Another active area of research in the lab is to understand the role of inter-organellar communication during immunity. As described above, chloroplast localized NRIP1 is recruited to the cytoplasm and nucleus to mediate pathogen recognition and activation of immune signaling. Interestingly, during the immune response chloroplasts send out dynamic tubular projections known as “stromules”. This suggests that chloroplasts have an additional function downstream of pathogen recognition. Stromules are induced during the beginning phases of PCD and pro-PCD cell-to-cell signals such as hydrogen peroxide (H2O2) and salicylic acid (SA) are sufficient to induce stromule formation. During the immune response, stromules form strong connections with nuclei. These connections precede the accumulation of chloroplast-localized NRIP1 and H2O2 in the nucleus. These data provide evidence that stromule to nuclei connections are involved in the signaling from chloroplasts to nuclei during a defense response. Furthermore, we showed that constitutive induction of stromules enhances PCD, proving that stromules function during the progression of PCD. We propose a model in which stromules are involved in transducing and amplifying pro-defense signals originating from chloroplasts (PMC4596411).
In a recent paper, we show that chloroplast stromules dynamically extend along microtubules and anchor to a fine network of actin microfilaments. Stromules appear to guide the movement of chloroplasts, and our work uncovered this hitherto unknown role of stromules and defined a newly discovered form of chloroplast movement. We show that stromules are anchored to actin filaments along nuclei, and these interactions guide the clustering of chloroplasts to nuclei during plant innate immunity (PMC5815851). Future studies will determine if stromules just guide chloroplast movement or provide a physical force that drives the movement.
