Search

Under heat stress, polyunsaturated acyl groups, such as α-linolenate (18:3) and hexadecatrienoate (16:3), are removed from chloroplastic glycerolipids in various plant species. Here, we showed that a lipase designated HEAT INDUCIBLE LIPASE1 (HIL1) induces the catabolism of monogalactosyldiacylglycerol (MGDG) under heat stress in Arabidopsis thaliana leaves. Using thermotolerance tests, a T-DNA insertion mutant with disrupted HIL1 was shown to have a heat stress-sensitive phenotype. Lipidomic analysis indicated that the decrease of 34:6-MGDG under heat stress was partially impaired in the hil1 mutant. Concomitantly, the heat-induced increment of 54:9-triacylglycerol in the hil1 mutant was 18% lower than that in the wild-type plants. Recombinant HIL1 protein digested MGDG to produce 18:3-free fatty acid (18:3-FFA), but not 18:0- and 16:0-FFAs. A transient assay using fluorescent fusion proteins confirmed chloroplastic localization of HIL1. Transcriptome coexpression network analysis using public databases demonstrated that the HIL1 homolog expression levels in various terrestrial plants are tightly associated with chloroplastic heat stress responses. Thus, HIL1 encodes a chloroplastic MGDG lipase that releases 18:3-FFA in the first committed step of 34:6 (18:3/16:3)-containing galactolipid turnover, suggesting that HIL1 has an important role in the lipid remodeling process induced by heat stress in plants.

To identify nuclear genes involved in plastid function, we analyzed Arabidopsis thaliana mutants with albino, pale green or variegated leaves using the Activator/Dissociation (Ac/Ds) transposon tagging system. In this study, we focused on mutants with a Ds insertion in the gene At1g32080 (AtLrgB), which encodes a homolog of the bacterial membrane protein LrgB. Although the detailed function of bacterial LrgB remains unclear, it is speculated that LrgB functions against cell death and lysis in cooperation with LrgA. Reverse transcription-PCR (RT-PCR) and promoter-GUS (β-glucuronidase) analyses showed that AtLrgB is expressed in leaves, stems and flowers, but not in roots. Moreover, its expression in leaves continued until senescence. We used three Ac/Ds-tagged mutants (atlrgB) that showed the same phenotypes. During the continuous observation of seedlings under short-day conditions, we found that the cotyledons and true leaves of the mutant plants during early development showed immediate greening, similar to wild-type plants, after which some parts showed a chlorotic phenotype. In contrast, true leaves at the late stage of plant development did not show degreening. When the atlrgB mutant was grown under continuous light, its chlorotic phenotype was suppressed. Transformation with normal AtLrgB restored these phenotypes. Trypan blue staining and electron microscopic observations indicated that chlorotic cell death occurred in the white sectors. The phenotypes of atlrgB resembled those in lesion mimic mutants, suggesting that AtLrgB functions against cell death, similar to the bacterial Lrg system.

We describe here the diversity of chloroplast proteins required for embryo development in Arabidopsis (Arabidopsis thaliana). Interfering with certain chloroplast functions has long been known to result in embryo lethality. What has not been reported before is a comprehensive screen for embryo-defective (emb) mutants altered in chloroplast proteins. From a collection of transposon and T-DNA insertion lines at the RIKEN chloroplast function database (http://rarge.psc.riken.jp/chloroplast/) that initially appeared to lack homozygotes and segregate for defective seeds, we identified 23 additional examples of EMB genes that likely encode chloroplast-localized proteins. Fourteen gene identities were confirmed with allelism tests involving duplicate mutant alleles. We then queried journal publications and the SeedGenes database (www.seedgenes.org) to establish a comprehensive dataset of 381 nuclear genes encoding chloroplast proteins of Arabidopsis associated with embryo-defective (119 genes), plant pigment (121 genes), gametophyte (three genes), and alternate (138 genes) phenotypes. Loci were ranked based on the level of certainty that the gene responsible for the phenotype had been identified and the protein product localized to chloroplasts. Embryo development is frequently arrested when amino acid, vitamin, or nucleotide biosynthesis is disrupted but proceeds when photosynthesis is compromised and when levels of chlorophyll, carotenoids, or terpenoids are reduced. Chloroplast translation is also required for embryo development, with genes encoding chloroplast ribosomal and pentatricopeptide repeat proteins well represented among EMB datasets. The chloroplast accD locus, which is necessary for fatty acid biosynthesis, is essential in Arabidopsis but not in Brassica napus or maize (Zea mays), where duplicated nuclear genes compensate for its absence or loss of function.

The chloroplast NAD(P)H dehydrogenase (NDH) complex functions in PSI cyclic and chlororespiratory electron transport in higher plants. Eleven plastid-encoded and three nuclear-encoded subunits have been identified so far, but the entire subunit composition, especially of the putative electron donor-binding module, is unclear. We isolated Arabidopsis thaliana crr23 (chlororespiratory reduction) mutants lacking NDH activity according to the absence of a transient increase in Chl fluorescence after actinic light illumination. Although CRR23 shows similarity to the NdhL subunit of cyanobacterial NDH-1, it has three transmembrane domains rather than the two in cyanobacterial NdhL. Unlike cyanobacterial NdhL, CRR23 is essential for stabilizing the NDH complex, which in turn is required for the accumulation of CRR23. Furthermore, CRR23 and NdhH, a subunit of chloroplast NDH, co-localized in blue-native gel. All the results indicate that CRR23 is an ortholog of cyanobacterial ndhL in Arabidopsis, despite its diversity of structure and function.

Genomic differences among past Streptococcus pyogenes serotype M3 strains isolated in 1973 and before from patients with streptococcal pharyngitis, recent (1990s) serotype M3 clinical isolates from patients with pharyngitis, and recent M3 isolates from patients with toxic shock-like syndrome were investigated by restriction landmark genomic scanning and by modified random-amplified polymorphic DNA-polymerase chain reaction. Similar polymorphic DNA fragments were identified between the older M3 isolates and the recent isolates; also, the recent M3 clinical isolates from patients with pharyngitis were genetically indistinguishable, by the methods used, from the M3 isolates of patients with toxic shock-like syndrome. Although nucleotide sequences of these regions showed no apparent homology with known virulence factors, the DNA fragments could distinguish the recent M3 strains from the past strains. These results suggested that the recent strains have emerged because of genetic divergence.