Alkaline extract of the seaweed Ascophyllum nodosum stimulates arbuscular mycorrhizal fungi and their endomycorrhization of plant roots

Sarah Hines,Katy Shotton,Timo Van Der Zwan,Balakrishnan Prithiviraj,Kevin Shiell

doi:10.1038/s41598-021-93035-9

Abstract

Ascophyllum nodosum extracts (ANE) are well-established plant biostimulants that improve stress tolerance and crop vigour, while also having been shown to stimulate soil microbes. The intersection of these two stimulatory activities, and how they combine to enhance plant health, however, remains poorly understood. In the present study, we aimed to evaluate: (1) the direct effect of ANE on the arbuscular mycorrhizal fungus Rhizophagus irregularis, and (2) whether ANE influences endomycorrhization in plants. ANE enhanced development of R. irregularis in vitro, showing greater spore germination, germ tube length, and hyphal branching. Greenhouse-grown Medicago truncatula drench-treated with ANE formed mycorrhizal associations faster (3.1-fold higher mycorrhization at week 4) and grew larger (29% greater leaf area by week 8) than control plants. Foliar applications of ANE also increased root colonization and arbuscular maturity, but did not appear to enhance plant growth. Nonetheless, following either foliar or drench application, M. truncatula genes associated with establishment of mycorrhizae were expressed at significantly higher levels compared to controls. These results suggest that ANE enhances mycorrhization through both direct stimulation of arbuscular mycorrhizal fungus growth and through stimulation of the plant’s accommodation of the symbiont, together promoting the establishment of this agriculturally vital plant–microbe symbiosis.

Highlights

Seaweed extracts, such as those of the brown seaweed Ascophyllum nodosum, have a long history of use as plant biostimulants[1]
We examined the effect of aqueous alkaline Ascophyllum nodosum extracts (ANE) on spore germination and development of the agriculturally important arbuscular mycorrhizal fungi (AMF) species Rhizophagus irregularis
R. irregularis spore germination significantly improved in the presence of 0.1 and 0.5 g/l ANE at day 4 relative to the water and nutrient controls (Fig. 1)

Summary

Introduction

Seaweed extracts, such as those of the brown seaweed Ascophyllum nodosum, have a long history of use as plant biostimulants[1]. A growing body of evidence demonstrates the beneficial influence of ANE on plant-associated microbiota, along with correlated benefits to plant health and y ield[5,17,18]. In this prior work, applications of ANE have been shown to broadly influence the structure and activity of microbial communities in the rhizosphere, increasing rhizosphere biodiversity, respiration, and metabolic activity. While methanolic extracts and neutral aqueous extracts of these seaweed species have been demonstrated to stimulate AMF growth and colonization, it remains unknown whether alkaline extracts of Ascophyllum nodosum possess similar bioactivity. In a greenhouse trial using the model legume Medicago truncatula, we studied the effect of foliar and drench applications of ANE on the colonization of R. irregularis, interrogated through root microscopy and expression analysis of M. truncatula genes involved in the signaling for and accommodation of endosymbiotic fungi

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