plant surface microbiology.pdf

6 Signalling in the Rhizobia–Legumes Symbioses 111
4 Signal Perception and Molecular Biology of Nodule
Initiation
On the molecular and cellular level a large number of responses of legume
roots to Nod factors (LCOs) are known (Table 3). In the epidermis and the
root hairs, ion fluxes, plasma membrane depolarization and accumulation of
calcium in the root hair tips have been observed within seconds. In the range
of minutes; cytoskeleton modifications, root hair deformation and specific
gene expression, e.g., for ENOD 12 are found as well as calcium 2+ spiking and
phospholipase C and D activation. In the range of hours to days, formation of
pre-infection threads and cell divisions in the nodule primordium can be
observed, together with the expression of other early nodulins such as ENOD
20. At the same time, in the vascular tissue, an inhibition of polar auxin transport
and a specific gene expression of ENOD 40 follow. Several of these reactions
are triggered by nanomolar concentrations of Nod factors.
Table 3. Responses of legume roots to Nod factors (modified from Cullimore et al. 2000;
Hartog et al. 2001; Hogg et al. 2002)
Tissue Responses Rapidity of Concen- Tested
response tration plant genera
of Nod and species
factors
applied
Epidermis Ion fluxes Seconds nM Medicago
and root Plasma membrane Seconds nM Medicago
hairs depolarization
Increase in intracellular pH Seconds nM Medicago
Accumulation of Ca 2+ in Seconds nM Medicago, Vigna
root hair tip
Ca 2+ spiking 10 min nM Medicago, Pisum
Gene expression Minutes–hours fM–pM Medicago
(e.g., ENOD12, RIP1)
Root hair deformation Minutes–hours nM–µM Many
Cytoskeleton modification Minutes–hours fM–pM Phaseolus, Vicia
Phospholipase C and Minutes–hours nM Vicia sativa
D activation
Cortex Gene expression (e.g., ENOD20) Hours–days pM Medicago
Formation of pre-infection Days nM–µM Vicia
threads
Cell division leading to nodule Days nM–µM Many
primordium formation
Competitive nodulation blocking Days nM Pisum
(Cnb)
Vasculature Inhibition of polar auxin Minutes Trifolium
transport
Gene expression (e.g., ENOD40) 24 h-days nM–µM Glycine, Vicia,
Medicago