Proximity driven plastid-nucleus relationships are facilitated by tandem plastid-ER dynamics.

Peri-nuclear clustering (PNC) of chloroplasts has largely been described in senescent and pathogen- or ROS- stressed cells. Stromules, tubular plastid extensions are also observed under similar conditions. Coincident observations of PNC and stromules associate the two phenomena in facilitating retrograde signaling between chloroplasts and the nucleus. However, PNC incidence in non-stressed cells under normal growth and developmental conditions, when stromules are usually not observed, remains unclear. Using transgenic Arabidopsis expressing different organelle-targeted fluorescent proteins we show that PNC is a dynamic subcellular phenomenon that continues in the absence of light and is not dependent on stromule formation. PNC is facilitated by tandem plastid-ER dynamics created through membrane contact sites between the two organelles. While PNC increases upon ER-membrane expansion, some plastids may remain in the peri-nuclear region due to their localization in ER-lined nuclear indentions. Moreover, some PNC plastids may sporadically extend stromules into ER-lined nuclear grooves. Our findings strongly suggest that PNC is not an exclusive response to stress caused by pathogens, high light or exogenous-H2O2 treatment and does not require stromule formation. However, morphological and behavioural alterations in ER and concomitant changes in tandem, plastid-ER dynamics play a major role in facilitating the phenomenon.