Soil Nitrogen Status Modifies Rice Root Response to Nematode-Bacteria Interactions in the Rhizosphere

Abstract

It has been hypothesized that faunal activity in the rhizosphere influences root growth via an auxin-dependent pathway. In this study, two methods were used to adjust nematode and bacterial populations within experimental soils. One is "exclusion", where soil mixed with pig manure was placed in two bags with different mesh sizes (1mm and 5μm diameter), and then surrounded by an outer layer of unamended soil resulting in soil with a greater populations of bacterial-feeding nematodes (1mm) and a control treatment (5μm). The second method is "inoculation", whereby autoclaved soil was inoculated with bacteria (E. coli and Pseudomonas) and Nematodes (Cephalobus and C. elegans). In order to detect the changes in the rice's perception of auxin under different nutrient and auxin conditions in the presence of soil bacterial-feeding nematodes, responses of soil chemistry (NH4+, NO3- and indole acetic acid (IAA)), rice root growth and the expression of an auxin responsive gene GH3-2 were measured. Results showed that, under low soil nutrient conditions (exclusion), low NO3- correlated with increased root branching and IAA correlated with increased root elongation and GH3-2 expression. However, under high soil nutrient conditions (inoculation), a high NH4+ to NO3- ratio promoted an increase in root surface area and there was an additional influence of NH4+ and NO3- on GH3-2 expression. Thus it was concluded that soil bacterial-feeding nematodes influenced soil nutritional status and soil IAA content, promoting root growth via an auxin dependent pathway that was offset by soil nitrogen status.

Fig 1. Dynamic of soil parameters, root architecture and relative GH3-2 expression in rice roots in the ‘exclusion’ experiment, after 0, 10, 20 and 30 days of plant growth.

Comparisons are shown for two soil types treated using either a 1mm or 5 μm, to allow or exclude (respectively) bacterial-feeding nematodes and where: (a) nematode abundance (b): NH₄⁺, (c) NO₃^-,(d) IAA content, (e) relative GH3-2 expression: (f) total root length; (g) root tips; (h) root average diameter.

Fig 2. Relationship between soil IAA content and (a) Nematode abundance or (b) relative GH3-2 expression in rice roots.

1mm: soil contains more bacterial-feeding nematodes than 5 μm: see text for details.

Fig 3

Dynamic of soil mineral nitrogen in the ‘inoculation’ experiment; (a) nematode abundance, (b): NH4+, (c) NO3-, before and 14 and 20 days after inoculation of sterilised soil with: CK = sterilized soil; N1 = Cephalobus; N2 = C. elegans; E = E. coli; EN1 = E.coli and Cephalobus; EN2 = E and N2; P = Pseudomonas; PN1 = P and N1; PN2 = P and N2; B = Burkholderia; BN1 = B and N1; BN2 = B and N2.

Fig 4. Effects of bacterial-feeding nematodes on soil IAA content, root architecture and and relative GH3-2 expression in rice root in the ‘inoculation’ experiment.

(a) IAA content; (b) relative GH3-2 expression, (c) total root length; (d) root tips; (e) root average diameter; (f) root surface area. Treatments are as Fig 3.

Fig 5

Overview of the effects of changes of faunal composition on soil chemistry and root parameters (a) the ‘exclusion’ dataset—Effect of treatment on parameter as supported by REML (See Table 5—Text and box colour, GREEN 1mm>5mm, BLUE 1mm<5mm, Dashed line only significant in interaction with DAYS). Lines and arrows interactions parameters correlations supported by REML (See–Table 2 lines and arrows—GREEN positive effect, BLUE Negative effect, Pale blue dotted line interaction only significant due to interaction with DAYS). Green dashed line interactions supported by correlations (Fig 3) (b-c) the ‘inoculation’ dataset (b) Effect of the presence of any bacteria and nematodes, (c) Effect of the presence of specifically IAA-producing bacteria (vs other bacteria and no bacteria) and nematodes. (See Table 6) Text colour = nematode effect, Underline colour = bacterial effect, box = significant interaction effect. GREEN = positive effect of the presence, BLUE = negative effect. BLACK = more complex interactions. Lines and arrows interactions parameters correlations supported by REML (See Table 4 lines and arrows—GREEN positive effect, BLUE Negative effect, BLACK significant interactions with either bacteria (but not specifically IAA producing bacteria or nematode). Green dashed line weaker potential interactions (see Table 4d) (d) Comparison of the main effects in the two experiments. GREEN—increased under higher faunal levels or positive interactions BLUE—decreased under higher faunal levels or negative interaction.