Balance and trade : charting the course of fungal symbiosis in sugarcane root systems

The root systems of sugarcane (Saccharum L. spp. hybrids) host a variety of symbiotic fungal organisms including arbuscular mycorrhizae (AM) fungi and dark septate endophytes (DSE). AM/DSE symbioses are widely reported to influence the productive capacity of a range of agricultural crops via enhanced nutrient assimilation and plant stress tolerance. Presently, the biological and functional characteristics AM/DSE in sugarcane have only been examined superficially and remain poorly understood. Before the potential benefits of AM/DSE symbioses can be exploited through management strategies, characterisation of the temporal changes in AM/DSE colonisation over the sugarcane crop cycle is required. This study investigated the colonisation dynamics of AM/DSE in sugarcane roots during 1^st and 2^nd ratoon crop phases. A field study was undertaken at two proximal sites with different cropping histories to assess AM/DSE colonisation of sett and shoot roots over 28 weeks of crop growth. At each observation, the functional structures produced by AM (arbuscules, hyphae, spores and vesicles) were delineated and quantified. Significant differences in the rate of AM/DSE colonisation over time and between root types were observed. There were no significant differences in AM sett root colonisation between sites. For both fungal types, differences in shoot root colonisation between sites were not significant. DSE sett root colonisation was significantly lower in the 2^nd ratoon compared to the 1^st ratoon crop. For both fungal types, sett root colonisation was significantly higher than for shoot roots at both sites. Maximum AM/DSE sett root colonisation occurred between 5–7 weeks, prior to the establishment of shoot roots and rapid developmental period of ratoon crops. While AM sett root colonisation was consistent across ratoon phases, there is a need to account for the differences in DSE sett root colonisation levels between 1^st and 2^nd ratoon crops. The composition of total AM colonisation at both sites was dominated by arbuscules and hyphae with comparatively lower levels of spores and vesicles. A diversity of AM fungal morphologies previously undocumented in sugarcane was observed co-occupying individual root systems. These data provide a basis for the development of appropriate sampling regimes in future studies investigating management impacts on AM/DSE symbioses in sugarcane cropping systems.