Abstract
Arbuscular mycorrhiza (AM) are mutualistic interactions formed between soil fungi and plant roots. AM symbiosis is a fundamental and widespread trait in plants with the potential to sustainably enhance future crop yields. However, improving AM fungal association in crop species requires a fundamental understanding of host colonisation dynamics across varying agronomic and ecological contexts. To this end, we demonstrate the use of betalain pigments as in vivo visual markers for the occurrence and distribution of AM fungal colonisation by Rhizophagus irregularis in Medicago truncatula and Nicotiana benthamiana roots. Using established and novel AM-responsive promoters, we assembled multigene reporter constructs that enable the AM-controlled expression of the core betalain synthesis genes. We show that betalain colouration is specifically induced in root tissues and cells where fungal colonisation has occurred. In a rhizotron setup, we also demonstrate that betalain staining allows for the noninvasive tracing of fungal colonisation along the root system over time. We present MycoRed, a useful innovative method that will expand and complement currently used fungal visualisation techniques to advance knowledge in the field of AM symbiosis.
Highlights
Arbuscular mycorrhiza (AM) fungi of the subphylum Glomeromycotina are soil fungi that engage in symbiosis with land plants [1]
To establish a reporter system that allows for the noninvasive visualisation of AM fungal colonisation in roots, we explored the use of betalain biosynthesis genes under the control of AM symbiosis–specific plant promoters
In M. truncatula, PT4 and BCP1 are expressed during root colonisation by AM fungi and are often used as markers to quantify the extent of AM symbiosis [10,11]
Summary
Arbuscular mycorrhiza (AM) fungi of the subphylum Glomeromycotina are soil fungi that engage in symbiosis with land plants [1]. Symbiotic associations with AM fungi date back to over 400 million years ago and can be formed by 70% to 72% of extant land plant species [2,3,4]. AM fungi are obligate biotrophs that receive all their carbon intake from the plant, which is estimated at up to 20% of the plant’s photosynthate [5]. The fungus assists the plant with the acquisition of mineral nutrients, mainly phosphorus, whose availability in soils is often a limiting factor for plant growth [6]. During AM symbiosis, fungal hyphae form dichotomously branched structures, named arbuscules, within root cortex cells.
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