The role of a receptor-like protein kinase (At-RLK3) in the perception of chemical activators in Arabidopsis thaliana

English: The involvement of a receptor-like protein kinase, At-RLK3 in the response of A. thaliana to four different plant activators was analyzed in this study. The activators were salicylic acid (SA), benzothiadiazole (8TH), acetylsalicylic acid (ASA) and 2,6-dichloroisonicotinic acid (INA). The activation of the defense response was determined measuring both the activity and expression on protein level of β-1,3-glucanases. SA, ASA, 8TH and INA treatments showed an early transient induction of β-1,3-glucanase activity after treatment. On protein level the majority of polypeptides were constitutively expressed in the SA, 8TH, ASA and INA treated plants, indicating that a pool of inactive β -1,3-glucanase proteins already present in the cells, were activated upon treatment. It was only in the SA and 8TH treated plants that newly synthesized polypeptides were visible during the later stages of the time studies. Once the activation of β -1,3-glucanases was established, the possible involvement of At-RLK3 in the activation of plant defense mechanisms by plant activators, was investigated. It was found that At-RLK3 mRNA accumulated rapidly within plants treated with SA, 8TH, ASA and INA. The results indicated a correlation between the onset of SAR in the plant and the expression of the At- RLK3 gene. Hydrogen peroxide acts as an early marker for the activation of defenses. The fact that H202 levels rose after At-RLK3 expression was already induced, led us to suggest that H202 did not act as the primary signal that binds to At-RLK3 but that it acts downstream of At-RLK3. All four treatments did however show the characteristic oxygen burst shortly after application. These results however need to be substantiated by repeating each experiment. In addition to being involved in defense signalling, a role for At-RLK3 in plant development was also suggested. It was found that the transfer of a second At- RLK3 gene to wild-type plants inhibited the expression of the native gene. These lower At-RLK3 levels led to an altered phenotype of the transgenic plant, implicating a possible role for At-RLK3 in the development of the plant. Elevated levels of At-RLK3 in transgenic plants led a higher induction of PR-2 expression after treatment with salicylic acid. A potential application for At-RLK3 in important crops when treated with plant activators could be envisaged.