After perception of specific biotic or abiotic stimuli, such as root colonization by rhizobacteria or selected chemicals, plants can enhance their basal resistance against pathogens. Due to its likely sustainability, this induced resistance will be valuable for disease management in agriculture. This study examined resistance against wheat leaf rust (Puccinia triticina) induced by Pseudomonas protegens CHA0 (CHA0) and β-aminobutyric acid (BABA). Seed dressing with CHA0 reduced the number of sporulating pustules on leaves, and expression of resistance was visible as necrotic or chlorotic leaf flecks. Beneficial effect of CHA0 on wheat seedlings growth was observed in when they were challenged or not with leaf rust. BABA was tested at 10, 15 or 20 mM, and a dose-dependent reduction of leaf rust infections was observed with greatest protection at 20 mM. However, BABA treatment repressed plant growth at 20 mM. Balancing the BABA impact on plant growth and its protective capacity, 15 mM of the compound was selected as suitable to protect wheat seedlings against leaf rust, with the least impact on vegetative host growth. Histological aspects of the pathogen infection process was studied to understand mechanisms of behind the observed resistance. The pre-entry process was not affected by the two resistance inducers, but both treatments reduced fungus penetration and haustorium formation. Timing and amplitude of the resistance reactions were different after bacterial or chemical induction, leading to different levels of resistance. During fungal colonization of host tissues, high deposition of callose and accumulation of H2O2 in both CHA0- and BABA-treated plants indicated important contributions to resistance.

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8The hemibiotrophic fungus Zymoseptoria tritici is the causative agent of Septoria tritici leaf blotch 9 (STB) disease of wheat (Triticum aestivum L.), the economically most damaging disease of wheat in 10 Europe. Today, ecofriendly plant protection methods compatible with sustainable agriculture are 11 strongly desirable. Here, we applied the two chemical inducers β-aminobutyric acid (BABA) and 12 benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH) and the two biotic inducers 13Pseudomonas protegens CHA0 (CHA0) and P. chlororaphis PCL1391 (PCL) on the roots of wheat 14 seedlings in order to test their ability to induce resistance against STB. At 21 days after inoculation, 15 only plants treated with BABA showed a smaller area covered by lesions and less pycnidia compared 16 to the untreated control plants. We evaluated spore germination and fungal development on inoculated 17 wheat leaves at early infection stages using calcofluor white staining. Overall, spores of Z. tritici 18 germinated less on plants soil-drenched with BABA and BTH and their hyphal growth was 19 significantly delayed. On the contrary, CHA0 and PCL seed treatments did not affect fungal growth in 20 wheat leaves. In conclusion, BABA efficiently enhanced plant resistance to Z. tritici, BTH delayed 21 fungal development at early stages while the two biotic inducers did not enhance resistance against 22 STB disease. 23

8After perception of specific biotic or abiotic stimuli, such as root colonization by rhizobacteria or 9 selected chemicals, plants are able to enhance their basal resistance against pathogens. Due to its 10 sustainability, such induced resistance is highly valuable for disease management in agriculture. Here 11 we study an example of resistance against wheat-leaf rust induced by Pseudomonas protegens CHA0 12 (CHA0) and β-aminobutyric acid (BABA), respectively. Seed dressing with CHA0 reduced the 13 number of sporulating pustules on the leaves and the expression of resistance was visible as necrotic 14 or chlorotic flecks. Moreover, a beneficial effect of CHA0 on growth was observed in wheat 15 seedlings challenged or not with leaf rust. BABA was tested at 10, 15 and 20 mM and a dose-16 dependent reduction of leaf rust infection was observed with the highest level of protection at 20 mM. 17However, BABA treatment repressed plant growth at 20 mM. Balancing between BABA-impact on 18 plant growth and its protective capacity, we selected 15 mM as suitable concentration to protect 19 wheat seedlings against leaf rust with the least impact on vegetative growth. To understand the 20 mechanisms behind the observed resistance, we have studied the histological aspects of the fungal 21 infection process. Our results showed that the p re-entry process was not affected by the two 22 resistance inducers. However, both treatments reduced fungal penetration and haustoria formation. 23The timing and the amplitude of the resistance reactions was different after bacterial or chemical 24 induction, leading to different levels of resistance to leaf rust. During fungal colonization of the 25 tissues, a high deposition of callose and the accumulation of H 2 O 2 in both CHA0-and BABA-treated 26 plants pointed to an important contribution to resistance. 27 Leaf rust, callose deposition, hydrogen peroxide (H 2 O 2 ), plant resistance inducers 29 INTRODUCTION 30 Plants dispose of several layers of sophisticated defense mechanisms to defend themselves against 31 pathogen attack. The first layer is given by preformed physical and chemical barriers that impede the 32 pathogen to penetrate into the plant and to initiate infection (Ferreira et al. 2006). Once the presence 33 of the pathogen has been detected, the plant activates further chemical and physical barriers that 34 block or at least delay the attack (second layer; Jones and Dangl (2006)). Defense success depends 35 on the readiness of the plant to detect the pathogen. In the case of the interaction between wheat and 36 the leaf rust pathogen (Puccinia triticina), the plant can detect specific fungal avirulence factors 37 (elicitors) with leaf rust resistance genes (Lr). This gene-by-gene interaction is a very rapid 38 recognition-reaction event leading to an elevated degree of resistance against the disease. However, 39 the avirulence patterns can change and the pathogen may become undetectable by the plant. This 40 2 resistance breakdown happened recently with yellow rust (Hovmøller ...