TABLE CONTENTS

SESSION TWENTY-NINE – SOIL FOOD WEBS AND RHIZOSPHERE
COMPLEXITY
CONVENORS: LILIANE RUESS & BRYAN GRIFFITHS
Multitrophic Interactions in the Rhizosphere Shaped by Root-feeding
Nematodes
Ruess, L.
Institute of Zoology, Darmstadt University of Technology, Schnittspahnstrasse 3, 64287 Darmstadt, Germany
Plant parasitic nematodes, often referred to as ‘hidden enemies’, are among the most
widespread and important herbivores causing crop loss. They influence host plants mainly
directly by their carbon demand. These primary effects expressed at the cellular, tissue and
whole plant level have been extensively investigated. However, indirect nematode-plant
interactions may be at least as important as direct ones, as:
1. Root feeders can facilitate the allocation of photoassimilate carbon to roots, resulting
in higher exudation rates.
2. Nematode induced leakage of plant metabolites from damaged root cells may increase
carbon translocation to the soil microbial biomass.
3. The enhanced carbon supply to the rhizosphere likely promotes microbial
populations, and fosters mineralisation processes and plant growth.
This indicates a tight link between root-feeding nematodes and soil microorganisms, and
parallels can be drawn between nematode-plant and microbial-plant interactions. Moreover it
implies that the activity of root feeders imposes similar mechanism on microbial turnover
than does microfaunal grazing. Recent studies support these hypotheses. They revealed
additional indirect effects such as changes in root morphology, which modified microbial
communities in the rhizoplane. Overall, these multitrophic interactions in the rhizosphere
were dependant on the specific nematode-plant association and the developmental stage of
the nematode in the host.
What Nematodes Do for the Microbial Loop in the Rhizosphere
Griffiths, B.S.
Teagasc, Environment Research Centre, Johnstown Castle, Wexford, Ireland
Conceptually the hypotheses for signaling and positive feedback loops in the rhizosphere are
well advanced. The stimulation of bacterial growth by root exudation attracts bacterialfeeding
fauna, of which nematodes are a significant component. Grazing causes a change in
the rhizosphere microbial community structure and is specifically thought to encourage the
activity of hormone producing bacteria around the roots. The subsequent stimulation of root
growth and lateral root initiation leads to a positive feedback – more exudation stimulates
more bacteria and leads to more fauna, etc. The practical demonstration of these phenomena
is harder to come by. Most convincingly shown for protozoa in gnotobiotic systems, recent
studies with bacterial-feeding nematodes in field soil with mixed microbial communities will
presented and discussed.
5 th International Congress of Nematology, 2008 104