The Expansin Gene Is a Direct Target of CsLOB1 in Citrus

Transcription activator-like effectors are key virulence factors of . They are secreted into host plant cells and mimic transcription factors inducing the expression of host susceptibility (S) genes. In citrus, is a direct target of PthA4, the primary effector associated with citrus canker symptoms. is a transcription factor, and its expression is required for canker symptoms induced by subsp. . Several genes are up-regulated by PthA4; however, only was described as an S gene induced by PthA4. Here, we investigated whether other up-regulated genes could be direct targets of PthA4 or CsLOB1. Seven up-regulated genes by PthA4 were investigated; however, an expansin-coding gene was more induced than . In transient expression experiments, we demonstrate that the expansin-coding gene, referred here to as (), is not a direct target of PthA4, but CsLOB1. Interestingly, was induced by CsLOB1 even without the predicted CsLOB1 binding site, which suggested that CsLOB1 has other unknown binding sites. We also investigated the minimum promoter regulated by CsLOB1, and this region and LOB1 domain were conserved among citrus species and relatives, which suggests that the interaction PthA4-- is conserved in citrus species and relatives. This is the first study that experimentally demonstrated a CsLOB1 downstream target and lays the foundation to identify other new targets. In addition, we demonstrated that the is a putative S gene indirectly induced by PthA4, which may serve as the target for genome editing to generate citrus canker-resistant varieties.