Rik H. M. Op den Camp,1
Huub J. M. Op den Camp,4
Ton Bisseling,1,5 and
1Department of Plant Sciences, Laboratory of Molecular Biology, Wageningen University, Droevendaalsesteeg 1, 6708 PB, Wageningen, The Netherlands; 2Department of Agricultural Biotechnologies, Universitá di Padova, Viale dell'Universitá 16, 35020 Legnaro (Padova), Italy; 3Department of Plant Sciences, Laboratory of Microbiology, Dreijenplein 10, Building Number 316, 6703 HB Wageningen, The Netherlands; 4Department of Microbiology, IWWR, Radboud University Nijmegen, Heyendaalseweg 135, 6525 AJ, Nijmegen, The Netherlands; 5College of Science, King Saud University, Post Office Box 2455, Riyadh 11451, Saudi Arabia
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Accepted 21 March 2012.
The non-legume genus Parasponia has evolved the rhizobium symbiosis independent from legumes and has done so only recently. We aim to study the promiscuity of such newly evolved symbiotic engagement and determine the symbiotic effectiveness of infecting rhizobium species. It was found that Parasponia andersonii can be nodulated by a broad range of rhizobia belonging to four different genera, and therefore, we conclude that this non-legume is highly promiscuous for rhizobial engagement. A possible drawback of this high promiscuity is that low-efficient strains can infect nodules as well. The strains identified displayed a range in nitrogen-fixation effectiveness, including a very inefficient rhizobium species, Rhizobium tropici WUR1. Because this species is able to make effective nodules on two different legume species, it suggests that the ineffectiveness of P. andersonii nodules is the result of the incompatibility between both partners. In P. andersonii nodules, rhizobia of this strain become embedded in a dense matrix but remain vital. This suggests that sanctions or genetic control against underperforming microsymbionts may not be effective in Parasponia spp. Therefore, we argue that the Parasponia-rhizobium symbiosis is a delicate balance between mutual benefits and parasitic colonization.
© 2012 The American Phytopathological Society