Abstract
Arbuscular mycorrhizal fungus (AMF), forming symbiosis with most terrestrial plants, strongly modulates root system architecture (RSA), which is the main characteristic of root in soil, to improve plant growth and development. So far, the studies of AMF on tea plant seedlings are few and the relevant molecular mechanism is not deciphered. In this study, the 6-month-old cutting seedlings of tea plant cultivar “Wancha No.4” were inoculated with an AMF isolate, Rhizophagus intraradices BGC JX04B and harvested after 6 months of growth. The indexes of RSA and sugar contents in root were determined. The transcriptome data in root tips of mycorrhizal and non-mycorrhizal cutting seedlings were obtained by RNA-sequence (Seq) analysis. The results showed that AMF significantly decreased plant growth, but increased the sucrose content in root and the higher classes of lateral root (LR) formation (third and fourth LR). We identified 2047 differentially expressed genes (DEGs) based on the transcriptome data, and DEGs involved in metabolisms of phosphorus (42 DEGs), sugar (39), lipid (67), and plant hormones (39) were excavated out. Variation partitioning analysis showed all these four categories modulated the RSA. In phosphorus (P) metabolism, the phosphate transport and release (DEGs related to purple acid phosphatase) were promoted by AMF inoculation, while DEGs of sugar transport protein in sugar metabolism were downregulated. Lipid metabolism might not be responsible for root branching but for AMF propagation. With respect to phytohormones, DEGs of auxin (13), ethylene (14), and abscisic acid (5) were extensively affected by AMF inoculation, especially for auxin and ethylene. The further partial least squares structural equation modeling analysis indicated that pathways of P metabolism and auxin, as well as the direct way of AMF inoculation, were of the most important in AMF promoting root branching, while ethylene performed a negative role. Overall, our data revealed the alterations of genome-wide gene expression in tea plant roots after inoculation with AMF and provided a molecular basis for the regulatory mechanism of RSA (mainly root branching) changes induced by AMF.
Highlights
The root plays a great important role in plant growth and development through absorbing mineral nutrients and water, anchoring the aboveground, storing carbohydrates and lipids, and secreting an enormous range of compounds with growth regulatory properties
The 6-month-old “Wancha No.4” (Wang et al, 2018) cutting seedlings with about 5 cm height were used as tea plant (Camellia sinensis L.) materials, which were inoculated with Arbuscular mycorrhizal fungus (AMF) in pot culture, and AMF isolate was Rhizophagus intraradices BGC JX04B provided by Beijing Academy of Agriculture and Forestry Sciences (Chen et al, 2017a)
AMF inoculation significantly increased sucrose content (p < 0.01) in the root compared with the C treatment, while no significant difference was found in the contents of reducing sugar and soluble sugar (Figure 1)
Summary
The root plays a great important role in plant growth and development through absorbing mineral nutrients and water, anchoring the aboveground, storing carbohydrates and lipids, and secreting an enormous range of compounds with growth regulatory properties. Among all the ingredients in RSA, root branching, including adventitious root (AR) and LR formation, is one of the most important to define the function of promoting plant growth. Plant roots with more branching, coming with more root tips and foraging area, could grow well in phosphorus-deficient soils through a citrate-enhanced uptake way (McKay Fletcher et al, 2020). RSA, showing great plasticity, is influenced by soil water and nutrient, soil texture, heterogeneity, and biotic interaction (Balliu et al, 2021; Lombardi et al, 2021), wherein arbuscular mycorrhizal fungus (AMF), which naturally forms a symbiosis with about 80% terrestrial plant species, could affect RSA by enhancing LR formation through the auxin pathway (Chen et al, 2017a). The successfully constructed mycorrhiza will promote plant growth, and elevate abiotic stress tolerance of most plants, including drought, water logging, low temperature, salinity, and so on, which was intensively reviewed by Begum et al (2019)
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