Abstract

Soybean stem and root rot disease, caused by Phytophthora sojae , is a considerable threat to soybean cultivation. P. sojae is known to secrete a diverse array of nearly 400 RxLR-dEER effectors. However, the functional roles and underlying mechanisms of most effectors remain largely unexplored. Plant U-box proteins (PUBs), a type of putative E3 ubiquitin ligase, play a crucial role in the plant immune regulation. Various of soybean PUBs were targeted by multiple of P. sojae effectors. In this study, it was determined that 14 of the 27 tested soybean PUBs interacted with Avr1d, and the expression levels of interacted PUBs appear to be higher than those of uninteracted PUBs when infected by P. sojae . However, Avr1d -knockout mutants retained full pathogenicity on the susceptible cultivar as the wild type did. Avh32 and Avh64, two effectors that shared some homology with Avr1d, were found to interact with GmPUB13, albeit at distinct regions. Interestingly, Avh64 could inhibit the ubiquitination activity. The triple mutants of Avr1d , Avh32 , and Avh64 still showed no decrease in pathogenicity on susceptible cultivars. These findings suggested functional redundancy among P. sojae effectors and indicated a saturation-attacking strategy employed by P. sojae “effectors squads” targeting soybean PUBs, which has great significance for understanding the functional mechanisms of Phytophthora secretomes and for controlling soybean Phytophthora stem and root rot.