and Zhiyi, 2005). The root exudates of leguminous plants contain some materials as signals to induce rhizobium to produce nodulation factors, which could, thereby, cause identification, infection, colonization and nodulation of rhizobium towards leguminous plants (Zhang and Zhang, 2005). However, few studies have reported the effects of ginseng root exudates on the chemotaxis of C. destructans, and ginseng rust rot disease is an urgent problem needing to be solved by ginseng-planting regions or countries (Lee et al., 2016).

In China, the growth and development stage of ginseng lies in the period from late May to late June when root exudates gradually increase, reaching the peak period of ginseng rust rot disease (Wang, 2001). Though there are lots of factors that affect the occurrence of C. destructans, the exudates of roots definitely play a significant role in it. Our study accurately sought to analyze that the fungus exhibited stronger positive chemotactic responses towards the low and middle concentration of ginseng root exudates in aqueous phase, butanol phase and petroleum ether, and under the optimal chemotactic parameters, the spores germination, mycelial growth rate and dried weights of C. destructans significantly increased, whereas all of them decreased as the increase of root exudates concentration. Li et al. (2009) reported some of the ginseng root exudates inhibited the colony growths of C. destructrans at a high concentration and accelerated the growths at a medium concentration.

Allelopathic effect on stress response can be dynamic and transformation, such as certain concentration can enhance allelopathic effect, but high concentration decreased allelopathy. It is because that the microorganisms converse between growth and defense by adjusting the external allelopathic material adaptability. Due to the presence of specific chemical receptor on the surface of the microorganisms membranes, it allows the microorganisms to detect changes in the concentration of chemical substances in the extracellular environment, and by intracellular delivery system will feel the transformation of chemical signals into intracellular signals and then effects the microorganisms taxis movement behavior.

Nicol et al. (2003) found that ginsenosides secreted by American ginseng root had positive influence in the species composition and growth of the soil fungal (Phytophthora cactorum and C. destructans) community in the effective concentrations, while it inhibited the growth of harzianum. When Phytophthora megasperma infects soybean, its zoospores would be strongly attracted by the daidzein and genistein which are secreted in the exudates, and the chemotactic effect happens even in the low concentration, but when Phytophthora megasperma infects alfalfa and douglas fir, it does not show this kind of chemotactic response (Nicol et al., 2003). Bagga and Straney (2000) reported that apigenin could effectively stimulate the germination of Nectria haematococca spores, and such stimulation could take place in lower concentrations, but for other flavonoids, such as luteolin and hesperetin, this impact could be triggered in higher concentrations, and the effect of flavonoids as stimulation factors of spores germination was related with inhibition activity of cAMP phosphatase, thus increasing the ability of cAMP.

Above all, we suggested that ginseng root exudates could be perceived, metabolized and transformed by the rhizo-fungus in the rhizosphere, and in addition, there might be an association between root exudates concentration effect and chemotactic response, colonization, reproduction, community building of soil microbial.

Previous research has shown that the morbidity of C. destructans isolated in P. ginseng is higher in the roots before and after growth stage than that during the growth stage, while during three periods, there is no obvious difference in the external factors, and thus the rhizosphere environment plays an important role in ginseng root exudates. Zhang et al. (2014) has reported that root exudates contain ginsenoside, organic acid, phenolic acid, and other secondary metabolites, some of which would cause the acid-base imbalance of soil and lead to the chemotaxis attraction of some soil microorganism swimming or colonizing on the surface of plant root. Larsen et al. (2004) has reported that chemotaxis of some soil microorganism are sensitive to temperature, increased by the rising of temperature, which is related to methylation and phosphorylation of receptor proteins on the intracellular membrane of soil microorganism because the kinetics of enzymes that take part in the process of methylation and phosphorylation is influenced by the temperature.

So we investigated the effect of and temperature of ginseng root exudates, which seems to be the important factors for chemotaxis of C. destructans. In this study, our findings suggested that when C. destructans was cultured in different and temperatures, the chemotaxis towards ginseng root exudates was different from that in rose bengal medium, i.e. it was higher in weak acid environment than that in the neutral and basic environment, meanwhile, chemotactic response was the highest at 20℃ compared with that at 10, 15 and 25℃.

The study was not only indirectly verified that root exudates in the rhizosphere environment acted as specific chemoattractant for pathogenic fungus (C. destructans) growth and multiplication, but it also indicated that the chemotaxis of C. destructans towards ginseng root exudates may supply a positive advantage indetermining the outcome of its colonization.

In this study, although three kinds of inducers couldn’t make C. destructans grew faster, they could release some volatile substances to forma gradient in the soil which could be detected by C. destructans, which led to the regulation of the mycelia activities, and the results were as follows; 1) Activities of mycelia on the side that receives ginseng root exudates was enhanced to promote the transmission capability of nutrition or water, thus the nutrition accumulating faster to reach a degree of saturation condition where more branches of mycelia grow to make the whole fungal colony look fluffier and firmer due to the repulsive interaction and larger angles of mycelia, 2) Another phenomenon was that ginseng root exudates may stimulate and resolve some intracellular molecules of C. destructans cells, and objectively improve the concentration of intracellular substances, which made the osmotic pressure of mycelia increase to absorb more water to grow fluffier and firmer. If there were any supportive materials such as membrane or medium around the mycelia, the mycelia easily touched upon the materials to grow along then, and however, if there were no such supportive materials, the growth of branches would be restricted, and as a result, the margin of bacterial colony would be on the decline. Thereby, from the macro view, the results were shown in Fig. 6 that mycelia grew toward ginseng root exudates, which formed the chemotactic response of C. destructans that we discovered. As for the formation mechanism, further studies are needed.