IN INOCULANTS Nodulaid - 17th International Nitrogen Fixation ...

17 th International Congress on Nitrogen Fixation
Fremantle, Western Australia
27 November – 1 December 2011
Session Details: Tuesday 29 November 2011
Authors:
Concurrent Session 5 – Bacteroids & Symbiosomes
1100 - 1230
Vanessa J Melino 1 , Elizabeth A Drew 2 , Gordon Thomson 1 , Rosemary White 3 , Wayne G
Reeve 1 , Ross A Ballard 2 , and Graham O‟Hara 1 .
1 Centre for Rhizobium Studies, Murdoch University, Murdoch, 6150, Western Australia
2 South Australian Research and Development Institute, Plant and Soil Health, Urrbrae,
5064, South Australia
3 Commonwealth Scientific and Industrial Research Organisation, Plant Industry,
Canberra, 2601, ACT
Presentation Title: Inside out: an in-depth characterisation of effective (Fix + ), sub-optimal (Fix partial ) and
ineffective (Fix - ) N2-fixation in Trifolium symbiosis
Presentation Time: 1200 - 1220
A major challenge to the formation of a successful symbiotic partnership between Trifolium spp. and Rhizobium
leguminosarum bv. trifolii (R.l.t) is the presence of naturalized soil bacteria which can cause the induction and
infection of root nodules without the benefits of nitrogen fixation (ineffective) or which may alternatively result in
poor nitrogen fixation (sub-optimal). Analysis of symbiotic performance and morphological examination of nodule
and bacteroid development enabled the classification of each of the 16 symbiotic combinations into one of four
groups: Effective ( Fix 90-100% ), Sub-optimal ( Fix 10-70% ), Ineffective-differentiated (Fix - ) and Ineffective-vegetative (Fix -
). The effective combinations are each within the nodule function range of 90-100% containing infection threads,
bacteroids in inter-zone II-III and III with 1-2 bacteroids per peribacteroid unit. Sub-optimal combinations result in
a varied rate of fixation from 10 to 70% and their nodules contain an abundance of senescing bacteroids present
either in an enlarged and developmentally early zone of senescence (IV) or where zonation is absent, they are
found adjacent to bacteroid-dense plant cells. Although differentiated Y-shaped bacteroids of effective nodules
were like-wise observed in sub-optimal nodules they were on average shorter in length. Ineffective-differentiated
combinations produce nodules which are absent of zonation, void of acetylene reduction activity and their plant
cells contain elongated but less swollen bacteroids alongside senescing bacteroids. Ineffective-vegetative
combinations produce nodules which are absent of bacteroids despite the presence of rhizobia-filled infection
threads. These findings demonstrate the diversity of symbiotic outcomes from combinations of symbionts within
the same genus, and the range of failures that can occur during infection. Future functional genomic efforts will
be directed towards studying genetic control of internalization of rhizobia by cortical cells (failed in the ineffectivevegetative
nodules), abortion of bacteroid differentiation post-elongation (observed in the ineffectivedifferentiated
nodules) and accelerated bacteroid senescence post-differentiation (observed in sub-optimal
nodules).
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2011