Role of plastids and mitochondria in the early development of seedlings in dark growth conditions

Abstract

Establishment of the seedlings is a crucial stage of the plant life cycle. The success of this process is essential for the growth of the mature plant. In Nature, when seeds germinate under the soil, seedlings follow a dark-specific program called skotomorphogenesis, which is characterized by small, non-green cotyledons, long hypocotyl, and an apical hook-protecting meristematic cells. These developmental structures are required for the seedlings to emerge quickly and safely through the soil and gain autotrophy before the complete depletion of seed resources. Due to the lack of photosynthesis during this period, the seed nutrient stocks are the primary energy source for seedling development. The energy is provided by the bioenergetic organelles, mitochondria, and etioplast (plastid in the dark), to the cell in the form of ATP through mitochondrial respiration and etio-respiration processes, respectively. Recent studies suggest that the limitation of the plastidial or mitochondrial gene expression induces a drastic reprogramming of the seedling morphology in the dark. Here, we discuss the dark signaling mechanisms involved during a regular skotomorphogenesis and how the dysfunction of the bioenergetic organelles is perceived by the nucleus leading to developmental changes. We also describe the probable involvement of several plastid retrograde pathways and the interconnection between plastid and mitochondria during seedling development. Understanding the integration mechanisms of organellar signals in the developmental program of seedlings can be utilized in the future for better emergence of crops through the soil.

Keywords: development; etioplasts; mitochondria; retrograde control; skotomorphogenesis.

Figure 1

Skotomorphogenesis is the dark-specific developmental program of seedlings. Seedlings growing in darkness present unique structures, including apical hook, long hypocotyl, and simple root architecture. At this stage, the seedlings possess etioplasts, small, round plastids that contain proto-thylakoids (PT), prolamellar bodies (PLB), starch, and plastoglobules. Light-dependent protochlorophyllide oxidoreductase (LPOR) binds protochlorophyllide and is coated on PLB membranes. Ribosomes are found in the PLB network, whereas ATP synthases are on the PT membranes.

Figure 2

PIFs are the key regulators of skotomorphogenesis. Left panel: during development in the dark, PHYTOCHROME INTERACTING FACTORs (PIFs) act with COP1-SPA complex (CUL4-DDB1-COP1-SPA) and CDD complex (CUL4-DDB1-COP10-DET1) to inhibit the activity of different transcription factors related to chloroplast biogenesis, like HY5, HYH, HFR1 and GLK2. The COP1-SPA complex also acts to stabilize PIFs by inactivating BIN2. The PIFs induce the expression of the genes related to different phytohormone signaling pathways. Right panel: soon after illumination, the cotyledons open and light perception triggers the activation of different photoreceptors. Light-activated cryptochromes (Cry) and phytochromes (from inactive Pr to the active form pfr) interact and/or induce the degradation of the PIFs in photobodies. Moreover, after being activated by the blue light, cryptochromes interfere with the COP1 complexes. All these events release and enhance the activity of different transcription factors like HY5 and GLKs, resulting in the expression of chloroplast biogenesis and function genes.

Figure 3

Functional link among PGE limitation, mitochondrial dysfunction, and the developmental response of etiolated seedlings. Under PGE-limited conditions (in the presence of rifampicin/spectinomycin) or under mitochondrial dysfunction (rpoTmp mutant), mitochondrial stress marker genes are induced at the transcript level. The expression of the nuclear stress markers is independent of the GUN1 (plastidial) and ANAC017 (mitochondrial) retrograde pathways in the PGE-limited conditions but is dependent on the ANAC017 retrograde pathway in the mitochondrial dysfunction condition. In mitochondrial stress conditions, the ER-localized ANAC017 proteins are cleaved and released for migration into the nucleus and transcriptional regulation. Under both plastidial and mitochondrial dysfunctional conditions, expression of AOX is increased at the mRNA and protein levels; consequently, the AOX-dependent respiratory capacity is also induced. Under PGE-limited conditions, ROS is significantly induced, particularly in the absence of AOX. A developmental response (twisting phenotype) that requires functional AOX is induced as an ultimate effect in both organellar dysfunction conditions. Whether the developmental response in PGE-limited conditions is induced directly by plastid signals or via mitochondria by AOX-generated signals is still a matter of investigation.

Figure 4

Functions of GUN1 in plastid biology and retrograde control of nuclear gene expression. Left panel: in the dark, by interacting with NEP, MORF2, FUG1, and PRPS1, GUN1 controls plastid gene expression (PGE). GUN1 is also involved in plastid protein import by interacting with HSC70-1 and the TIC/TOC complex. When GUN1 is not present, preproteins and, consequently, cytosolic chaperones (HSP90 and HSP70) are accumulated in the cytosol. The HSP90 is thought to be involved in the induction of the expression of the PhANGs, either by repressing negative or activating positive transcription factors. GUN1 can also bind the haem and the enzymes of the TBP, such as the D-subunit of Mg-chelatase (CHLD) and FC1, and thus may affect the flux through the TBP. Moreover, GUN1-mediated signals might influence the expression of GLK1 and GLK2, which modifies the expression of the PhANGs and the essential TBP genes. Most of the molecular functions of GUN1 were identified using the plastid translation inhibitor lincomycin (LIN) or the inhibitor of the carotenoid biosynthetic pathway, norflurazon (NF). Right panel: in the light, levels of GUN1 proteins are strongly decreased by the action of the plastid CLPC1 protease, leading to the expression induction of PhANGs.

Figure 5

Candidates for retrograde signaling pathways acting during PGE-limited conditions during shoto-morphogenesis. In PGE-limited conditions, distinct plastid metabolites might accumulate and activate the corresponding signaling pathway, as (1) 3′-phosphoadenosine 5′-phosphate (PAP); (2) β-cycloidal (β-cyc), that is the product of the cleavage of β-carotene by singlet oxygen and results in the oxidative signaling, (3) the 2-C-methyl-D-erythosphate (MEcPP) that is required for regulation of auxin-related genes, and (4) malate. It has been proposed that plNAD-MDH reduces NADH excess levels in plastids by converting oxaloacetate (OAA) into malate that is then exported to the cytoplasm to be imported into the mitochondria. In mitochondria, malate is converted back to OAA by mMDH, leading to the production of NADH. NADH can provide electrons to the electron transport chain and, if in excess, can ultimately lead to the production of mitochondrial reactive oxygen species (ROS). ROS can further act as a signaling molecule to modulate nuclear gene expression and, at last, to define the structure of the seedling.