  Citation: Saha, H.; Kaloterakis, N.; Harvey, J.A.; Van der Putten, W.H.; Biere, A. Effects of Light Quality on Colonization of Tomato Roots by AMF and Implications for Growth and Defense. Plants 2022, 11, 861. https://doi.org/10.3390/ plants11070861 Academic Editors: Mónica Sebastiana and Rui Malhó Received: 14 February 2022 Accepted: 22 March 2022 Published: 24 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). plants Article Effects of Light Quality on Colonization of Tomato Roots by AMF and Implications for Growth and Defense Haymanti Saha 1, * , Nikolaos Kaloterakis 1,2,3 , Jeffrey A. Harvey 1,4 , Wim H. Van der Putten 1,5 and Arjen Biere 1 1 Department of Terrestrial Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Droevendaalsesteeg 10, 6708 PB Wageningen, The Netherlands; n.kaloterakis@fz-juelich.de (N.K.); j.harvey@nioo.knaw.nl (J.A.H.); w.vanderputten@nioo.knaw.nl (W.H.V.d.P.); a.biere@nioo.knaw.nl (A.B.) 2 Soil Biology Group, Wageningen University and Research, Droevendaalsesteeg 2, 6708 PB Wageningen, The Netherlands 3 Institute of Bio- and Geosciences, Agrosphere (IBG-3), Forschungszentrum Jülich GmbH, Wilhelm-Johnen-Straße, 52428 Jülich, Germany 4 Department of Ecological Sciences, Section Animal Ecology, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands 5 Laboratoryof Nematology, Wageningen University and Research, Droevendaalsesteeg 2, 6708 PB Wageningen, The Netherlands * Correspondence: h.saha@nioo.knaw.nl; Tel.: +31-645036538 Abstract: Beneficial soil microbes can enhance plant growth and defense, but the extent to which this occurs depends on the availability of resources, such as water and nutrients. However, relatively little is known about the role of light quality, which is altered during shading, resulting a low red: far-red ratio (R:FR) of light. We examined how low R:FR light influences arbuscular mycorrhizal fungus (AMF)-mediated changes in plant growth and defense using Solanum lycopersicum (tomato) and the insect herbivore Chrysodeixis chalcites. We also examined effects on third trophic level interactions with the parasitoid Cotesia marginiventris. Under low R:FR light, non-mycorrhizal plants activated the shade avoidance syndrome (SAS), resulting in enhanced biomass production. However, mycorrhizal inoculation decreased stem elongation in shaded plants, thus counteracting the plant’s SAS response to shading. Unexpectedly, activation of SAS under low R:FR light did not increase plant susceptibility to the herbivore in either non-mycorrhizal or mycorrhizal plants. AMF did not significantly affect survival or growth of caterpillars and parasitoids but suppressed herbivore-induced expression of jasmonic acid-signaled defenses genes under low R:FR light. These results highlight the context- dependency of AMF effects on plant growth and defense and the potentially adverse effects of AMF under shading. Keywords: arbuscular mycorrhizal fungi; context dependency; light quality; plant defense; plant microbe insect interactions; Solanum lycopersicum (tomato) 1. Introduction In natural ecosystems, ecological communities are characterized by multitrophic interactions, e.g., between plants and their above- and belowground beneficial insects and microbes, pests and pathogens [1,2]. An important groups of beneficial plant root symbionts are Arbuscular Mycorrhizal Fungi (AMF). The beneficial potential of AMF for plant growth and fitness has received considerable attention over the past decades. AMF form hyphal networks with plant roots that enhance the access of roots to a large soil surface area, often resulting in enhanced plant growth and a positive mycorrhizal growth response (MGR) [3]. In addition to providing nutritional benefits, AMF colonization is also known to enhance tolerance to abiotic stresses such as drought and salinity and to improve direct and indirect plant defenses against pathogens and pests [4–8]. One of the Plants 2022, 11, 861. https://doi.org/10.3390/plants11070861 https://www.mdpi.com/journal/plants