The Nutrient And Auxin Mechanisms Of The Effects Of Bacterial-Feeding Nematodes On Rice Root Growth

Bacterial-feeding nematodes, one of the primary grazers of soil bacteria, have been shown to affect root growth of different plant species. Most researches realized that the effects of bacterial-feeding nematodes on plant root growth resulted from direct or indirect nutrient effects (mainly nitrogen mineralization). Bacterial-feeding nematodes grazing on bacteria accelerate bacterial turnover and increase the turnover of soil organic matter, increased the nitrogen mineralization and improved the supply of inorganic nitrogen, and subsequently the plant growth was stimulated. Several researchers, however, have claimed that this process may not fully explain the mechanism of bacterial-feeding nematode stimulating plant growth. Considering the similarly of the physiology and zoology between bacterial-feeding nematodes and protozoa, and that the bacterial-feeding nematodes occur at equal or greater biomass in the rhizosphere than protozoa, it is believed that the activity of bacterial-feeding nematodes in the rhizosphere will also stimulate root proliferation by hormonal effects as seen for protozoan grazing.Two ways, which are soil microbilal and plant, were constructed to study the mechanisms of the effect of bacterial-feeding nematode on plant root growth. First, a gnotobiotic microcosm experiments were conducted to study the interactions between different bacterial-feeding nematodes and different IAA-producing bacteria and their effects on soil IAA content and nitrogen mineralization; Then, with the same system to study the effects of interactions between different bacterial-feeding nematodes and different IAA-producing bacteria on rice root growth and rice root intrinsic gene responding to alteration of IAA content in soil, which is then illustrate the mechanisms of the effect of bacterial-feeding nematodes on plant root growth. Then, experiments with natural soil (soil didn’t sterile but directly incubated for enriched in soil native nematodes) were manipulated to determine the effects of bacterial-feeding nematodes on the root development of rice seedlings. Nematode grazing changing the microbial community was also discussed by physiology method Biolog and molecule method PCR-DGGE; Moreover, in order to illustrate the auxin effect of interactions between bacterial-feeding nematodes and bacteria on plant growth, the relative expression of the early auxin response genes GH3-2 in rice root was also measured by quantitative Real-time PCR. The main results were as follows:1. The effect of bacterial-feeding nematodes (Cephalobus sp.) on the number、activity and IAA production of IAA-producing bacteria (P and B) in gnotobiotic microcosm experimentThe results showed that:Grazing on bacteria did not stimulate the growth of the bacteria; however, Grazing on bacteria stimulated bacteria activity and the ability of producing IAA significantly. They were influenced by bacteria species and sampling time; the presence of nematodes increased the activity of P bacteria after 8 days, while after 20 days for B bacteria; the nematodes increased soil IAA cntent after 14 days for P bacteria and 20 days after incubation for B bacteria. Meanwhile, the number of nematodes was influenced by bacteria species. The number of Cephalobus sp. in P bacteria was higher in B bacteria, but the number of the nematode was 2 times more than initial for both treatments.2. The effect of different bacterial-feeding nematodes (Cephalobus sp. and C. elegans) on IAA-producing bacteria (P and B) and the consequence of these interactions on rice plant growth in gnotobiotic microcosm experimentThe results showed that the presence of nematodes affected the activity of IAA-producing acteria significantly, and the effects were varied with nematodes species, bacteria species and sampling time. C. elegans (N2) increased bacterial activity and soil IAA content much earlier than Cephalobus sp. (N1), which may be explained by different characteristic of nematode from different family. The effect of bacteria on the number of nematodes was different between different treatments. The number of nematodes in P treatment was higher than that in B treatment, and C. elegans was more than Cephalobus sp.. The presence of nematodes influences the root growth of rice including the total length, surface area and average diameter; the effects were varied with bacteria species and sampling time. The higher nitrogen uptake by the plant in the presence of nematodes than absence of nematodes, and the nitrogen content was higher in B treatment than that in P treatment. However, there was no significantly effect of nematode species on nitrogen uptake by the plant during the whole incubation.3. The effect of bacterial-feeding nematodes on soil IAA content and GH3-2 expression in rice root in gnotobiotic microcosm experiment The presence of nematodes increased soil IAA content, but the effect varied with different nematode and bacteria. Consistent with soil IAA content, the nematodes increased the GH3-2 expression in rice root. The correlations analysis showed that IAA correlate with the GH3-2 expression in rice root (r=0.663, P=0.019). Combined these results with the effects of bacterial-feeding nematodes on plant root proliferation and to some extent illustrated that bacterial-feeding nematodes affect plant root growth likely through the hormonal effects by intrinsic genetic program in plant root.4. The effect of bacterial-feeding nematodes on rice root growthRice seedlings grown in the soils containing more bacterial-feeding nematodes developed a highly branched root system with longer roots; and the effects varied with sampling days. That may due to the difference growth period and sensitivity ability to variable environment of different species plant.5. The effect of bacterial-feeding nematodes on soil nitrogen mineralizationIn the system with rice, soil minerial nitrogen contents for both treatments with more bacterial-dfeeding nematodes and few bacterial-dfeeding nematodes were all increased during 0～10th day, and then decreased during 10～30th day. Soil minerial nitrogen contents in SM1 treatment were significant higher than SM5 treatment at day 10, with the increasing days of incubation, soil minerial nitrogen contents in SM1 treatment decreased (82.6%) more than that in SM5(50.5%). The results indicated that the nematodes increased soil nitrogen mineralization, increased the inorganic N available for uptake by the plant, thus decreased the content in soil. The results also showed that rice may be fond of different form of nitrogen, which induced the difference between NH4+-N and NO3--N contents in soil.6. The effect of bacterial-feeding nematodes on rhizosphere microbial community structure (Biolog and PCR-DGGE)The results showed that:Bacterial-feeding nematodes affect microbial activity (indicated by AWCD) and significantly changed the soil microbial community structure that the soil microbe’s functions shifted evidently based on the sole carbon use pattern derived form Biolog method, and the effects varied with sampling time. The presence of nematodes significantly increased bacterial diversity in rice system. The results of DGGE showed that the richness index and diversity index, evenness index of soil microbe was increased during 10-20th day, decreased during 20th-30th day, and the diversity of soil microbe in SM1 were significantly higher than that in SM5 at day 20. The UPGMA analysis showed that the microbial community is similar between SM1 and SM5 at day 20 and 30, while different from day10. It is similar with the results got from Biolog. DGGE fingerprint showed that the nematode affect soil bacterial community structure via appearances or disappearance bands or alteration band intensity.7. The effect of bacterial-feeding nematodes on soil IAA content and GH3-2 expression in rice rootCompared to the control soils (SM5), the soils containing more bacterial-feeding nematodes (SM1) significantly enhanced contents of IAA in soil planted with rice. The correlations analysis between IAA content in soil and GH3-2 expression measured by PT-PCR in rice root showed that IAA significantly correlated with the GH3 expression (P=0.0261, r=0.865), indicating that the GH3-2 was up regulated expression in rice root by increased exogenesis IAA. Combined these results with the effects of bacterial-feeding nematodes on plant root proliferation and to some extent illustrated that bacterial-feeding nematodes affect plant root growth likely through the hormonal effects by intrinsic genetic program in plant root.To summarize, the research discussed two main mechanisms of the effect of bacterial-feeding nematodes on plant growth by two ways, one is rhizosphere soil bacterial community structure, and the other is isintrinsic genetic program in plant root system. These findings supported theoretic knowledge and technical to host soil nematodes as an important bio-resource in soil nutrient management and plant growth. The present study may be valid not only for theory, but also for future application in agriculture.