Integrated analysis of the metabolome, transcriptome and miRNome reveals crucial roles of auxin and heat shock proteins in the heat stress response of loquat fruit

Loquat (Eriobotrya japonica) is a subtropical tree that blooms in autumn, and its fruits ripen during late spring. Sunburn caused by high temperature and high light intensity is one of the most severe abiotic stress factors that decreases loquat productivity and fruit quality and shelf life, greatly inhibiting sustainable development of this fruit crop industry. To date, little is known about fruit heat tolerance regulation, and investigation of heat stress responses and thermotolerant improvement is important for fruit crops. ‘Wuduiyadanben’ loquat fruits under 40 °C stress were collected for physiological and comparative sequencing analyses; hormone changes in loquat fruit in response to heat stress were investigated using liquid chromatography-mass spectrometry (LC-MS) analysis. Several discriminatory hormone metabolites were identified, including fourteen cytokinins (CK), eight auxins, five jasmonic acids (JA), three gibberellin acids (GA), two abscisic acids (ABA), two salicylic acids (SA), one ethylene (ETH) and one strigolactone (SL). Transcriptomics analysis suggested that genes enriched in ‘Plant hormone signal transduction’, ‘Protein processing in endoplasmic reticulum’, ‘Photosynthesis’, ‘Plant-pathogen interaction’ and ‘Oxidative phosphorylation’ pathways are involved in the heat stress response. Additionally, small RNA analysis further revealed that miRNAs, including miRNA160-x, miRNA393-x, miR6187-x, miR8726-y and novel-m0057–5p, might act together with auxin signaling and heat shock proteins to regulate genes in the heat stress response process. In summary, this study provides a comprehensive understanding of the complex molecular mechanisms underlying the thermal stress responses of loquat fruit.