Downy Mildew Resistance 6-like (DMR6-like) genes are identified as salicylic acid (SA) hydroxylases and negative regulators of plant immunity. Previously, we identified two rice DMR6-like genes, OsF3H03g, and OsF3H04g, that act as susceptible targets of transcription activator-like effectors (TALEs) from Xanthomonas oryzae pv. oryzicola (Xoc), which causes bacterial leaf streak (BLS) in rice. Furthermore, all four homologs of rice DMR6-like proteins were identified to predominantly carry the enzyme activity of SA 5-hydroxylase (S5H), negatively regulate rice broad-spectrum resistance, and cause the loss of function of these OsDMR6s, leading to increased resistance to rice blast and bacterial blight (BB). Here, we curiously found that an OsF3H04g knock-out mutant created by T-DNA insertion, osf3h04g, was remarkedly susceptible to BLS and BB and showed an extreme reduction in SA content. OsF3H04g knock-out rice lines produced by gene-editing were mildly susceptible to BLS and reduced content of SA. To explore the susceptibility mechanism in OsF3H04g loss-of-function rice lines, transcriptome sequencing revealed that another homolog, OsS3H, had induced expression in the loss-of-function OsF3H04g rice lines. Furthermore, we confirmed that a great induction of OsS3H downstream and genomically adjacent to OsF3H04g in osf3h04g was primarily related to the inserted T-DNA carrying quadruple enhancer elements of 35S, while a slight induction was caused by an unknown mechanism in gene-editing lines. Then, we found that the overexpression of OsS3H increased rice susceptibility to BLS, while gene-editing mediated the loss-of-function OsS3H enhanced rice resistance to BLS. However, the knock-out of both OsF3H04g and OsS3H by gene-editing only neutralized rice resistance to BLS. Thus, we concluded that the knock-out of OsF3H04g activated the expression of the OsS3H, partially participating in the susceptibility to BLS in rice.

Rice–Xanthomonas oryzae pv. oryzicola (Xoc) is one of the commonly used rice models of host–pathogen interactions. Xoc causes bacterial leaf streak (BLS) and has quarantine status. As a Gram-negative pathogen, Xoc usually employs type III secretion effectors (T3SEs), including transcription activator-like effectors (TALEs) and non-TALEs, to interfere with the innate immunity of rice. However, few major resistance genes corresponding to Xoc are found in rice cultivations; only Rxo1-AvrRxo1 and Xo1-TALEs interactions have been discovered in rice–Xoc. In this review, we focus on the role of the T3S system (T3SS) in Xoc virulence and consider the reported non-TALEs, including AvrRxo1, AvrBs2, XopN, XopC2, XopAP, and XopAK, as well as TALEs including Tal2g/Tal5d, Tal2h, Tal2a, Tal7, Tal10a, TalI, Tal2b, and Tal2c. Interestingly, AvrRxo1, XopC2, and XopAP disturb stomatal opening to promote infection through targeting diverse signaling pathways in rice. Otherwise, Tal2b and Tal2c, respectively, activate two rice salicylic acid (SA) hydroxylation genes to redundantly suppress the SA-mediated basal defense, and TalI, which has unknown targets, suppresses the SA signaling pathway in rice. In addition, other Xoc virulence factors are discussed. In conclusion, several T3SEs from Xoc interfere with similar defense pathways in rice to achieve invasion, providing an outlook for the control of this disease through manipulating the conserved pathways.