Abstract
Fungi in the genus Cercospora cause crop losses world-wide on many crop species. The wide host range and success of these pathogens has been attributed to the production of a photoactivated toxin, cercosporin. We engineered tobacco for resistance to Cercospora nicotianae utilizing two strategies: 1) transformation with cercosporin autoresistance genes isolated from the fungus, and 2) transformation with constructs to silence the production of cercosporin during disease development. Three C. nicotianae cercosporin autoresistance genes were tested: ATR1 and CFP, encoding an ABC and an MFS transporter, respectively, and 71cR, which encodes a hypothetical protein. Resistance to the pathogen was identified in transgenic lines expressing ATR1 and 71cR, but not in lines transformed with CFP. Silencing of the CTB1 polyketide synthase and to a lesser extent the CTB8 pathway regulator in the cercosporin biosynthetic pathway also led to the recovery of resistant lines. All lines tested expressed the transgenes, and a direct correlation between the level of transgene expression and disease resistance was not identified in any line. Resistance was also not correlated with the degree of silencing in the CTB1 and CTB8 silenced lines. We conclude that expression of fungal cercosporin autoresistance genes as well as silencing of the cercosporin pathway are both effective strategies for engineering resistance to Cercospora diseases where cercosporin plays a critical role.
Highlights
Fungi in the genus Cercospora cause devastating crop losses on a wide range of crop plants world-wide including sugar beet, corn, and coffee as well as many vegetable and ornamental species [1]
Independent transformations were done with three genes from C. nicotianae previously shown to be involved in cercosporin autoresistance in the fungus: ATR1, encoding a putative ABC transporter [27]; CFP, encoding a putative MFS transporter [12, 27]; and 71cR, encoding a hypothetical protein [29]
Transformants with ATR1 and CFP utilized haploid plants generated by crossing N. tabacum with Nicotiana africana [40]
Summary
Fungi in the genus Cercospora cause devastating crop losses on a wide range of crop plants world-wide including sugar beet, corn, and coffee as well as many vegetable and ornamental species [1]. The wide host range and lack of disease resistance in many host species has been attributed to the fungus’ production of a photoactivated perylenequinone toxin, cercosporin (for review see [2,3,4]). In host plants the cercosporin-generated ROS cause peroxidation of the host cell membrane lipids, leading to membrane breakdown, death of host cells and leakage of nutrients needed by the fungus for tissue colonization [9,10,11]. The importance of light, required for cercosporin photoactivation, in disease symptom development has been documented in both coffee and sugar beet [18, 19]
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