Humic Substances from Different Sources Modulate Salicylic Acid-Mediated Defense in Plants Infected by Powdery Mildew

Area of Science:

• Plant Pathology
• Agricultural Science
• Biochemistry

Background:

• Modern agriculture's reliance on chemical inputs raises environmental and health concerns.
• Biostimulants, particularly humic substances (HS), offer sustainable alternatives by modulating plant growth and defense.
• Understanding HS structural impact on plant defense pathways is crucial for sustainable agriculture.

Purpose of the Study:

• To evaluate the effects of vermicompost (Vc) and peat (Pt) derived HS on the salicylic acid (SA)-mediated defense pathway in tomato plants infected with powdery mildew (Oidium sp.).
• To characterize HS structural properties using solid-state ¹³C CPMAS NMR.
• To assess the impact of HS on defense-related enzymatic activities and gene expression.

Main Methods:

• Characterization of HS from vermicompost and peat using solid-state ¹³C CPMAS NMR.
• Infection of tomato plants (Solanum lycopersicum cv. Micro-Tom) with Oidium sp.
• Spectrophotometric determination of enzymatic activities (lipoxygenase, peroxidase, phenylalanine ammonia lyase, beta 1,3-glucanase).
• RT-qPCR to quantify gene expression levels of SA signaling and defense genes.

Main Results:

• Both Vc-HS and Pt-HS significantly reduced disease severity in tomato plants.
• HS treatment activated key SA-related defense genes, including NPR1, PR1, PR2, and PR5.
• Peat-derived HS (Pt) provided greater protection and defense gene induction compared to vermicompost-derived HS (Vc).
• HS amplified defense gene expression and enzymatic activities more significantly under pathogen infection than in non-infected plants.

Conclusions:

• Structural differences in HS influence the plant's defense response to pathogens.
• HS, particularly peat-derived, are effective in enhancing plant immunity and reducing disease severity.
• HS represent a promising sustainable strategy to improve plant defense mechanisms in agriculture, reducing reliance on agrochemicals.