Auxin and nitric oxide control indeterminate nodule formation.
BACKGROUND
Rhizobia symbionts elicit root nodule formation in leguminous plants. Nodule development requires local accumulation of auxin. Both plants and rhizobia synthesise auxin. We have addressed the effects of bacterial auxin (IAA) on nodulation by using Sinorhizobium meliloti and Rhizobium leguminosarum bacteria genetically engineered for increased auxin synthesis.


RESULTS
IAA-overproducing S. meliloti increased nodulation in Medicago species, whilst the increased auxin synthesis of R. leguminosarum had no effect on nodulation in Phaseolus vulgaris, a legume bearing determinate nodules. Indeterminate legumes (Medicago species) bearing IAA-overproducing nodules showed an enhanced lateral root development, a process known to be regulated by both IAA and nitric oxide (NO). Higher NO levels were detected in indeterminate nodules of Medicago plants formed by the IAA-overproducing rhizobia. The specific NO scavenger cPTIO markedly reduced nodulation induced by wild type and IAA-overproducing strains.


CONCLUSIONS
The data hereby presented demonstrate that auxin synthesised by rhizobia and nitric oxide positively affect indeterminate nodule formation and, together with the observation of increased expression of an auxin efflux carrier in roots bearing nodules with higher IAA and NO content, support a model of nodule formation that involves auxin transport regulation and NO synthesis.
Pii
Youry
Y
Dipartimento Scientifico Tecnologico, University of Verona, Verona, Italy. pii@sci.univr.it
Crimi
Massimo
M
Cremonese
Giorgia
G
Spena
Angelo
A
Pandolfini
Tiziana
T
D001692:Biological Transport / Q000502:physiology; D007210:Indoleacetic Acids / Q000378:metabolism*; D009569:Nitric Oxide / Q000378:metabolism*; D009876:Operon; D030781:Organisms, Genetically Modified / Q000378:metabolism; D027805:Phaseolus / Q000382:microbiology; D010937:Plant Growth Regulators / Q000378:metabolism*; D016961:Rhizobium leguminosarum / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D053204:Root Nodules, Plant / Q000254:growth & development / Q000378:metabolism / Q000382:microbiology*; D016962:Sinorhizobium meliloti / Q000235:genetics / Q000378:metabolism / Q000502:physiology*
D007210:Indoleacetic Acids; D010937:Plant Growth Regulators; D009569:Nitric Oxide; C030737:indoleacetic acid