Natural variations of TFIIAγ gene and LOB1 promoter contribute to citrus canker disease resistance in Atalantia buxifolia

Abstract

Citrus canker caused by Xanthomonas citri subsp. citri (Xcc) is one of the most devastating diseases in citrus industry worldwide. Most citrus cultivars such as sweet orange are susceptible to canker disease. Here, we utilized wild citrus to identify canker-resistant germplasms, and found that Atalantia buxifolia, a primitive (distant-wild) citrus, exhibited remarkable resistance to canker disease. Although the susceptibility gene LATERAL ORGAN BOUNDARIES 1 (LOB1) could also be induced in Atalantia after canker infection, the induction extent was far lower than that in sweet orange. In addition, three of amino acids encoded by transcription factor TFIIAγ in Atalantia (AbTFIIAγ) exhibited difference from those in sweet orange (CsTFIIAγ) which could stabilize the interaction between effector PthA4 and effector binding element (EBE) of LOB1 promoter. The mutation of AbTFIIAγ did not change its interaction with transcription factor binding motifs (TFBs). However, the AbTFIIAγ could hardly support the LOB1 expression induced by the PthA4. In addition, the activity of AbLOB1 promoter was significantly lower than that of CsLOB1 under the induction by PthA4. Our results demonstrate that natural variations of AbTFIIAγ and effector binding element (EBE) in the AbLOB1 promoter are crucial for the canker disease resistance of Atalantia. The natural mutations of AbTFIIAγ gene and AbLOB1 promoter in Atalantia provide candidate targets for improving the resistance to citrus canker disease.

Antioxidant Metabolites in Primitive, Wild, and Cultivated Citrus and Their Role in Stress Tolerance

“…Flavonoids are involved in the processes of plant resistance against pathogens, pollination attraction, and seed dispersal facilitation; possess antibacterial, antifungal, and antiviral activities; and can scavenge ROS and defend against insects and pests [ 91 , 92 ]. Flavonoid biosynthesis is increased significantly after serve abiotic (metal toxicity, drought, wounding, high-light stress, chilling, salt stress, radiation, and nutrient deficiency [ 19 , 65 , 84 ]) and biotic stresses (e.g., bacterial, fungal, and viral infection) in Citrus species [ 15 ]. The genes involved in the biosynthesis of antioxidant flavonoids are demonstrated in Figure 3 .…”

“…The primitive citrus species that have a high level of metabolites are tolerant to various abiotic and biotic stresses; the wild citrus species ( C. medica and C. latipes ) possessing moderate levels of metabolites are semi-tolerant to stresses; and the cultivated citrus species that contain fewer metabolites are more prone to stresses. High levels of metabolites are positively correlated with abiotic [ 13 ] and biotic stress tolerance in citrus [ 15 ].…”

“…In citrus, the concentration of antioxidant metabolites is positively correlated with stress tolerance (Figure 4). biosynthesis is increased significantly after serve abiotic (metal toxicity, wounding, high-light stress, chilling, salt stress, radiation, and nutrient de [19,65,84]) and biotic stresses (e.g., bacterial, fungal, and viral infection) in Citru [15]. The genes involved in the biosynthesis of antioxidant flavonoids are demo in Figure 3.…”

“…Citrus species possess diverse and uneven levels of metabolites [ 12 ]. Some primitive and wild citrus species (e.g., Atalantia buxifolia and C. latipes ) have high levels of metabolites (particularly phenolics and flavonoids) and are tolerant to various abiotic and biotic stresses, whereas the cultivated citrus species (e.g., C. sinensis and Cleopatra mandarin) containing less total metabolites are susceptible to abiotic and abiotic stresses [ 13 , 15 ]. The leaves, fruit juice, and phloem sap of cultivated citrus species showed a rapid increment in the antioxidant flavonoid and volatile compounds in response to biotic stress [ 16 ].…”

“…The genomic diversity and dissimilar levels of metabolites among Citrus species provide a promising opportunity to breed citrus cultivars with increased contents of metabolites [ 15 , 19 ]. However, the biosynthetic mechanisms of these antioxidant metabolites (e.g., flavonoids) in citrus are scarcely understood [ 1 ].…”