5.2. Ammonia transport
Ammonium/ammonia (NH4
+/NH3) is an important nitrogen
fertilizer for crops. Whereas NH4
+ transporters have been identified in
plants since long, NH3 was initially suggested to cross membrane by
free diffusion [88]. Yet, several TIP2 homologs of Arabidopsis and
wheat were found to have a remarkable permeability to NH3, and may
therefore participate in NH3 compartmentalization in vacuoles
[88–90]. This idea remains to be assessed at the whole plant level and,
for instance, overexpression of AtTIP2;1 or AtTIP2;3 in Arabidopsis failed
to enhance whole plant NH4
+/NH3 accumulation [90].
The symbiotic interaction of plants with soil microorganisms may
provide an interesting context to understand the role of aquaporins in
plant NH4
+/NH3 acquisition. Firstly, mycorrhizal symbiosis causes
significant changes in aquaporin expression in host plants [91–93] and
specific aquaporins of legumes, such as GmNOD26, are expressed in
the peribacteroid membrane of N2-fixing root nodules [3]. Secondly,
recent work has demonstrated that GmNOD26 is a NH3-transporting
aquaporin that binds cytosolic glutamine synthase to create a
metabolite funnel and possibly enhance ammonia assimilation
efficiency [94,95]. Thirdly, some of fungal aquaporins expressed in
ectomycorrhiza showed a high NH3 permeability [96], and could
contribute to NH3 export from the fungal cytoplasm into the plant
apoplasm. Interestingly, a high-affinity NH4
+ transporter was found to
be specifically expressed in arbuscular mycorrhizal (AM) roots of Lotus
japonicus [97]. The interplay between these two types of NH4
+ or NH3
transport proteins may provide a basis to explain the fungus-based
nitrogen nutrition of plants in symbiotic roots.