MeBIK1, a novel cassava receptor-like cytoplasmic kinase, regulates PTI response of transgenic Arabidopsis

Cassava bacterial blight is the most destructive disease in cassava, causing a significant reduction in its production. The innate immunity response, which has a broad spectrum and a persistent effect, is the basal defence of plants in response to pathogens. Isolation and identification of innate immune-related genes in cassava will contribute to understanding the disease resistance mechanism. In Arabidopsis, the receptor-like cytoplasmic kinase (RLCK) AtBIK1 is known to be an important signal mediator in pathogen-associated molecular pattern-triggered immunity (PTI) response, forming a signal complex from various receptors including the flagellin receptor FLS2, the chitin receptor CERK1 and the receptor for bacterial EF-Tu EFR (Zhang et al. 2010). In the present study, we selected a candidate receptor-like cytoplasmic kinase gene, MeBIK1, from the cassava genome. MeBIK1 encodes a 409 amino acid polypeptide comprising a typical serine/threonine protein kinase domain, and is located on the cell membrane. MeBIK1 gene expression was significantly increased upon stimulation with flagellin (flg22) and peaked at 1 h. In vitro genetic complementation experiment showed that MeBIK1 complemented the reduced pathogen-associated molecular pattern-triggered immunity (PTI) response in Arabidopsis bik1 mutant. Arabidopsis MeBIK1 overexpression lines OX1 demonstrated a strong resistance to Xanthomonas axonopodis pv. manihotis HN01, whereas its sensitivity to Pseudomonas syringae pv. tomato DC3000 was enhanced. The peak level of reactive oxygen species (ROS) burst was reached in different Arabidopsis plants (bik1, OX1 and wild type) at 12 min after induction with flg22. However, the OX1 showed significantly higher ROS levels than the control and mutant, whereas the lowest level of ROS burst was found in the bik1 mutant. These results indicate that cassava MeBIK1 has a similar function as Arabidopsis AtBIK1 and improves disease resistance in transgenic Arabidopsis by regulating the PTI response.