Unraveling Quinoa (Chenopodium quinoa Willd.) Defense Against Downy Mildew (Peronospora variabilis): Comparative Molecular Analysis of Resistant “Hualhuas” and Susceptible “Real” Cultivars

Quinoa (<i>Chenopodium quinoa</i> Willd.) is a new, promising non-conventional useful crop; however, its susceptibility to downy mildew, caused by <i>Peronospora variabilis</i>, is a key obstacle limiting its productivity in Egypt. Identifying and utilizing resistant quinoa cultivars appear to be reliable and cost-efficient strategies for controlling downy mildew, particularly in resource-limited farmers' fields. This study aimed to evaluate the differential resistance of the Peruvian "<i>Hualhuas</i>" and Bolivian "<i>Real</i>" quinoa cultivars to <i>P. variabilis</i> infection under laboratory conditions to provide precise insight into their basic defense mechanism(s). Inoculated "<i>Hualhuas</i>" plants displayed complete resistance against <i>P. variabilis</i>, with no visible symptoms (incompatible reaction), while those of "<i>Real</i>" plants revealed high susceptibility (compatible reaction), with typical downy mildew lesions on their leaf surfaces. Disease incidence reached about 66% in the inoculated "<i>Real</i>" plants, with most inoculated leaves having lesions of grades 4 and 5 covering up to 90% of their leaf surfaces. Susceptibility indices reached up to 66% in the inoculated "<i>Real</i>" plants. Resistance to <i>P. variabilis</i> observed in the "<i>Hualhuas</i>" plants may have been largely attributed to elevated endogenous H₂O₂ levels, increased peroxidase (POX) activity and abundance, enhanced phenylalanine ammonia-lyase (PAL) activity and expression, as well as the upregulation of the pathogen-related protein 10 gene (<i>PR-10</i>). The results of this study indicate that the quinoa cultivar "<i>Hualhuas</i>" not only is a promising candidate for sustainable control of quinoa downy mildew but also, through a deep understanding of its molecular resistance mechanisms, would provide a possible route to enhance downy mildew resistance in other genotypes.