Chloroplast biogenesis during seedling development of angiosperms is a rapid and highly dynamic process that parallels the light-dependent photomorphogenic programme. Pre-treatments of dark-grown seedlings with lincomyin or norflurazon prevent chloroplast biogenesis upon illumination yielding albino seedlings. A comparable phenotype was found for the Arabidopsis mutant plastid-encoded polymerase associated protein 7 (pap7) being defective in the prokaryotic-type plastid RNA polymerase. In all three cases the defect in plastid function has a severe impact on the expression of nuclear genes representing the influence of retrograde signaling pathway(s) from the plastid. We performed a meta-analysis of recently published genome-wide expression studies that investigated the impact of the aforementioned chemical and genetic blocking of chloroplast biogenesis on nuclear gene expression profiles. We identified a core module of 152 genes being affected in all three conditions. These genes were classified according to their function and analyzed with respect to their implication in retrograde signaling and chloroplast biogenesis. Our study uncovers novel genes regulated by retrograde biogenic signals and suggests the action of a common signaling pathway that is used by signals originating from plastid transcription, translation and oxidative stress.

Plant seeds do not contain differentiated chloroplasts. Upon germination the seedling, thus, need to gain photoautotrophy before storage energies are depleted. This requires the coordinated expression of photosynthesis genes encoded in nuclear and plastid genomes. Chloroplast biogenesis needs to be additionally coordinated with the light regulation network that controls seedling development. This coordination is achieved by nucleus-to-plastid signals called anterograde and plastid-to-nucleus signals coined retrograde. Retrograde signals sent from plastids during intial chloroplast biogenesis are also called biogenic signals. They have been recognized as highly important for proper chloroplast biogenesis and for seedling development. The molecular nature, transport, targets and signalling function of biogenic signals are, however, under debate. Several studies disproved the involvement of a number of key components that were at the base of initial models of retrograde signalling. New models now propose major roles for a functional feedback between plastid and cytosolic protein homeostasis in signaling plastid dysfunction as well as the action of dually localized nucleo-plastidic proteins that coordinate chloroplast biogenesis with light-dependent control of seedling development. This review provides a survey of the developments in this research field, summarizes the unsolved questions, high-lights several recent advances and discusses potential new working modes.

varieties, transgenic or knockout lines able to maintain their green color longer than wild-type plants. In these plants, long-lasting leaf coloration is correlated to durable chlorophyll accumulation compared to wild-type plants or standard varieties, and it is often associated with delayed senescence 52. In Arabidopsis, senescence mechanisms induce the expression of ORESARA1 (ORE1) and ORE1 SISTER1 (ORS1) genes. ORE1 activates program cell death and ORS1 participates to salt-induced senescence; the corresponding knockout mutant plants display a stay-green phenotype and delayed senescence 19,20,22,26. Conversely, the disruption of VND-INTERACTING2 (VNI2) and JUNGBRUNNEN1 (JUB1)-which also encode for two NAC proteins-causes early senescence while their overexpression induces a stay-green phenotype 24,53. Recently, it was demonstrated that transgenic tomato lines, with reduced accumulation of SlNAP2 messenger (Solanum lycopersicum NAC-like, activated by Apetala3/ Pistillata), display a stay-green phenotype even upon ABA (abscisic acid) application 33. In this manuscript, we describe the role of Solyc12g036480, which encodes a NAC transcription factor able to modulate leaf senescence in tomato. We demonstrate that Solyc12g036480 downregulation, achieved via Virusinduced gene silencing (VIGS), confers longer life span and delayed overall senescence in tomato plants; for this reason we named this gene HḖBĒ (HEB) after the Greek youth goddess. Results and discussion HEB expression analyses. The tomato NAC TFs family counts 101 members and only few of them have been functionally characterized. As yet, tomato NAC proteins have been described as involved in defense responses, stomata opening and closure, drought tolerance, flower-boundary morphogenesis, leaf senescence and fruit ripening 33,54-57. Among these 101 NAC members, we have selected Solyc12g036480/HEB for a deeper characterization. According to the transcriptome data collection of the Tomato Genome Consortium, HEB is equally transcribed in leaves and roots, but from the experimental data of Huang and Schiefelbein, HEB messenger is not detected in roots 58,59. In order to define temporally and spatially HEB expression pattern, quantitative Real-Time PCRs (qRT-PCRs) were performed. Expression analyses were carried out using organs dissected by Micro-tom plants; UBIQUITIN 3 (UBI3) and ELONGATION FACTOR 1α (EF1α) were used as reference genes 60. HEB transcript was found in young and old leaves and in young floral buds, but its mRNA is barely detected in roots, stem, mature green and red ripe fruits [developmental stages as described in 61 (Fig. 1)].