Given the major role of arbuscular mycorrhizal fungi (AMF) in ecosystems there is an urgent need to understand how these two components of AMF will respond to global environmental change, in which one key variable is temperature. In nature, AMF often experience large temporal (daily and seasonal) and spatial variations in temperature (particularly near the soil surface), which are likely strongly to affect key metabolic processes such as respiration. In both the IRM and ERM, respiration provides the energy (ATP and reducing equivalents) necessary for growth and cellular maintenance. Respiration also provides the ATP necessary for carbon and nutrient exchange between the IRM phase and surrounding roots. Although we know that AMF growth is temperature sensitive, we do not know how short- and long-term changes in temperature will affect respiration rates of either the IRM or ERM phases of AMF. In plants, temperature mediated differences in respiration are often modelled as a simple exponential function of temperature with a constant Q10 (the proportional change in respiration per 10oC rise in temperature). However, we now know that Q10 values calculated over short time periods are highly variable, particularly when substrate availability and/or the demand for respiratory ATP changes. Given their contrasting physical locations (one inside the root, the other out in the surrounding soil) and differences in their respective functions (with resultant differences in the demand for respiratory energy), the temperature response of respiration is likely to differ between IRM and ERM. A constant Q10 value cannot be applied to all systems because, in plants, respiration often acclimates (with the result that cold and warm-grown plants exhibit similar rates of respiration when measured at their respective growth temperature). Respiratory acclimation also occurs in soils (reflecting CO2 release by roots, AMF & other heterotrophs), as shown by the response of a tall grass prairie system to artificial warming. Global circulation models (GCMs) that fail to take into account variability in respiratory Q10 values and acclimation over-estimate annual respiratory CO2 release into the atmosphere and consequently over-estimate the extent to which atmospheric CO2 concentrations will rise over long periods. Given that respiration by AMF often represents a significant proportion of soil respiration, understanding (1) how respiration of the ERM responds to temperature and (2) how colonisation of roots by IRM impacts on the temperature response of root respiration, is vital if GCMs are to predict more accurately future rates of CO2 release by soils into the atmosphere.
Arbuscular mycorrhizal fungi (AMF) are among the most widespread and ecologically significant of all soil micro-organisms. These soil fungi form symbiotic associations with around two-thirds of all land plants (some 200 000 species) in which both partners benefit. The plant obtains enhanced phosphorus uptake and other benefits such as increased pathogen resistance while the fungus receives a supply of carbon from its host plant. In addition, arbuscular mycorrhizal fungi (AMF) are known to influence plant community structure and nutrient cycling in natural ecosystems as well as releasing large amounts of respiratory CO2 into the surrounding soil, which ultimately diffuses into the atmosphere (contributing to the greenhouse effect). It is therefore surprising that we know relatively little about how this influential group of fungi respond to conditions in the environment. Temperature is one such key environmental variable because it effects metabolic processes, can show wide variation (i.e. day and night, winter and summer) and is predicted to increase as a result of global climate change. Although we do know how plants respond to variations in temperature, most studies on have been conducted using non-mycorrhizal (i.e. uncolonised) plants whereas in the natural environment AMF colonisation is the normal condition. In this proposal we will investigate how the AMF is influenced by temperature by measuring respiration. We will examine how AMF influence the respiration response of the plants root (compared to uncolonised roots). In addition, the AMF has two phases, one inside the root, and the other external to the root responsible for exploring the soil for nutrients. If both these phases respond to temperature in the same manner is currently unknown. We will vary the temperature experienced by the external phase independently of that of the plant so that the response of the AMF can be examined directly. We will also test if the fungus is able to adjust (i.e. acclimate) to changes in temperature such that respiration is maintained in a steady state rather than changing.
Plantago lanceolata seedlings were colonised or un-colonized with AM fungi, Glomus hoi, and were initially grown in pots at a constant temperature of 21oC. The impact of cold acclimation on respiration rates in colonised/un-colonized plants was assessed via transferring 21oC-grown plants to 7oC for 10 days. Respiration rates were measured as O2 uptake [in the absence/presence of exogenous substrates and uncoupler (CCCP)] or CO2 release. Rates of O2 uptake in the presence of glucose/CCCP provide an estimate of overall respiratory capacity. Capacity of one of the final stage of the cytochrome pathway (the energy producing pathway of mitochondria) was determined via measurement of cytochrome c oxidase (COX) capacity in whole tissue extracts. Finally, the impact of colonization on relative abundance of COX and alternative oxidase protein was determined by immunoblotting. In preliminary experiments, it was found that removal of extra-radical mycelium (ERM) had little effect on measured rates of O2 uptake by colonized roots; thus, respiration rates exhibited by mycorrhizal roots was assumed to reflect O2 uptake by the combination of roots and intra-radical mycelium (IRM), without contribution by ERMs.
We conclude that AMF colonization has a marked stimulatory effect on O2 uptake (but with little impact on overall respiratory capacity) in Plantago lanceolata roots; the decrease in COX protein in colonized roots suggests that the increase in overall flux was not associated with an increase in the capacity for ATP synthesis. The stimulatory effect of colonization on the rate of in vivo O2 uptake decreased as roots aged/grew bigger & respiratory capacity (and in vivo activity) also declined. Log-log plots of respiration against root fresh mass revealed that the slopes were near identical in mycorrhizal and non-mycorrhizal roots, and that the y-axis intercept was greater in the colonized roots; thus the proportional stimulation of respiration by mycorrhizal colonization is constant throughout root development. This finding has important implications for large scale models that seek to predict rates of below-ground plant respiration in the absence and presence of functioning mycorrhizal symbioses. AMF colonization was found to have no effect on the short-term temperature dependence of root respiration, with near identical Q10 values exhibited by mycorrhizal and non-mycorrhizal roots, therefore, we conclude that AMF colonisation does not alter the short-term temperature dependence of respiration by roots. Exposure of non-colonized roots to 7oC for 10 days resulted in a marked increase in in vivo O2 uptake (measured at 21oC), irrespective of the root age/size (i.e. non-colonized roots cold acclimated). Similarly, cold-acclimated non-mycorrhizal roots exhibited substantial increases in respiration rates when measured at the low growth temperature (7oC), with the result that respiration was near homeostatic in cold acclimated and warm-grown roots. Little acclimation was observed in roots colonized mycorrhizal fungi. This inability to increase respiration rates upon sustained exposure to cold likely reflects the fact that mycorrhizal respiration was running at near full capacity before shifting to the cold. Thus, we conclude that colonization restricts the ability of roots to dynamically adjust to long-term decreases in growth temperature.
|Effective start/end date
|28/08/06 → 27/08/09