Histopathological aspects of induced resistance by Pseudomonas protegens CHA0 and β-aminobutyric acid in wheat against Puccinia triticina

After perception of specific biotic or abiotic stimuli, such as root colonization by rhizobacteria or selected chemicals, plants are able to enhance their basal resistance against pathogens. Due to its sustainability, such induced resistance is highly valuable for disease management in agriculture. Here we study an example of resistance against wheat-leaf rust induced by Pseudomonas protegens CHA0 (CHA0) and {beta}-aminobutyric acid (BABA), respectively. Seed dressing with CHA0 reduced the number of sporulating pustules on the leaves and the expression of resistance was visible as necrotic or chlorotic flecks. Moreover, a beneficial effect of CHA0 on growth was observed in wheat seedlings challenged or not with leaf rust. BABA was tested at 10, 15 and 20 mM and a dose-dependent reduction of leaf rust infection was observed with the highest level of protection at 20 mM. However, BABA treatment repressed plant growth at 20 mM. Balancing between BABA-impact on plant growth and its protective capacity, we selected 15 mM as suitable concentration to protect wheat seedlings against leaf rust with the least impact on vegetative growth. To understand the mechanisms behind the observed resistance, we have studied the histological aspects of the fungal infection process. Our results showed that the p re-entry process was not affected by the two resistance inducers. However, both treatments reduced fungal penetration and haustoria formation. The timing and the amplitude of the resistance reactions was different after bacterial or chemical induction, leading to different levels of resistance to leaf rust. During fungal colonization of the tissues, a high deposition of callose and the accumulation of H2O2 in both CHA0-and BABA-treated plants pointed to an important contribution to resistance.