The role of arbuscular mycorrhizas in plant zinc and phosphorus nutrition

Abstract

Arbuscular mycorrhizas (AM) are an association formed between terrestrial plants and a specialised group of soil fungi. The primary benefit to plants of forming AM is that of enhanced soil nutrient uptake (especially phosphorus [P]), although there are many other benefits to plants of forming AM. Soil zinc (Zn) deficiency is a widespread and increasingly important issue that can have detrimental effects upon plant and human nutrition. There is an opportunity to maximise the efficiency of AM Zn uptake to address the issue of plant Zn deficiency. However, for this we need to improve our understanding of the role of AM in plant Zn uptake. This includes investigation into the interactions between Zn and P when taken up by AM, and investigation into the activity of the mycorrhizal pathways on Zn and P uptake. One of the challenges associated with studying AM is establishing an effective non-mycorrhizal control. The approach used in this study to overcome this issue is the use of a mycorrhiza-defective tomato (Solanum lycopersicum L.) mutant (rmc) and its wild-type progenitor (76R). The mutant has reduced mycorrhizal colonisation (typically less than 1% root length colonised). When compared to its wild-type, rmc provides an ideal non-mycorrhizal control in studies of AM. Furthermore, in studies focused on the mycorrhizal pathway of P uptake, a mutant of Medicago truncatula that is defective in the gene controlling for mycorrhizal P uptake (mtpt4) was compared to its wild-type. To trace nutrient uptake via the mycorrhizal pathway, I used radioactive isotopes of P (33P and 32P) and Zn (65Zn), in conjunction with buried hyphal compartments, accessible only by the external hyphae of AMF. The combination of these techniques provides an effective way to directly quantify the contribution by AM to plant P or Zn content, and compare it to that of the direct uptake pathway. In the studies described in this thesis, the mycorrhizal plants generally had higher densities of nutrients (especially P) in their tissues and were larger than the non-mycorrhizal plants. This was especially the case when concentrations of P and/or Zn were low in the soil. We found that P and Zn uptake by AM was highly interactive and had an effect on the nutrition, growth, and resource allocation of the plants. There was generally an antagonistic effect of P upon Zn uptake when they were both applied as fertilisers, however, the extent of the antagonistic effect was dependent on the Zn fertiliser material applied. As P and Zn concentration increased in the soil, the benefit of being mycorrhizal was reduced for Zn uptake but generally maintained for P uptake. Furthermore, once soil Zn concentrations reached toxic conditions, there was evidence of the “protective effect” of AM, whereby AM reduced the uptake of Zn compared to the non-mycorrhizal state. We discovered that up to a third of a plant’s Zn is delivered via the mycorrhizal pathway on a Zn-deficient soil, but this contribution decreased significantly with increasing soil Zn concentration. With regards to P uptake by AM, we hypothesised that the mycorrhizal pathway uptake of P would have a local, as well as a long-distance effect, upon direct pathway P uptake (via roots). While this hypothesis was supported by the physiological data, the expression of mycorrhizal and direct pathway P transporter genes did not clearly agree with the physiological data, and further study into this hypothesis will be required. In conclusion, in studies of AM and plant Zn nutrition, it is important to consider the effect of soil P concentration, as it highly modifies the relationship between AM and Zn. Furthermore, the mycorrhizal pathway of uptake for Zn can appear to be active or inactive based on comparison of plant Zn content between mycorrhizal and non-mycorrhizal plants, however, the actual activity can be very different. Mycorrhiza-defective or mycorrhiza-uptake-pathway-defective mutant plant genotypes and radioactive isotope labeling techniques will both be very useful in future studies of the role of AM in plant Zn and P nutrition.