Abstract
Arbuscular mycorrhizal fungi (AMF) establish symbiosis with most terrestrial plants, and greatly regulate lateral root (LR) formation. Phosphorus (P), sugar, and plant hormones are proposed being involved in this regulation, however, no global evidence regarding these factors is available so far, especially in woody plants. In this study, we inoculated trifoliate orange seedlings (Poncirus trifoliata L. Raf) with an AMF isolate, Rhizophagus irregularis BGC JX04B. After 4 months of growth, LR formation was characterized, and sugar contents in roots were determined. RNA-Seq analysis was performed to obtain the transcriptomes of LR root tips from non-mycorrhizal and mycorrhizal seedlings. Quantitative real time PCR (qRT-PCR) of selected genes was also conducted for validation. The results showed that AMF significantly increased LR number, as well as plant biomass and shoot P concentration. The contents of glucose and fructose in primary root, and sucrose content in LR were also increased. A total of 909 differentially expressed genes (DEGs) were identified in response to AMF inoculation, and qRT-PCR validated the transcriptomic data. The numbers of DEGs related to P, sugar, and plant hormones were 31, 32, and 25, respectively. For P metabolism, the most up-regulated DEGs mainly encoded phosphate transporter, and the most down-regulated DEGs encoded acid phosphatase. For sugar metabolism, the most up-regulated DEGs encoded polygalacturonase and chitinase. For plant hormones, the most up-regulated DEGs were related to auxin signaling, and the most down-regulated DEGs were related to ethylene signaling. PLS-SEM analysis indicates that P metabolism was the most important pathway by which AMF regulates LR formation in this study. These data reveal the changes of genome-wide gene expression in responses to AMF inoculation in trifoliate orange and provide a solid basis for the future identification and characterization of key genes involved in LR formation induced by AMF.
Highlights
Root is essential for plant growth and development, serving a multiple functions including anchorage, absorption of mineral nutrients and water, and production of exudates with growth regulatory properties (Bailey et al, 2002; Chen et al, 2017)
Gene network was obtained to uncover the mechanism of the adventitious root formation of carnation (Dianthus caryophyllus L.) cutting in response to exogenous auxin (Villacorta-Martín et al, 2015). These results demonstrate the potential application of RNA-Seq technique to reveal the comprehensive pathway by which arbuscular mycorrhizal fungi (AMF) regulates lateral root (LR) formation
Compared with the non-mycorrhizal treatment (C), AMF inoculation significantly increased the growth of trifoliate orange seedlings, as revealed by the increase in either shoot FW or root FW, but decreased the R/S (Table 1, Figure 1A)
Summary
Root is essential for plant growth and development, serving a multiple functions including anchorage, absorption of mineral nutrients and water, and production of exudates with growth regulatory properties (Bailey et al, 2002; Chen et al, 2017). As an example for the latter, a mutualistic association develops between roots and arbuscular mycorrhizal fungi (AMF) (Smith and Read, 2008), which comprehensively affect plant RSA, especially the LR formation Fusconi (2014) proposed a comprehensive model that AMF affect LR formation probably through three pathways: regulating phosphorus (P) metabolism (increased absorption), regulating carbohydrate metabolism (changed carbohydrate partitioning and pathway), and regulating plant hormone metabolism (modulation of concentration, transport and sensitivity) in host plants. This proposal has been partially verified (Jiang et al, 2015; Chen et al, 2017), a comprehensive verification is needed and the contribution of each pathway is to be quantified
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