“Plant disease has a major negative impact on crops. It was estimated that up to 40% of plant productivity in Africa and Asia, and about 20% in the developed world, is lost to pests and pathogens.”

Mitogen-activated protein kinase (MAPK) cascades are important pathways downstream of sensors/receptors that regulate cellular responses to both external and endogenous stimuli in eukaryotes. SIPK and WIPK, two tobacco MAPKs, were implicated in signaling plant disease resistance response based on correlative evidence. This paper provided, for the first time, the loss-of-function evidence supporting the role of these two MAPKs and their upstream MAPKK, NtMEK2, in signaling plant disease resistance. Tobacco resistance against tobacco mosaic virus (TMV) is mediated by N resistance gene in tobacco. When the expression of SIPK, WIPK, or NtMEK2 was suppressed by virus-induced gene silencing (VIGS), N gene-mediated resistance against TMV was severely compromised. In this paper, we also showed that the activation of NtMEK2-SIPK/WIPK cascade leads to hypersensitive response (HR)-like cell death based on gain-of-function transgenic tobacco that conditionally expresses an active NtMEK2. HR cell death, a type of plant programmed cell death, plays an important role in restricting the spread of pathogens in plants and is a hallmark of plant resistance against pathogens.

Recent genetic studies greatly enriched our knowledge about how plants recognize the invading pathogens. However, the signaling pathways downstream of the sensing step remain to be defined. Protein phosphorylation and dephosphorylation are implicated in the process based on correlative evidence from inhibitor and biochemical analyses. This study provided genetic evidence that plant MAPK pathways, represented by tobacco NtMEK2-SIPK/WIPK, are involved in signaling plant disease resistance by activating various defense responses including HR cell death.

In addition to the loss- and gain-of-function data, we also demonstrated the role of the conserved kinase interaction motif (KIM) in NtMEK2 in this paper. Mutation of the conserved basic amino acids in this motif, or the deletion of N-terminal 64 amino acids containing this motif significantly compromised or abolished the ability of NtMEK2 to activate SIPK/WIPK in vivo. These mutants are also defective in interacting with SIPK and WIPK, suggesting protein-protein interaction is required for the functional integrity of this MAPK cascade.

Plant disease has a major negative impact on crops. It was estimated that up to 40% of plant productivity in Africa and Asia, and about 20% in the developed world, is lost to pests and pathogens. Plants fend off pathogens by deploying a battery of active defense responses when they are challenged by pathogens. Understanding the regulation of plant disease resistance may eventually lead to crops with enhanced pathogen resistance and improved yield.

This paper is a fine example of collaborative research. An earlier study performed by myself when I was a post-doc with Dr. Daniel Klessig (then at the Waksman Institute, Rutgers University) implicated SIPK and WIPK, two mitogen-activated protein kinases, in N gene-mediated tobacco resistance against TMV (Zhang, S., and Klessig, D.F., Proc. Natl. Acad. Sci. USA 95: 7433-7438, 1998.). This conclusion was mainly based on correlative biochemical evidence. To provide genetic evidence, my lab used a conditional gain-of-function approach and found that the activation of SIPK and WIPK by an active NtMEK2, the upstream MAPKK of SIPK and WIPK, leads to HR-like cell death in the absence of TMV infection. HR cell death is a hallmark of plant resistance to pathogen. Dr. Hailing Jin (currently an Assistant Professor at University of California, Riverside) in Dr. Barbara Baker’s lab (University of California, Berkeley & Plant Gene Expression Center) silenced SIPK, WIPK, and their upstream NtMEK2 using VIGS, and found that N gene-mediated resistance against TMV was compromised in these plants. Putting these two pieces of information together, we provided genetic evidence that NtMEK2-SIPK/WIPK cascade plays an important role in tobacco resistance to TMV in the paper.