Melanoidins promote plant disease resistance, immunity and growth, through the salicylic acid pathway

• Environmental ramifications of the continued use of chemical pesticides have increased the need to identify environmentally friendly disease control options. • Melanoidins, macromolecular products of the Maillard reaction, were hypothesized to be amongst the building blocks of humic substances. However, to date, the effects of melanoidins on plant growth or disease resistance were unknown. • Here, we examined the potential of melanoidins to induce plant disease resistance and growth, finding that melanoidins promoted cellular immunity, disease resistance, and plant growth, at a level comparable to other natural inducers. • Using transcriptomics and genetic defense mutants, we observed that melanoidins promote immunity and disease resistance primarily through the salicylic acid pathway. • Our results support the notion that melanoidins can find use as naturally derived disease control and growth promoting agents, and could potentially be combined with JA elicitors, to maximize the plant immune response. Environmental ramifications of the continued use of chemical pesticides have increased the need to identify non-toxic, naturally occurring substances that can serve as environmentally friendly disease control. Following pathogen infection, plants can develop enhanced resistance to further attack. This is known as induced resistance. Induced resistance is broadly divided into systemic acquired resistance (SAR) and induced systemic resistance (ISR). SAR generally relies on salicylic acid (SA), is commonly triggered by local infection, and can provide long-term resistance to subsequent infection. ISR is often described as being initiated by beneficial soilborne microorganisms, and as being regulated by jasmonic acid (JA) and ethylene (ET). Inducers can be chemical or biological, and usually they operate by activating systemic immunity. Among the natural inducers are biochar and humic substances, which have established positive effects on plant disease resistance and plant growth. Melanoidins, macromolecular products of the Maillard reaction, were long ago hypothesized to be amongst the building blocks of humic substances. However, to date, no information has been available regarding the specific effects of melanoidins on plant growth or disease resistance. Here, we examined the potential of melanoidins to induce disease resistance and growth. We found that melanoidins promoted cellular immunity and disease resistance against Botrytis cinerea at a level comparable to other natural inducers. Using transcriptomics and genetic defense mutants, we observed that melanoidins promote immunity and disease resistance through a swift and transient effect on the transcriptome, primarily through the salicylic acid pathway. Our results support the notion that melanoidins can find use as naturally derived disease control and growth promoting agents, and could potentially be combined with JA elicitors, to maximize the plant immune response.