Potassium Phosphite Activates Components Associated with Constitutive Defense Responses in Coffea arabica Cultivars.

Phosphites have been used as inducers of resistance, activating the defense of plants and increasing its ability to respond to the invasion of the pathogen. However, the mode of action of phosphites in defense responses has not yet been fully elucidated. The objective of this study was to evaluate the effect of potassium phosphite (KPhi) in coffee cultivars with different levels of resistance to rust to clarify the mechanism by which KPhi activates the constitutive defense of plants. To this end, we studied the expression of genes and the activity of enzymes involved in the defense pathway of salicylic acid (SA) and reactive oxygen species (ROS), in addition to the levels of total soluble phenolic compounds and soluble lignin. Treatment with KPhi induced constitutive defense responses in cultivars resistant and susceptible to rust. The results suggest that KPhi acts in two parallel defense pathways, SA and ROS, which are essential for the induction of systemic acquired resistance (SAR) when activated simultaneously. The activation of the mechanisms associated with defense routes demonstrates that KPhi is a potential inducer of resistance in coffee plants.

Constitutive Defense Strategy of Coffee Under Field Conditions: A Comparative Assessment of Resistant and Susceptible Cultivars to Rust.

Coffea arabica is the most economically important coffee species worldwide. However, its production is severely limited by diseases such as rust. The mechanisms underlying constitutive defense responses in coffee are still poorly understood, compared with induced defense mechanisms. We aimed to characterize constitutive defense responses of thirteen cultivars of C. arabica. Cultivars were classified under field conditions according to the level of resistance to rust: resistant (R), moderately resistant (MR), and susceptible (S). Based on this classification, the stability of eight reference genes (RGs) was evaluated. The most stable RGs were EF1α, APT1, and 24S. We also evaluated the expression of CaWRKY1, CaPAL1, CaCAD1, and CaPOX1, and activities of PAL, CAD, and POX, which are involved in lignin biosynthesis, and leaf content of total phenolic compounds and lignin. Gene expression and enzymatic activity were not correlated with defense metabolites in the R cultivar group but showed a negative correlation with phenolic compounds in MR cultivars. Cultivar S showed positive correlations of gene expression and enzyme activity with phenolic compounds. These results may assist coffee breeding programs regarding selection of genotypes and in optimization of rust resistance.