Lifespan and functionality of mycorrhizal fungal mycelium are uncoupled from host plant lifespan

Abstract

Arbuscular mycorrhizal fungi (AMF) are obligate symbionts, living in associations with the roots of most land plants. AMF produce wide networks of extraradical mycelium (ERM) of indeterminate length, spreading from host roots into the surrounding soil and establishing belowground interconnections among plants belonging to the same or to different taxa. Whether their lifespan and functionality are limited by host plant viability or can be extended beyond this limit is unknown. To address this issue, we performed time-course studies to investigate viability and functionality of ERM produced in an in vivo whole-plant system by Funneliformis mosseae and Rhizoglomus irregulare, after shoot detachment. Our data revealed that viability and functionality of F. mosseae and R. irregulare extraradical hyphae were uncoupled from host plant lifespan. Indeed, ERM spreading from roots of intact or shootless plants showed comparable levels of viability, similar structural traits and ability to establish mycorrhizal symbioses with new plants, as long as five months after shoot removal. Our findings expand the current knowledge on AMF biology and life cycle, providing data on ERM long-term survival in the soil of two Glomeracean species, functional to the prompt establishment of mycorrhizal symbioses and to the maintenance of soil biological fertility.

Figure 1

Fluorescein diacetate staining showing viability of Cichorium intybus roots, 1 (a), 2 (b), 3 (c) and 4 (d) days after shoot detachment from roots; scale bars = 120 µm (a) and 90 µm (b–d).

Figure 2

Membranes bearing Cichorium intybus roots colonised by Funneliformis mosseae IMA1 and Rhizoglomus irregulare IMA6, stained for succinate dehydrogenase (SDH) activity 4 months after shoot detachment. (a,b) Dead roots with SDH-positive intraradical fungal structures; scale bars = 120 µm (a) and 70 µm (b). (c) Viable (SDH-positive) arbuscule within a dead root cell; scale bar = 35 µm. (d) Viability of IMA1 extraradical hyphae emerging from a dead colonised root indicated by the deposition of blue formazan salts; scale bar = 100 µm. (e) SDH-positive extraradical network of IMA1 spreading on the membrane; scale bar = 120 µm.

Figure 3

Percentage of viable extraradical mycelial length (means ± standard error of means) recorded in extraradical networks formed by Funneliformis mosseae IMA1 (a) and Rhizoglomus irregulare IMA6 (b) in symbiosis with Cichorium intybus, at variable times after shoot removal (shootless plants) or in intact plants.

Figure 4

Percentage of viable extraradical mycelial length (means ± standard error of means) recorded close to roots (near-roots zone) or in the peripheral zone of extraradical networks formed by Funneliformis mosseae IMA1 (a) and Rhizoglomus irregulare IMA6 (b) in symbiosis with Cichorium intybus, at variable times after shoot removal (shootless plants) or in intact plants. At each time point, asterisks indicate viability values of ERM produced by roots of shootless plants which differ significantly (P < 0.01) from those of ERM produced by roots of intact plants.

Figure 5

Light micrograph showing retraction septa (arrows) formed in peripheral hyphae devoid of protoplasm (succinate dehydrogenase activity staining) of extraradical mycelium spreading from Funneliformis mosseae IMA1 colonised roots, 4 months after Cichorium intybus shoot detachment; scale bar = 10 µm.