Chloroplast Control of Leaf Senescence

Abstract

SummaryChloroplasts have an active role in the induction and course of senescence through signals, derived from reactive oxygen species (ROS), exiting to the cytosol. In this way, leaf chloroplasts mimic the role of mitochondria in the apoptosis of non-photosynthetic organs. Oxylipins, magnesium-protoporphyrin IX (Mg-ProtoIX) and proteins are the best studied signal candidates that exit from chloroplasts that, through a poorly-understood network of transduction intermediates, inhibit the expression of genes encoding components of the photosynthetic machinery and activate the expression of senescence-associated genes (SAG) in the nucleus, ultimately resulting in activation of programmed cell death (PCD). There are many similarities in the mechanisms of production and scavenging of ROS in chloroplasts during stress defense and in senescence. However, there are also differences and one significant difference is that the feed-back effect of chloroplast signals on the ROS level in defense responses changes to a feed-forward effect that favors further increases of ROS in the PCD associated with leaf senescence or the hypersensitive response (HR). Therefore, a key question related to the induction of leaf senescence resides in the molecular basis of the shift from a feed-back to a feed-forward suicide response of the network that regulates the chloroplast ROS level. Up-regulation of nuclear genes encoding components of the thylakoid Ndh complex and of lipoxygenase (LX) and down-regulation of the nuclear genes for chloroplastic superoxide dismutase (SOD) seem to be among the changes responsible for the feed-forward increase of ROS associated with (and probably determining) PCD, but further investigations are needed to identify the intermediates and their cause-effect relations in the signal transduction network. These investigations must include hormones, H2O2, proteins and microRNAs, whose levels and interactions are also affected by environmental factors acting inside and outside the chloroplast.