Effect of tillage and crop on arbuscular mycorrhiza

Large-scale inoculation with arbuscular mycorrhizal fungi (AMF) is generally impractical in most regions and we have little understanding of the factors that determine inoculation success. Nevertheless, the ability to take full advantage of indigenous AMF for sustainable production needs to be developed within cropping systems. We used part of a long-term field experiment to understand the influence of tillage and the preceding crop on AMF colonization over the growing season. Arbuscular mycorrhiza colonization rate was more affected by treatment (tillage or the combination of crop and preceding crop) than by the total number of AMF spores in the soil. Conventional tillage (CT) had a statistically significant negative effect (P £ 0.05) on spore numbers isolated from the soil, but only in the first year of study. However, the AMF colonization rate was significantly reduced by CT, and the roots of wheat, Triticum aestivum, L, cv. Coa after sunflower, Helianthus annuus L., were less well colonized than were those of triticale, X Triticosecale Wittmack, cv. Alter after wheat, but the affect of tillage was more pronounced than was the effect of crop combination. Under no-till there was a significant increase in AMF colonization rate throughout the sampling period in both wheat and triticale,indicating that the extraradical mycelium previously produced acted as a source of inoculum. In general, triticale showed greater AMF colonization than wheat, despite the preceding crop being less mycotrophic. Under these experimental conditions, typical of Mediterranean agricultural systems, AMF colonization responded more strongly to tillage practices than to the combination of crop and preceding crop

NO-TILL PROVIDES THE OPPORTUNITY TO MANAGE UNDERGROUND INTERACTIONS BETWEEN ARBUSCULAR MYCORRHIZAL FUNGI, WEEDS AND CROP PLANTS UNDER MEDITERRANEAN CONDITIONS

Early colonization of crop roots by arbuscular mycorrhiza (AM) is considered beneficial but its importance likely depends on the possible stresses faced by the host plant. Manganese toxicity is one such stress that AM can alleviate. Colonization initiated by extraradical mycelium (ERM) is faster than other sources of inoculum. No-till creates the possibility of encouraging inoculation via this source. At seeding time the ERM available for colonizing plants under no-till would have developed in association with previous crops or those weeds that germinated after the first autumn rain. However, the long, hot and dry summer under Mediterranean conditions might reduce the effectiveness of the ERM to colonize the new crop. The hypothesis that an intact ERM developed by weeds can affect the earlier AM colonization of wheat and alleviate Mn toxicity was tested in a pot experiment. Two mycotrophic (Ornithopus compressus L., Lolium rigidum Gaudin), and one non-mycotrophic (Silene galica L) weed species were grown for 7 weeks before being controlled with Glyphosate (the ERM remaining intact) or by mechanical disturbance (which also disrupted the ERM). Wheat was then planted and allowed to grow for 21 days. AM colonization, plant dry weight and shoot nutrient content were evaluated for both weeds and wheat. When an intact mycelium was present at the seeding of wheat (treatments with Ornithopus compressus and Lolium rigidum controlled by Glyphosate) there was a threefold increase in the AM colonization rate and growth of the crop compared with results for all the other treatments. The enhanced growth of wheat was associated with an alleviation of Mn toxicity, consistent with the hypothesis. However, there was a significant difference of the wheat growth after Ornithopus and Lolium (1.9 times), suggesting functional diversity within mycotrophic weeds and crops

Impact of tillage system on arbuscular mycorrhiza fungal communities in the soil under Mediterranean conditions.

A more diverse arbuscular mycorrhiza (AM) fungal community should be more versatile and resilient to variation in environmental conditions over space and time. To evaluate the effect of no-till and conventional tillage systems, AM fungal diversity was assessed as part of a long term field experiment by sequencing of DNA, extracted from soil, that encoded the large ribosomal sub-unit and was obtained by nested-PCR. In comparison with no-till, conventional tillage decreased AM fungal diversity by 40%. Differences between treatments in the frequency of the operational taxonomic units (OTUs) present in soil, confirm that AM fungi are differently vulnerable to soil disturbance

Summer survival of arbuscular mycorrhiza extraradical mycelium and the potential for its management through tillage options in Mediterranean cropping systems

The potential to manage arbuscular mycorrhizal (AM) colonisation within Mediterranean agricultural systems likely depends on the summer survival of the extraradical mycelium. To investigate this further a three-stage experiment was undertaken. The first stage was the creation of two contrasting levels of extraradical mycelium development, achieved by two contrasting levels of soil disturbance (typifying full tillage or no-till).. In the second stage this differential mycelial inoculum was subjected to Mediterranean summer temperature and soil water regimes, representing the post-harvest fallow. During the third stage, corresponding to the next growing season, its survival was evaluated without further soil disturbance (typifying no-till conditions) using wheat as host crop. The results clearly indicated that the extraradical mycelium survived the prevailing summer conditions. Knowing that extraradical mycelium can survive over the Mediterranean summer encourages the use of tillage systems that minimize mechanical disturbance of the soil, such as no-till. Knowledge gained in this study suggests that by making the appropriate choice of crops to establish a mycorrhizal-supportive rotation there can be opportunities for agro-ecosystem management to benefit from the symbiotic relationship

Soil and weed management for enhancing arbuscular mycorrhiza colonisation of wheat

Tillage and weed control are critical components of cropping systems that need to be combined such that crops benefit from reduced competition. However, weeds may also contribute to the biological diversity within the agro-environment. This greenhouse study investigated whether common weeds of arable cropping systems were suitable host plants for arbuscular mycorrhizal fungi (AMF), allowing the development of extraradical mycelium (ERM) that can contribute to the early colonization of a following wheat crop, especially in the absence of soil disturbance. Weeds were allowed to grow for up to 2 months before being controlled by soil disturbance or herbicide application (glyphosate or paraquat). Pregerminated wheat seeds were then planted. Chemical control of the weeds prior to sowing enhanced the early arbuscular mycorrhiza (AM) colonization rate of wheat roots, whereas mechanical disturbance was less acceptable as a method of weed control for rapid AM colonization. The type of herbicide (contact or systemic) had no impact on colonization of the wheat crop. Enhanced AM colonization promoted early P acquisition and growth of the crop. Appropriate management of weeds emerging between two consecutive cropping seasons coupled with no-till soil management could ensure a quick and efficient AM colonization of the following wheat plants

The importance of no-till in the development of cropping systems to maximise benefits of arbuscular mycorrhiza symbiosis

The symbiosis of arbuscular mycorrhizal fungi (AMF) with plant roots presents several benefits to agricultural systems, including improved soil structure, resistance of crops to drought and soil pathogens, and its importance will increase in the future due to the scarcity of P reserves in the world. No-till is a powerful tool to manage AMF in the cropping systems as it allows the use of ERM developed in the previous year and the ERM developed by weeds in the beginning of the cropping year, as a major source of inoculum for the new crop. Therefore the design of crop rotations to take full advantage of AMF symbiosis, like an enhanced ability to uptake P, must consider the tillage system to be adopted and an appropriate weeds management

Practical exploitation of mycorrhizal fungi in agricultural systems

Improving the sustainability of agricultural systems requires a more efficacious use of soil resources. Mycorrhizas are known to contribute to host plant P acquisition and protection against both biotic and abiotic stresses, such as soil-borne diseases and toxic metal ions. However, practical exploitation of the mutualistic relationship is rarely considered in agricultural systems, allegedly owing to the cost of inoculation and the requirement for timely colonisation. To overcome these limitations, the presence of an extensive extraradical mycelium (ERM) from indigenous arbuscular mycorrhizal fungi (AMF) could be used as the preferential source for colonisation of a crop plant. Colonisation of crop roots starting from an intact ERM takes place faster and generally forms a more effective mycorrhizal association than when initiated from other propagules such as spores and root fragments. We report on the ability of an intact ERM developed by indigenous AMF population on mycotrophic plants (Developers) to significantly improve the AMF colonisation of wheat, subterranean clover and maize allowing for a better performance of the crop. This mechanism allowed the protection of wheat and subterranean clover from excessive Mn concentration in the shoots or in the roots, as in the case of the clover, leading to a greater growth of the crop (2.7 and 4.7 times respectively). Using the same strategy to promote the AMF colonisation of maize, the crop was able to exhibit high levels of colonization, even up to 45 kg P.ha-1 of applied P, and the plants took advantage in terms of P use efficiency. Our results indicate that the use of intact ERM as preferential AMF propagule is a valid strategy to increase the role of this symbiosis under marginal or more intensive cropping systems, through simple adaptations to both crop rotations and tillage practices

One-pot access to L-5,6-dihalotryptophans and L-alknyltryptophans using tryptophan synthase

The authors thank the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013/ERC grant agreement no 614779, and the University of St Andrews for a studentship (to D. R. M. Smith).We report, for the first time, the use of tryptophan synthase in the generation of L- dihalotryptophans and L-alkynyltryptophans. These previously unpublished compounds will be useful tools in the generation of probes for chemical biology, in biosynthetic diversification and as convenient building blocks for synthesis.PostprintPeer reviewe

Symbiosis Specificity of the Preceding Host Plant Can Dominate but Not Obliterate the Association Between Wheat and Its Arbuscular Mycorrhizal Fungal Partners

The symbiosis established between arbuscular mycorrhizal fungi (AMF) and roots of most land plants plays a key role in plant nutrient acquisition and alleviation of environmental stresses. Despite the ubiquity of the symbiosis, AMF and host species display significant specificity in their interactions. To clarify preferential associations between wheat (Triticum aestivum) and AMF, we characterized root AMF communities in the transition from two first host species, ryegrass (Lolium rigidum) and yellow-serradella (Ornithopus compressus), grown separately or together, to a second host (wheat), by sequencing the large subunit ribosomal DNA (LSU rDNA) gene. The response of AMF communities in wheat to prior soil disturbance – and consequently of the mycelial network [intact extraradical mycelium (ERM) vs. disrupted mycelium] established with either of the first hosts – was also investigated. Since the outcome of a specific host– symbiont interaction depends on the molecular responses of the host plant upon microbial colonization, we studied the expression of six key symbiosis-related genes in wheat roots. AMF communities on L. rigidum and O. compressus roots were clearly distinct. Within an undisturbed ERM, wheat AMF communities were similar to that of previous host, and O. compressus-wheat-AMF interactions supported a greater growth of wheat than L. rigidum-wheat-AMF interactions. This effect declined when ERM was disrupted, but generated a greater activation of symbiotic genes in wheat, indicating that plant symbiotic program depends on some extent on the colonizing symbiont propagule type. When a mixture of L. rigidum and O. compressus was planted, the wheat colonization pattern resembled that of O. compressus, although this was not reflected in a greater growth. These results show a lasting effect of previous hosts in shaping wheat AMF communities through an efficient use of the established ERM, although not completely obliterating host–symbiont specificity

Intact extraradical mycelium - a strategy for arbuscular mycorrhizal fungi in agricultural systems.

Arbuscular mycorrhiza are known to contribute to host plant P acquisition and protection against both biotic and abiotic stresses. However the practical exploitation of this mutualistic relationship is rarely considered in agricultural systems, allegedly owing to the cost of inoculation and the requirement for timely colonisation. AMF colonisation starting from an intact extra-radical mycelium (ERM) takes place faster than when initiated from other AMF propagules. We report on the ability of an intact ERM developed by indigenous AMF population on mycotrophic plants (developers) to significantly improve the subsequent AMF colonisation of wheat, subterranean clover and maize. This mechanism allowed the protection of wheat and subterranean clover from excessive Mn, leading to a greater growth of the crop (2.7 and 4.7 times, respectively). Using the same strategy to promote the AMF colonisation of maize, the crop was able to exhibit high levels of colonisation, even up to 45 kg of P per ha of applied P, and the plants took advantage in terms of P use efficiency. Our results indicate that the use of intact ERM as preferential source of AMF inoculum is a valid strategy to increase the role of this symbiosis under marginal or more intensive cropping systems. This strategy can easily be implemented in cropping systems through simple adaptations to both crop rotation and tillage practices