Humic substances trigger plant immune responses

Background Huanglongbing (HLB) is a devastating citrus disease. Before callose deposition, the bacterial infection causes oxidative stress, starting cell damage. Humic substances are among the most efficient antioxidants found in nature. Furthermore, it is also previously reported that humic substances can induce a phenylpropanoid metabolism contributing to the production of antimicrobial compounds. It has been noted by technicians and growers in the Brazil's main citrus belt that orchards continuously treated with humic substances can live well with the symptoms of HLB. Methods We treated two young citrus orchards (Citrus sinensis cultivars Baía and Pera) with soluble humic substances (HS) isolated from peat either combined or not with a plant growth-promoting bacteria consortium. The activity of key enzymes related to the response against biotic stresses, such as peroxidase (POX), β-1,3-glucanase (PR) and phenylalanine ammonia-lyase (PAL), as well as the differential transcription level of target genes linked to plant stress response by qRT-PCR was monitored for six months. Results No differences were obtained between sole humic substances and their application combined with plant growth-promoting bacteria, indicating that the microbial consortium had no modulatory effect on HS-treated plants. The treatments promoted the activities of POX, PAL and β-1,3-glucanase, which remained significantly higher concerning the control throughout the evaluation period. In addition, treatments positively regulated the transcription levels of CsPR-7, CsPR-3 and CsPR-11 genes. The experimental data were qualitatively similar to those found in commercial orange orchards treated continuously for 12 and 6 years with humic substances, where the activity and transcription levels were also more significant than for untreated plants. Conclusion Our results indicate that humic substances can trigger immune-mediated responses in plants and they can be used as a natural chemical priming agent to mitigate disease symptoms and contribute to more resilient citrus cultivation. Graphical Abstract