A multilayered regulatory mechanism for the autoinhibition and activation of a plant CC ‐ NB ‐ LRR resistance protein with an extra N‐terminal domain

Summary The tomato resistance protein Sw‐5b differs from the classical coiled‐coil nucleotide‐binding leucine‐rich repeat ( CC ‐ NB ‐ LRR ) resistance proteins by having an extra N‐terminal domain ( NTD ). To understand how NTD , CC and NB ‐ LRR regulate autoinhibition and activation of Sw‐5b, we dissected the function(s) of each domain. When viral elicitor was absent, Sw‐5b LRR suppressed the central NB ‐ ARC to maintain autoinhibition of the NB ‐ LRR segment. The CC and NTD domains independently and additively enhanced the autoinhibition of NB ‐ LRR . When viral elicitor was present, the NB ‐ LRR segment of Sw‐5b was specifically activated to trigger a hypersensitive response. Surprisingly, Sw‐5b CC suppressed the activation of NB ‐ LRR , whereas the extra NTD of Sw‐5b became a positive regulator and fully activated the resistance protein, probably by relieving the inhibitory effects of the CC . In infection assays of transgenic plants, the NB ‐ LRR segment alone was insufficient to confer resistance against Tomato spotted wilt tospovirus ; the layers of NTD and CC regulation on NB ‐ LRR were required for Sw‐5b to confer resistance. Based on these findings, we propose that, to counter the negative regulation of the CC on NB ‐ LRR , Sw‐5b evolved an extra NTD to coordinate with the CC , thus developing a multilayered regulatory mechanism to control autoinhibition and activation.