Search

SUMMARY: Chloroplast pre‐ribosomal RNA (rRNA) undergoes maturation, which is critical for ribosome assembly. While the central and auxiliary factors in rRNA maturation have been elucidated in bacteria, their mode of action remains largely unexplored in chloroplasts. We now reveal chloroplast‐specific factors involved in 16S rRNA maturation, Arabidopsis thaliana orthologs of bacterial RsmD methyltransferase (AtRsmD) and ribosome maturation factor RimM (AtRimM). A forward genetic screen aimed to find suppressors of the Arabidopsis yellow variegated 2 (var2) mutant defective in photosystem II quality control found a causal nonsense mutation in AtRsmD. The substantially impaired 16S rRNA maturation and translation due to the mutation rescued the leaf variegation phenotype by lowering the levels of chloroplast‐encoded proteins, including photosystem II core proteins, in var2. The subsequent co‐immunoprecipitation coupled with mass spectrometry analyses and bimolecular fluorescence complementation assay found that AtRsmD interacts with AtRimM. Consistent with their interaction, loss of AtRimM also considerably impairs 16S rRNA maturation with decelerated m2G915 modification in 16S rRNA catalyzed by AtRsmD. The atrimM mutation also rescued var2 mutant phenotypes, corroborating the functional interplay between AtRsmD and AtRimM towards modification and maturation of 16S rRNA and chloroplast proteostasis. The maturation and post‐transcriptional modifications of rRNA are critical to assembling ribosomes responsible for protein translation. Here, we revealed that the cooperative regulation of 16S rRNA m2G915 modifications by AtRsmD methyltransferase and ribosome assembly factor AtRimM contributes to 16S rRNA maturation, ribosome assembly, and proteostasis in chloroplasts. Significance Statement: The maturation and post‐transcriptional modifications of ribosomal RNA (rRNA) are critical to assembling ribosomes responsible for protein translation. Here, we revealed that the cooperative regulation of 16S rRNA m2G915 modifications by AtRsmD methyltransferase and ribosome assembly factor AtRimM contributes to 16S rRNA maturation, ribosome assembly, and proteostasis in chloroplasts. [ABSTRACT FROM AUTHOR]

Summary: The photosynthetic bacterial phycobiliprotein lyases, also called CpcT lyases, catalyze the biogenesis of phycobilisome, a light‐harvesting antenna complex, through the covalent attachment of chromophores to the antenna proteins. The Arabidopsis CRUMPLED LEAF (CRL) protein is a homolog of the cyanobacterial CpcT lyase. Loss of CRL leads to multiple lesions, including localized foliar cell death, constitutive expression of stress‐related nuclear genes, abnormal cell cycle, and impaired plastid division. Notwithstanding the apparent phenotypes, the function of CRL still remains elusive. To gain insight into the function of CRL, we examined whether CRL still retains the capacity to bind with the bacterial chromophore phycocyanobilin (PCB) and its plant analog phytochromobilin (PΦB). The revealed structure of the CpcT domain of CRL is comparable to that of the CpcT lyase, despite the low sequence identity. The subsequent in vitro biochemical assays found, as shown for the CpcT lyase, that PCB/PΦB binds to the CRL dimer. However, some mutant forms of CRL, substantially compromised in their bilin‐binding ability, still restore the crl‐induced multiple lesions. These results suggest that although CRL retains the bilin‐binding pocket, it seems not functionally associated with the crl‐induced multiple lesions. Significance Statement: Our study revealed that the Arabidopsis CRUMPLED LEAF (CRL) protein retains a bilin‐binding pocket similar to its homolog cyanobacterial bilin lyase. The subsequent reverse genetic approach establishes that the retained bilin‐binding aptitude is not essential for CRL function in Arabidopsis, assuring that CRL evolved with a different function. [ABSTRACT FROM AUTHOR]

Necrotrophic fungal pathogens produce toxic compounds that induce cell death in infected plants. Often, the primary targets of these toxins and the way a plant responds to them are not known. In the present work, the effect of tenuazonic acid ( TeA), a non-host-specific toxin of A lternaria alternata, on A rabidopsis thaliana has been analysed. TeA blocks the QB-binding site at the acceptor side of photosystem II ( PSII). As a result, charge recombination at the reaction centre ( RC) of PSII is expected to enhance the formation of the excited triplet state of the RC chlorophyll that promotes generation of singlet oxygen (1 O 2). 1 O 2 activates a signalling pathway that depends on the two EXECUTER ( EX) proteins EX1 and EX2 and triggers a programmed cell death response. In seedlings treated with TeA at half-inhibition concentration 1 O 2-mediated and EX-dependent signalling is activated as indicated by the rapid and transient up-regulation of 1 O 2-responsive genes in wild type, and its suppression in ex 1/ ex 2 mutants. Lesion formation occurs when seedlings are exposed to higher concentrations of TeA for a longer period of time. Under these conditions, the programmed cell death response triggered by 1 O 2-mediated and EX-dependent signalling is superimposed by other events that also contribute to lesion formation. [ABSTRACT FROM AUTHOR]

Summary Retrograde plastid-to-nucleus signaling tightly controls and coordinates the nuclear and plastid gene expression that is required for plastid biogenesis and chloroplast activity. As chloroplasts act as sensors of environmental changes, plastid-derived signaling also modulates stress responses of plants by transferring stress-related signals and altering nuclear gene expression. Various mutant screens have been undertaken to identify constituents of plastid signaling pathways. Almost all mutations identified in these screens target plastid-specific but not extraplastidic functions. They have been suggested to define either genuine constituents of retrograde signaling pathways or components required for the synthesis of plastid signals. Here we report the characterization of the constitutive activator of AAA-ATPase ( caa33) mutant, which reveals another way of how mutations that affect plastid functions may modulate retrograde plastid signaling. caa33 disturbs a plastid-specific function by impeding plastid division, and thereby perturbing plastid homeostasis. This results in preconditioning plants by activating the expression of stress genes, enhancing pathogen resistance and attenuating the capacity of the plant to respond to plastid signals. Our study reveals an intimate link between chloroplast activity and the susceptibility of the plant to stress, and emphasizes the need to consider the possible impact of preconditioning on retrograde plastid-to-nucleus signaling. [ABSTRACT FROM AUTHOR]

The conditional flu mutant of Arabidopsis thaliana generates singlet oxygen (1O2) in plastids during a dark-to-light shift. Seedlings of flu bleach and die, whereas mature plants stop growing and develop macroscopic necrotic lesions. Several suppressor mutants, dubbed singlet oxygen-linked death activator ( soldat), were identified that abrogate 1O2-mediated cell death of flu seedlings. One of the soldat mutations, soldat10, affects a gene encoding a plastid-localized protein related to the human mitochondrial transcription termination factor mTERF. As a consequence of this mutation, plastid-specific rRNA levels decrease and protein synthesis in plastids of soldat10 is attenuated. This disruption of chloroplast homeostasis in soldat10 seedlings affects communication between chloroplasts and the nucleus and leads to changes in the steady-state concentration of nuclear gene transcripts. The soldat10 seedlings suffer from mild photo-oxidative stress, as indicated by the constitutive up-regulation of stress-related genes. Even though soldat10/ flu seedlings overaccumulate the photosensitizer protochlorophyllide in the dark and activate the expression of 1O2-responsive genes after a dark-to-light shift they do not show a 1O2-dependent cell death response. Disturbance of chloroplast homeostasis in emerging soldat10/ flu seedlings seems to antagonize a subsequent 1O2-mediated cell death response without suppressing 1O2-dependent retrograde signaling. The results of this work reveal the unexpected complexity of what is commonly referred to as ‘plastid signaling’. [ABSTRACT FROM AUTHOR]

Plants under oxidative stress suffer from damages that havebeen interpreted as unavoidable consequences of injuriesinflicted upon plants by toxic levels of reactive oxygen species(ROS). However, this paradigm needs to be modified. Inactivationof a single gene, EXECUTER1, is sufficient to abrogate stressresponses of Arabidopsis thaliana caused by the release ofsinglet oxygen: External conditions under which these stressresponses are observed and the amounts of ROS that accumulate inplants exposed to these environmental conditions do not directlycause damages. Instead, seedling lethality and growth inhibitionof mature plants result from genetic programs that are activatedafter the release of singlet oxygen has been perceived by theplant. [ABSTRACT FROM AUTHOR]