Mycorrhizal inoculation modulates root morphology and root phytohormone responses in trifoliate orange under drought stress

Phytohormones can be responsible for activating tolerance responses of drought stress (DS). The present study was done to evaluate the effects of an arbuscular mycorrhizal fungus (AMF), Funneliformis mosseae, on root morphology and root phytohormones levels in trifoliate orange (Poncirus trifoliate) seedlings exposed to well-watered (WW, 75% of maximum water holding capacity) and drought stress (55% of maximum water holding capacity). The six-week DS treatment strongly restricted root mycorrhizal colonization by 27.7%. The DS treatment caused the decrease of total plant biomass and root morphological traits, but the AMF inoculation significantly increased total plant biomass arid root total length, projected area, surface area, average diameter, volume, and number of 1st, 2nd, and 3rd lateral root under WW and DS conditions. AMF plants exhibited significantly higher leaf water potential than non-AMF plants exposed to WW and DS. AMF colonization notably regulated the changes in root phytohormone levels: the increase of indole-3-acetic acid (IAA), abscisic acid (ABA), methyl jasmonate (MeJA), and zeatin riboside (ZR) levels under WW condition, and the increase of IAA, ABA, MeJA, ZR, and brassinosteroids (BRs) concentrations under DS conditions. These results concluded that AMF enhanced drought tolerance in trifoliate orange through modulation of root phytohormones and root morphology.