Hyaloperonospora arabidopsidis

Hyaloperonospora arabidopsidis is a species from the family Peronosporaceae. It is an obligate parasite and the causal agent of the downy mildew of the plant model organism Arabidopsis thaliana. While H. arabidopsidis has for a long time been subsumed under Peronospora parasitica (now Hyaloperonospora parasitica), recent studies have shown that H. parasitica is restricted to Capsella bursa-pastoris as a host plant. Like the other Hyaloperonospora species, H. arabidopsidis is highly specialized, and it is so far known with certainty only from Arabidopsis thaliana as a host plant. Hyaloperonospora arabidopsidis encodes 10 different noncytotoxic NLPs (HaNLPs) that do not cause necrosis. We discovered that these noncytotoxic NLPs, however, act as potent activators of the plant immune system in Arabidopsis thaliana. Ectopic expression of HaNLP3 in Arabidopsis triggered resistance to H. arabidopsidis, activated the expression of a large set of defense-related genes, and caused a reduction of plant growth that is typically associated with strongly enhanced immunity. N- and C-terminal deletions of HaNLP3, as well as amino acid substitutions, pinpointed to a small central region of the protein that is required to trigger immunity, indicating the protein acts as a microbe-associated molecular pattern (MAMP).