Stromule extension along microtubules coordinated with actin-mediated anchoring guides perinuclear chloroplast movement during innate immunity

Abstract

Dynamic tubular extensions from chloroplasts called stromules have recently been shown to connect with nuclei and function during innate immunity. We demonstrate that stromules extend along microtubules (MTs) and MT organization directly affects stromule dynamics since stabilization of MTs chemically or genetically increases stromule numbers and length. Although actin filaments (AFs) are not required for stromule extension, they provide anchor points for stromules. Interestingly, there is a strong correlation between the direction of stromules from chloroplasts and the direction of chloroplast movement. Stromule-directed chloroplast movement was observed in steady-state conditions without immune induction, suggesting it is a general function of stromules in epidermal cells. Our results show that MTs and AFs may facilitate perinuclear clustering of chloroplasts during an innate immune response. We propose a model in which stromules extend along MTs and connect to AF anchor points surrounding nuclei, facilitating stromule-directed movement of chloroplasts to nuclei during innate immunity.

Chloroplast clustering around the nucleus is a general response to pathogen perception in Nicotiana benthamiana

“…Chloroplast clustering around the nucleus has been previously observed during ETI triggered by the p50 protein from TMV (Caplan et al , ; Kumar et al , ). Considering this, we wondered whether expression of Rep could result in the activation of immune responses in N. benthamiana .…”

“…Fondong et al , ; Jelenska et al , , ; Li et al , ; Liu et al , ; Park et al , ; Petre et al , ; Rodriguez‐Herva et al , ; Rosas‐Diaz et al , ; de Torres Zabala et al , ). Intriguingly, chloroplasts can establish contact with the nucleus, through clustering around it and extending tubular stroma‐filled protrusions called stromules (Natesan et al , ), under different circumstances, including the activation of ETI triggered by the p50 protein from Tobacco mosaic virus (TMV) (Caplan et al , , ; Kumar et al , ; Kwok and Hanson, ). Although the exact role of this physical interaction between chloroplasts and nucleus is at present unclear, it has been suggested that this connection could enable the transfer of molecules from the plastids (e.g.…”

“…During plant–pathogen interactions, chloroplasts act as a source of signalling and/or defensive compounds, including ROS, SA, JA and cytoplasmic calcium bursts, and are required for transcriptional reprogramming and for the plant to mount a full defence response (Nomura et al , ; Park et al , ; Pogorelko et al , ; Sowden et al , ; de Torres Zabala et al , ). It has been previously shown that, upon activation of ETI through expression of the p50 protein from TMV, chloroplasts tend to cluster around the nucleus and emit stromules (Caplan et al , ; Kumar et al , ); interestingly, infection by the geminivirus AbMV also results in the induction of stromules (Krenz et al , , ). Here, we show that chloroplast perinuclear clustering is a general response to the perception of a biotic threat, occurring both during PTI and ETI, and in response to bacteria and viruses.…”

“…These results suggest the idea that ROS might be somehow perceived by the plastids, directly or indirectly, and elicit their migration towards the nucleus. On the other hand, recent work by Kumar et al () has shed light onto questions two and three, proving that chloroplasts move associated to microtubules, but anchor themselves to the nucleus through actin filaments. It is to be expected that these latest discoveries will pave the way for future studies, which will hopefully help unravel the functional relevance of this intriguing phenomenon.…”

Genetic and physical interactions between the organellar mechanosensitive ion channel homologs MSL1, MSL2, and MSL3 reveal a role for inter‐organellar communication in plant development

“…While individual reactions that take place in the plastid or mitochondrion benefit from their compartmentalization, broad metabolic processes are coordinated between them and the rest of the cell (Rolland et al, 2012;Schrader & Yoon, 2007;Sweetlove & Fernie, 2013). Furthermore, plastids and mitochondria physically interact with multiple other cellular compartments, including the nucleus, peroxisomes, and the ER (Barton, Wozny, Mathur, Jaipargas, & Mathur, 2018;Jaipargas, Mathur, Bou Daher, Wasteneys, & Mathur, 2016;Kumar et al, 2018;Kwok & Hanson, 2004;Mueller & Reski, 2015).…”