Root-hair growth and development regulated by soil microbes is associated with auxin. In this background, we hypothesized that mycorrhizal fungal inoculation induces greater root-hair growth through stimulated auxin synthesis and transport under water stress conditions. Trifoliate orange (Poncirus trifoliata) was inoculated with an arbuscular mycorrhizal (AM) fungus (Funneliformis mosseae) under well-watered (WW) and drought stress (DS) for 9 weeks. Compared with non-AM seedlings, AM seedlings displayed significantly higher density, length, and diameter of root hairs and root indoleacetic acid (IAA) level, whereas lower total root IAA efflux, regardless of soil moisture status. Root PtYUC3 and PtYUC8 involved in IAA biosynthesis were up-regulated by mycorrhization under WW and DS, whereas AM-modulated expression in PtTAA1, PtTAR2, PtYUC4, and PtYUC6 depended on status of soil moisture. Mycorrhizal inoculation down-regulated the transcript level of root auxin efflux carriers like PtPIN1 and PtPIN3, whereas significantly up-regulated the expression of root auxin-species influx carriers like PtABCB19 and PtLAX2 under DS. These results indicated that AMF-stimulated greater root-hair growth of trifoliate orange under DS that is independent on AMF species is related with mycorrhiza-modulated auxin synthesis and transport, which benefits the host plant to enhance drought tolerance.

A pot experiment was used to evaluate the effects of an arbuscular mycorrhizal fungus (AMF) Funneliformis mosseae on plant growth performance, root-hair growth, and root hormone levels in trifoliate orange (Poncirus trifoliata) seedlings under well-watered and drought stress (DS). A 9-week mild DS treatment significantly reduced mycorrhizal colonization of 2^nd- and 3^rd-order lateral roots. Root mycorrhizal colonization was relatively higher in the 2^nd- and 3^rd-order lateral root than in the taproot and the 1^st-order lateral root under WW and DS. AMF seedlings exhibited significantly higher root-hair density, length (except for the taproot) and diameter in taproot and 1^st-, 2^nd-, and 3^rd-order lateral roots under WW, and considerably higher root-hair density (except for 1^st-order lateral root), length (except for 2^nd-order lateral root) and diameter under DS. Mycorrhizal inoculation remarkably increased root abscisic acid (ABA), indole-3-acetic acid (IAA), methyl jasmonate, and brassinosteroids (BRs) concentrations under DS, in company with the decrease in root zeatin riboside and gibberellins levels and root IAA effluxes. Root-hair traits were significantly positively correlated with root colonization and root ABA and BRs levels. It is concluded that mycorrhizal plants possessed better root-hair growth to adapt mild DS, which is associated with mycorrhizal colonization and endogenous hormone changes.

This study aimed to evaluate effects of Funneliformis mosseae on plant growth and root exudate compositions and contents, soil and root phosphatase activity, soil glomalin concentrations, and thus soil aggregate stability and distribution in trifoliate orange under well-watered (75% of maximum water holding capacity) and drought stress (55% of maximum water holding capacity) conditions. After eight weeks of drought treatment, mycorrhizal fungal inoculation improved plant growth and exhibited altered composition of root exudates than non-inoculated treatment. Mycorrhizal fungal inoculation dramatically increased the relative abundance of phenolics (e.g., 2 H,8 H-Benzo[1,2-b:3,4-b’] dipyran-2-one,8,8-dimethyl), terpenoids (e.g., geijerene), and acids (n-hexadecanoic acid), while notably reduced the relative abundance of alkanes (e.g., tridecane, 2-methyl-), esters (e.g., hexanedioic acid and dimethyl ester), and amides (e.g.,13-docosenamide) in root exudates. Mycorrhizal fungal colonization profoundly increased easily extractable and total glomalin-related soil protein levels under two soil water regimes, which cemented soil macroaggregate (2–4 mm size) formation, thereby, improving soil aggregate stability. Mycorrhizal fungal-inoculated plants represented higher soil acid, alkaline and total phosphatase activities, irrespective of well-watered and drought treatment. The results suggest that mycorrhizal plants had improved root microenvironment to mitigate drought damage through changes in root exudate components along with glomalin, phosphatase, and soil aggregate stability in the mycorrhizosphere.

Sucrose and proline metabolisms are often associated with drought tolerance of plants. This study was conducted to investigate the effects of two arbuscular mycorrhizal fungi (AMF) species (Funneliformis mosseae and Paraglomus occultum) on root biomass, lateral root number, root sucrose and proline metabolisms in trifoliate orange (Poncirus trifoliata) seedlings under well-watered (WW) or drought stress (DS). All the AMF treatments significantly increased root dry weight, taproot length, and the number of lateral roots in 1st, 2nd, and 3rd class under WW and DS. Mycorrhizal seedlings conferred considerably higher fructose and glucose concentrations but lower sucrose accumulation, regardless of soil water status. Under DS, F. mosseae treatment significantly increased root sucrose synthase (SS, degradative direction) and sucrose phosphate synthase (SPS) activity but deceased root acid invertase (AI) and neutral invertase (NI) activity, and P. occultum inoculation markedly increased root AI, NI, SS, and SPS activities. AMF treatments led to a lower proline accumulation in roots, in company with lower activities of Δ1-pyrroline-5-carboxylate synthetase (P5CS), δ-ornithine aminotransferase (OAT), Δ1-pyrroline-5-carboxylate reductase (P5CR), and proline dehydrogenase (ProDH) in roots. It appears that the AM symbiosis induced greater root development and sucrose and proline metabolisms to adapt DS.