Apple replant disease (ARD) is a complex agricultural problem caused by multiple stressors that can lead to increased reactive oxygen species (ROS) levels and limited nutrient utilization in plants.  However, existing countermeasures cannot effectively address this challenge.  Here, we used Malus hupehensis as a test organism to investigate whether the pleiotropic molecule dopamine can alleviate ARD using pot experiments.  Exogenous application of 100 μmol L^–1 dopamine significantly promoted the growth of apple seedlings in the replanted soil, with a relative growth rate increase of 17.44%.  Our results revealed two major pathways by which dopamine regulates ARD resistance in apple trees.  First, dopamine effectively reduces the level of ROS and activates the expression of genes related to nitrogen (N) transport and metabolism.  Among those genes, MdNLP5, MdNRT1.1, MdNLP2, MdNRT2.5, MdNLP3, MdNRT2.4, MdNADH-GAGOT, and MdFd-GAGOT were strongly regulated by dopamine.  These regulatory effects promoted the uptake and utilization of soil N by the plants.  Second, dopamine improved the physical and chemical properties, enhanced microbial community diversity, and promoted mutual cooperation between microbial communities in the soil.  Furthermore, dopamine altered the microbial structure of rhizosphere soil (upregulating Clostridiales, Gaiellales, Sordariales and Mortierellales; downregulating Micrococcales, Longimicrobiales, Hypocreales and Cystobasidiales).  Notably, dopamine significantly upregulated the abundances of Gaiella and Mortierella, both of which were positively correlated with soil urease activity, soil available N content, plant growth and N uptake.  Dopamine also significantly downregulated the abundance of the plant pathogen Gibberella (by 11.71-fold) in replant soil.  Our results provide insights into the mechanisms by which dopamine promotes ARD resistance, and can promote the sustainable development of the apple industry.

Melatonin and dopamine can potentially prevent waterlogging stress in apples.  The current study investigated the mechanism by which melatonin and dopamine alleviate apple waterlogging stress.  This study demonstrated that melatonin and dopamine alleviated waterlogging by removing reactive oxygen species (ROS), and that the nitric oxide (NO) content and nitrate reductase (NR) activity were significantly correlated.  Melatonin and dopamine were also found to recruit different candidate beneficial endophytes (melatonin: Novosphingobium, Propionivibrio, and Cellvibrio; dopamine: Hydrogenophaga, Simplicispira, Methyloversatilis, Candidatus_Kaiserbacteria, and Humicola), and these endophytes were significantly and positively correlated with plant growth.  Network analyses showed that melatonin and dopamine significantly affected the endophytic bacterial and fungal communities under waterlogging stress.  The metabolomic results showed that melatonin and dopamine led to waterlogging resistance by upregulating the abundance of beneficial substances such as amino acids, flavonoids, coumarins, and organic acids.  In addition, melatonin and dopamine regulated the physicochemical properties of the soil, which altered the endophyte community and affected plant growth.  The co-occurrence network demonstrated close and complex relationships among endophytes, metabolites, soil, and the plants.  Our results demonstrate that melatonin and dopamine alleviate waterlogging stress in apples by recruiting beneficial endophytes to enhance physiological resilience.  This study provides new insights into how melatonin and dopamine alleviate stress and a theoretical basis for synergistic beneficial microbial resistance to waterlogging stress.

Fatty acid-binding proteins (FABPs) are a family of lipid chaperones, which contribute to systemic metabolic regulation through diverse lipid signalings.  In this study, a midgut-specific FABP gene (Slfabp2) was cloned from Spodoptera litura.  RT-PCR and Western blot analysis indicated that RNA and protein levels of SlFABP2 gradually increased and reached a peak at the prepupal stage and maintained a high level during the pupal stage.  The expression of SlFABP2 protein was induced by starvation treatment.  In vitro binding assay revealed that the recombinant SlFABP2 had high affinities of binding long-chain fatty acids, such as palmitic acid, arachidonate and oleic acid.  The results suggest that SlFABP2 may have a unique function that transports intracellular fatty acids and can regulate the metabolism of lipids in metamorphosis.  This work provides experimental clues for understanding the potential function of SlFABP2 in fatty acid metabolism in S. litura.