Research PaperRespiratory ATP cost and benefit of arbuscular mycorrhizal symbiosis with Nicotiana tabacum at different growth stages and under salinity

- Symbiosis with Rhizophagus irregularis affects both respiration and ATP synthesis under P deficiency and saline soils.
- At early stages, symbiosis represented an ATP cost for the tobacco leaves which was detrimental for shoot growth.
- At mature stages, symbiosis represented an ATP benefit, which allowed faster growth.
- Under salinity, AOP contributed in the acclimation of ATP production to the same level of NM plants.

Growth and maintenance partly depend on both respiration and ATP production during oxidative phosphorylation in leaves. Under stress, ATP is needed to maintain the accumulated biomass. ATP production mostly proceeds from the cytochrome oxidase pathway (COP), while respiration via the alternative oxidase pathway (AOP) may decrease the production of ATP per oxygen consumed, especially under phosphorus (P) limitation and salinity conditions. Symbiosis with arbuscular mycorrhizal (AM) fungi is reputed by their positive effect on plant growth under stress at mature stages of colonization; however, fungal colonization may decrease plant growth at early stages. Thus, the present research is based on the hypothesis that AM fungus colonization will increase both foliar respiration and ATP production at mature stages of plant growth while decreasing them both at early stages. We used the oxygen-isotope-fractionation technique to study the in vivo respiratory activities and ATP production of the COP and AOP in AM and non-AM (NM) tobacco plants grown under P-limiting and saline conditions in sand at different growth stages (14, 28 and 49 days). Our results suggest that AM symbiosis represents an ATP cost detrimental for shoot growth at early stages, whilst it represents a benefit on ATP allowing for faster rates of growth at mature stages, even under salinity conditions.