Interactions between hy1 and gun mutants of Arabidopsis, and their implications for plastid/nuclear signalling

Abstract

SummaryLhcb and other nuclear genes for chloroplastic proteins are regulated by several signals. Among them, light and retrograde signals from the plastid itself appear to act through closely related mechanisms. To investigate this interaction, we analysed an Arabidopsis mutant, hy1, de®cient in plastidic heme oxygenase. hy1 is defective in phytochrome chromophore biosynthesis, which has other indirect effects on tetrapyrrole metabolism. We generated double mutants between hy1-6.2, genetically a null mutation, and three known gun (genome uncoupled) mutants, defective in retrograde plastid signalling. Recent molecular evidence shows GUN5 to be involved in tetrapyrrole metabolism (N. Mochizuki and J. Chory, manuscript in preparation). We observed hy1gun4 to be semi-albino plants, and hy1gun5 albino lethal, in a high-light-sensitive manner. Both double mutants showed defective greening and chloroplast development, and expressed Lhcb at reduced levels speci®cally in high light. Their degree of`genome uncoupling' (Lhcb expression in the absence of functional chloroplasts) was similar to that observed in single mutants. These results can be interpreted as a metabolic (rather than genetic) interaction between HY1 and GUN4 or GUN5, and this in turn supports the involvement of tetrapyrroles as plastid signals. The tetrapyrrole precursor 5-aminolevulinic acid (ALA) inhibited Lhcb expression in hy1. Surprisingly, ALA also rescued photomorphogenesis of hy1. We speculate that either one tetrapyrrole intermediate, which can accumulate anomalously in hy1, or an altered ratio between two intermediates, plays a role as a repressor of Lhcb expression. gun1 did not exacerbate the plastid or Lhcb expression phenotype of hy1. This can be interpreted as a role for gun1 strictly on the same pathway as hy1 or, more likely, as evidence for the existence of at least one separate, non-tetrapyrrole related plastid signal.

Seedlings Lacking the PTM Protein Do Not Show a genomes uncoupled (gun) Mutant Phenotype

“…The other GUNs are all related to the tetrapyrrole pathway (Mochizuki et al, 2001;Larkin et al, 2003;Woodson et al, 2011). Further analysis of these mutants has supported the idea that tetrapyrroles are important for plastid signaling (Vinti et al, 2000;Strand et al, 2003;Moulin et al, 2008;Mochizuki et al, 2008;Voigt et al, 2010), and our current understanding is that the synthesis of heme by ferrochelatase 1 results in a positive signal that promotes expression of nuclear-encoded chloroplast genes (Woodson et al, 2011;Terry and Smith, 2013).…”

GUN4-Porphyrin Complexes Bind the ChlH/GUN5 Subunit of Mg-Chelatase and Promote Chlorophyll Biosynthesis inArabidopsis

“…Indeed, we found that, like the chlorophyll-deficient gun4-1 (Vinti et al, 2000;Mochizuki et al, 2001;Larkin et al, 2003) that causes a Leu-to-Phe (L88F) substitution (Larkin et al, 2003), the F191A-and R211A-expressing lines accumulate 23 to 48% less chlorophyll per mg fresh weight than the wild type when these seedlings were grown for 7 d in 100 mmol m 22 s 21 white light (Figure 2;see Supplemental Figures 1A and 1B online). Under these conditions, gun4-1 accumulated 67% less chlorophyll per mg fresh weight than the wild type (Figure 2; see Supplemental Figures 1A and 1B online), which is consistent with previous analyses of gun4-1 (Vinti et al, 2000;Mochizuki et al, 2001). …”

“…These findings implicated Mg-chelatase, Mg-PPIX biosynthesis, or chloroplastic tetrapyrrole metabolism in the plastid-to-nucleus signaling that regulates photosynthesis-related gene expression (Mochizuki et al, 2001). Data generated using distinct experimental approaches provide evidence that Mg-chelatase, Mg-PPIX biosynthesis, or chloroplastic tetrapyrrole metabolism can affect gene expression in Arabidopsis and other plants (Oster et al, 1996;Papenbrock et al, 2000b;Vinti et al, 2000;La Rocca et al, 2001McCormac and Terry, 2002;Strand et al, 2003;Alawady and Grimm, 2005;Gadjieva et al, 2005;Ankele et al, 2007;Zhang et al, 2011), Chlamydomonas reinhardtii (Johanningmeier and Howell, 1984;Kropat et al, 1997Kropat et al, , 2000Falciatore et al, 2005;Vasileuskaya et al, 2005;von Gromoff et al, 2008), and Synechocystis (Osanai et al, 2009). Mg-PPIX also regulates DNA replication in Cyanidioschyzon merolae and tobacco (Nicotiana tabacum) BY-2 cell cultures (Kobayashi et al, 2009).…”

Light Controls Cytokinin Signaling via Transcriptional Regulation of Constitutively Active Sensor Histidine Kinase CKI1

“…Arabidopsis (Arabidopsis thaliana) seeds of lines ProCKI1:uidA (Hejátko et al, 2003, the phyA-211 mutant (Reed et al, 1994;N6223), the phyA-211 phyB-9 double mutant (Strasser et al, 2010), 35S:CKI1(CKI1 2-2; Hejátko et al, 2009), Col-0 (the background ecotype of all the above lines; N1092), hy1-1 (NW67, background ecotype Ler; Koornneef et al, 1980), hy1-100 [also known as hy6 (cs236); background Col-0, LOS5; Chory et al, 1989] and hy1.62 (background Col-0, PoCA108; Vinti et al, 2000), and ft-1 (Lee et al, 2000) were used. ProCKI1:uidA was introduced to the phyA-211 and hy1-1 by crossing.…”