Editors' ChoicePlant Immunity

Immunity pipelines

Sci. Signal.  14 Jul 2015:
Vol. 8, Issue 385, pp. ec191
DOI: 10.1126/scisignal.aac9914

Stromules are dynamic tubular projections formed by plastids, such as chloroplasts, and they have been hypothesized to mediate interorganellar communication (see Gu and Dong). Caplan et al. found that viral or bacterial infection of Nicotiana benthamiana leaves induced chloroplasts to form stromules that contacted the nucleus. The binding of pathogen components to nucleotide-binding and leucine rich–repeat proteins (NLRs) initiates effector-triggered immunity (ETI), one of the outputs of which is the induction of programmed cell death (PCD) at the site of infection. Transient expression of the tobacco mosaic virus effector p50 induced stromules in cells undergoing PCD and in neighboring cells, but not in cells far from the site of PCD, suggesting that short-range signals produced during infection may be responsible for stromule induction. Application of hydrogen peroxide (H2O2) or salicylic acid, both of which are produced in chloroplasts of infected cells, also induced stromule formation. Transient expression of p50 caused N receptor–interacting protein 1 (NRIP1) to translocate from chloroplasts to the nucleus, where it is required for p50-mediated activation of the NLR receptor N. Experiments with various fluorescently tagged forms of NRIP1 indicated that, during the immune response, NRIP1 preferentially accumulated in nuclei that were contacted by stromules. H2O2 produced in chloroplasts also moved into nuclei during p50-induced immunity. Expression of peptides that interfered with the proper insertion of proteins into the outer chloroplast membrane prevented stromule formation in response to transient expression of p50. Plants lacking the chloroplast outer membrane protein CHUP1 constitutively formed stromules and exhibited increased PCD in response to transient p50 expression compared to controls. These findings imply that infection-induced stromules contribute to the nuclear accumulation of factors necessary for ETI and suggest that chloroplast factors may initiate or control PCD.

J. L. Caplan, A. S. Kumar, E. Park, M. S. Padmanabhan, K. Hoban, S. Modla, K. Czymmek, S. P. Dinesh-Kumar, Chloroplast stromules function during innate immunity. Dev. Cell 34, 45–57 (2015). [PubMed]

Y. Gu, X. Dong, Stromules: Signal conduits for plant immunity. Dev. Cell 34, 3–4 (2015).[PubMed]