Fungal pathogens secrete effector molecules into host plant cells to suppress host immunity to colonize plants. Ongoing efforts are being made to identify and characterize effector proteins in many fungal plant pathogens. Nevertheless, the precise biological and biochemical functions of many effectors, such as their trafficking from the pathogen to the host, have yet to be fully understood. In this study, we show that an effector candidate, matured PstCTE1 of Puccinia striiformis f. sp. tritici, localizes to chloroplasts when expressed in planta. It has no conserved transit signal region that can be detected by widely accepted prediction tools including TargetP and ChloroP, it must be carrying a unique localization signal. We have shown that N-terminal tagged red fluorescent protein has no effect on the chloroplast localization of PstCTE1, suggesting a new chloroplast translocation mechanism. We also observed the entrance of the candidate effector to the chloroplast even with the construct having the intact signal peptide on the N-terminus of the transit peptide region. Possibly due to overexpression of the protein in N. benthamina, accumulation in the ER (cytoplasm) was obvious. As previously reported, PstCTE1, similar to effector proteins, may either escape from the secretory pathway by retrograde transport, or translation may occur at alternative sites. This would result in a truncated and/or non-functional signal peptide at the N-terminus in a non-host model system (Nicotiana benthamiana), if it is not re-entering the cell from the apoplast. Our study adds PstCTE1 to the pool of few candidate effectors, experimentally shown to target the chloroplast.

真菌病原体将效应分子分泌到宿主植物细胞中，以抑制宿主免疫定植的植物。正在努力确定和鉴定许多真菌植物病原体中的效应蛋白。然而，许多效应子的精确生物学和生化功能，例如它们从病原体到宿主的运输，还没有被完全理解。在这项研究中，我们表明效应候选者，成熟的Puccinia striiformis f的PstCTE1 。sp。小麦，在植物中表达时定位于叶绿体。它没有保守的运输信号区域，无法被包括TargetP和ChloroP在内的广泛接受的预测工具检测到，它必须携带唯一的定位信号。我们已经显示，N末端标记的红色荧光蛋白对PstCTE1的叶绿体定位没有影响，提示了新的叶绿体转运机制。我们也观察到即使在转运肽区域的N-末端具有完整的信号肽的构建体，候选效应子也进入了叶绿体。可能是由于本氏烟草中蛋白质的过表达，在ER（细胞质）中的积累是明显的。如先前报道，与效应子蛋白相似，PstCTE1可能通过逆行转运从分泌途径中逃逸，或者翻译可能发生在其他位点。如果它没有从质外体中重新进入细胞，这将在非宿主模型系统（本氏烟草）中的N端导致截短和/或无功能的信号肽。我们的研究将PstCTE1添加到了少数候选效应子的库中，实验证明它们可靶向叶绿体。