Alternaria alternata stimulates blackhead disease development of ‘Korla’ fragrant pear (Pyrus bretschneideri Rehd) by regulating energy status and respiratory metabolism

Blackhead disease of ‘Korla’ fragrant pear (Pyrus bretschneideri Rehd), caused by the fungus Alternaria alternata (A. alternata), limits postharvest pear fruit storage and transportation. Herein, we explored the effects of A. alternata infection on respiratory metabolism and energy status of pear fruit, and their correlations with disease development. Compared with control fruit, A. alternata-inoculated fruit displayed a higher respiration rate and disease index. Infection increased the gene expression and enzyme activities of phosphohexose isomerase (PGI), alternative oxidase (AOX), succinate dehydrogenase (SDH) and cytochrome C oxidase (CCO) in pear fruit, and decreased those of 6-phosphogluconate dehydrogenase (6PGDH) and glucose-6-phosphate dehydrogenase (G6PDH) in the late storage period. Infection reduced nicotinamide adenine dinucleotide kinase (NADK) activity and expression level in fruit, inhibited NADP accumulation, and increased NAD, NADH, and NADPH contents in the early storage period. Infection also lowered adenosine triphosphate (ATP), adenosine diphosphate (ADP) and energy charge (EC) levels as well as ATPase activity, and enhanced adenosine monophosphate (AMP) content and PbAtpB expression levels, resulting in decreased fruit energy status. All these results suggest that disease development may be connected to the enhancement of the Embden-Meyerhof-Parnas (EMP) pathway, the tricarboxylic acid (TCA) cycle, the cytochrome pathway (CCP), the alternative pathway (AP), the increased then decrease in the pentose phosphate pathway (PPP), and reduced energy status in pear fruit.