1 Professor, 3 Professor (e mail: wuqiangsh@163.com) College of Horticulture and Gardening, Yangtze University, Jingzhou, Hubei 434025, China; 2 Principal Scientist, Soil Science (email: aksrivas2007@gmail.com), National Research Centre for Citrus, Nagpur 440 010, Maharashtra, India. 50 Indian Journal of Agricultural Sciences 84 (11): 1342–46, November 2014/Article Effect of auxin inhibitor and AMF inoculation on growth and root morphology of trifoliate orange (Poncirus trifoliata) seedlings CHUN-YAN LIU 1 , A K SRIVASTAVA 2 and QIANG-SHENG WU 3 College of Horticulture and Gardening, Yangtze University, Jingzhou, Hubei 434025, China Received: 13 May 2014; Revised accepted: 6 June 2014 ABSTRACT Auxins are considered as one of the important plant hormones coordinating the signals in regulation of plant development including the transformation in root morphology. Arbuscular mycorrhizal fungi (AMF) produce small amount of auxins, the role of which is yet not known in root and mycorrhizal development. The present study was undertaken to evaluate the effects of AMF ( Glomus versiforme Karst.) and an auxin inhibitor (P- Chlorophenoxyisobutyric acid, PCIB, 10 mM) on plant growth and root development of four-month-old mycorrhized trifoliate orange [Poncirus trifoliata (L.) Raf.] seedlings. Root mycorrhizal colonization and number of entry points and vesicles were significantly inhibited by exogenous PCIB treatment, suggesting that auxins are involved in establishment of AMF-host plant symbiosis. Application of PCIB significantly decreased the plant height, stem diameter, leaf number, and shoot and root fresh weight. On the other hand, AMF colonization increased the plant height, shoot and root fresh weight, thereby, suggesting that AMF colonization alleviated the negative effects of PCIB. Similarly, PCIB application significantly decreased taproot length, root average diameter, roots projected area, surface area of roots, root volume, root length under 0–1 cm category, and total root length. These root traits significantly improved upon inoculation with AMF. The study, hence, suggested that inoculation with AMF negated the adverse effects of PCIB through AMF-induced auxin production that actively participated in both root and mycorrhizal plant development. Key words: Arbuscular mycorrhizal fungi, P-Chlorophenoxyisobutyric acid, Plant growth, Root morphology, Trifoliate orange Arbuscular mycorrhizal fungi (AMF) are probably most abundant in agricultural soils, and can form a symbiotic association (arbuscular mycorrhiza, AM) with the roots of ~80% of terrestrial plants (Srivastava et al. 2002). The AM- symbiosis obtains plant carbohydrates to complete its life cycle, and AMs would enhance plant growth and development by acquisition of phosphate and other mineral nutrients from the soil (Porras-Sorianoa et al. 2009, Srivastava 2009, Srivastava and Ngullie 2009). Inoculation with AMF by and large affects the root traits of host plant, including taproot length, number of lateral roots, and root volume (Wu et al. 2008; Wang et al. 2014). Colonization with Glomus intraradices on the other hand, is reported to significantly increase the number of lateral roots in rice (Gutjahr et al. 2009). AMF colonization also promoted the formation of lateral roots of trifoliata orange seedlings, induced more fine roots and less coarse roots according to Yao et al. (2009). Auxins play an important role in plant development by inducing branching in lateral roots, responsible for providing water and nutrients (Zandonadi et al. 2010). Auxins are produced by spores of AMF (Splivallo et al. 2009), which help the host plant to improve the configuration of root system (Ludwig-Muller 2010). The AM-induced auxin production is anticipated to trigger the modification of root systems of the host plant. Until now, the relevant mechanisms responsible for AM-induced root modification are poorly known. P-Chlorophenoxyisobutyric acid (PCIB), an inhibitor of indole-3-acetic acid (IAA), has been widely used to inhibit auxin action by means of competing with auxin at the binding site of the auxin receptor (Xie et al. 2000, Oono et al. 2003). Studies showed that PCIB notably inhibited the formation of Arabidopsis root, including lateral root development, gravitropic response, and primary root growth (Oono et al. 2003). Biswas et al. (2007) further revealed that PCIB inhibited Arabidopsis root growth by reducing the size of root meristem. While, Zandonadi et al. (2010) reported that PCIB decreased the lateral root formation and the primary root length of Zea mays by 50- 55%. These studies offered a strong clue that could be used to clarify the roles of auxin development in root systems and further the possible impact of AMF. In recent years, massive efforts have been made worldover to study the interaction effect of AMF on plant root development.