Lights Off for Arbuscular Mycorrhiza: On Its Symbiotic Functioning under Light Deprivation

Abstract

Plants are often exposed to shade over different time scales and this may substantially affect not only their own growth, but also development and functioning of the energetically dependent organisms. Among those, the root symbionts such as arbuscular mycorrhizal (AM) fungi and rhizobia represent particularly important cases—on the one hand, they consume a significant share of plant carbon (C) budget and, on the other, they generate a number of important nutritional feedbacks on their plant hosts, often resulting in a net positive effect on their host growth and/or fitness. Here we discuss our previous results comparing mycorrhizal performance under different intensities and durations of shade (Konvalinková et al., 2015) in a broader context of previously published literature. Additionally, we review publicly available knowledge on the root colonization and mycorrhizal growth responses in AM plants under light deprivation. Experimental evidence shows that sudden and intensive decrease of light availability to a mycorrhizal plant triggers rapid deactivation of phosphorus transfer from the AM fungus to the plant already within a few days, implying active and rapid response of the AM fungus to the energetic status of its plant host. When AM plants are exposed to intensive shading on longer time scales (weeks to months), positive mycorrhizal growth responses (MGR) are often decreasing and may eventually become negative. This is most likely due to the high C cost of the symbiosis relative to the C availability, and failure of plants to fully compensate for the fungal C demand under low light. Root colonization by AM fungi often declines under low light intensities, although the active role of plants in regulating the extent of root colonization has not yet been unequivocally demonstrated. Quantitative information on the rates and dynamics of C transfer from the plant to the fungus is mostly missing, as is the knowledge on the involved molecular mechanisms. Therefore, these subjects deserve particular attention in the future.

Light and plant growth‐promoting rhizobacteria effects on Brachiaria brizantha growth and phenotypic plasticity to shade

“…This could have interfered in the quorum sensing, inhibiting the mutual interaction under full sun (Goh et al., ; Venturi & Keel, ). This response supports the evidence that light can interfere with mutualistic interaction, which in turn varies with hosts and beneficial micro‐organisms (Konvalinková & Jansa, ). Under limited light conditions, microbial root symbionts can create additive costs, resulting in decreased plant fitness, as observed in Vatica albiramis (Saner et al., ) and in Datura stramonium (Aguilar‐Chama & Guevara, ), where growth promotion is positively related to light intensity.…”

“…Growth of plants inoculated only with B. pyrrocinia was positively related to high light intensities. A possible cause for this response was a likely higher demand in photoassimilates for the symbiosis of this micro‐organism (Aguilar‐Chama & Guevara, ; Konvalinková & Jansa, ). A contrasting response occurred in plants inoculated only with P. fluorescens , which fostered higher growth mainly under restricted light intensities.…”

“…Beneficial micro-organisms are known to improve nutrient uptake, phosphorus solubilization, phytohormone production and disease resistance by elicited induced systemic resistance or systemic acquired resistance. Also, beneficial micro-organisms modify the phenotypic plasticity of plants, by mitigating the negative impact of abiotic stresses (Goh, Vallejos, Nicotra, & Mathesius, 2013;Paredes & Lebeis, 2016;Vacheron et al, 2013;Vimal, Singh, Arora, & Singh, 2017), including light limitation (Konvalinkov a & Jansa, 2016).…”

Tracking plant preference for higher‐quality mycorrhizal symbionts under varying CO2 conditions over multiple generations

“…We aimed to determine if over time, and across CO 2 ‐levels we would observe a relative increase of R. irregularis in the mixed AMF communities. To test this, we calculated the log response ratio of the abundances of both fungi log ( R. irr / G. agg ) This is a metric of the relative success of both fungi, with higher positive values indicating a relatively higher abundance of R. irregularis and negative values indicating G. aggregatum being more successful in colonizing plant roots (Hedges, Gurevitch, & Curtis, ; Hoeksema et al., ; Konvalinková & Jansa, ). We generated a linear model of this metric as response variable, with CO 2 ‐level and generation as explanatory variables, allowing us to test the relative performance of both fungi across generations and CO 2 ‐levels, including the potential for shifts over time in the relative success of the higher quality AM fungi.…”

“…Yet, the outcome of symbiotic partnerships can be highly context‐dependent, varying from strongly beneficial to both partners (mutualism) to a net fitness cost for one of the partners (parasitism) (Bronstein, ; Chamberlain, Bronstein, & Rudgers, ; Hoeksema et al., ). A key factor driving such context‐dependence is variation in the environmental availability of the symbiotically provided resources (Bever, ; Konvalinková & Jansa, ; de Mazancourt & Schwartz, ; Shantz & Burkepile, ; Weese et al, ). For instance, if a legume grows in a high‐nitrogen habitat, it can be cheaper to acquire nitrogen from the soil directly, than to invest carbon in nitrogen‐fixing rhizobial symbionts (Heath & Tiffin, ; Lau et al., ).…”