Integrated Transcriptomic and Biochemical Analyses Reveal the Root Development-Promoting Mechanism of Piriformospora indica on Blueberry Under Tap Water Irrigation

<i>Piriformospora indica</i>, a broad-spectrum plant growth-promoting fungus, has been successfully applied in blueberry (<i>Vaccinium corymbosum</i> L.). In this study, through an integrated transcriptomic and biochemical analyses, we investigated the effects of <i>P. indica</i> colonization on blueberry root growth under long-term tap water (EC ≈ 1500 μs/cm) irrigation. Comparative transcriptomic analysis revealed that <i>P. indica</i> colonization greatly influenced the expression of genes involved in RNA biosynthesis, solute transport, response to external stimuli, phytohormone action, carbohydrate metabolism, cell wall organization, and secondary metabolism pathways. Consistently, the fungal colonization significantly improved the nutrient absorption ability, and increased the contents of sucrose, starch, trehalose, total phenolic, total flavonoids, and indole-3-acetic acid (IAA), while suppressing the accumulations of jasmonic acid (JA), abscisic acid (ABA), 1-aminocyclopropane-1-carboxylic acid (ACC), and strigolactone (SL) in blueberry roots. Quantitative real-time PCR verification also confirmed the fungal influences on genes associated with these pathways/parameters, such as auxin homoeostasis-associated <i>WAT1</i>, cell wall metabolism-related <i>EXP</i>, phenylpropanoid biosynthesis-related <i>PAL</i> and <i>CHS</i>, carotenoid degradation-related <i>CCD8</i>, transportation-related <i>CNGC</i>, trehalose metabolism-related <i>TPP,</i> and so on. Our study demonstrated that <i>P. indica</i> improved blueberry adaptability to mild salt stress by synergistically regulating cell wall metabolism, secondary metabolism, stress responses, hormone homeostasis, sugar and mineral element transportation, and so on.