Type III Secretion Effectors of Xanthomonas oryzae pv. oryzicola: The Arsenal to Attack Equivalent Rice Defense for Invasion

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.