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]

Oxidative post-translational modifications of specific chloroplast proteins contribute to the initiation of retrograde signaling. The Arabidopsis thaliana EXECUTER1 (EX1) protein, a chloroplast-localized singlet oxygen (1O 2) sensor, undergoes tryptophan (Trp) 643 oxidation by 1O 2 , a chloroplast-derived and light-dependent reactive oxygen species. The indole side chain of Trp is vulnerable to 1O 2 , leading to the generation of oxidized Trp variants and priming EX1 for degradation by a membrane-bound FtsH protease. The perception of 1O 2 via Trp643 oxidation and subsequent EX1 proteolysis facilitate chloroplast-to-nucleus retrograde signaling. In this study, we discovered that the EX1-like protein EX2 also undergoes 1O 2 -dependent Trp530 oxidation and FtsH-dependent turnover, which attenuates 1O 2 signaling by decelerating EX1-Trp643 oxidation and subsequent EX1 degradation. Consistent with this finding, the loss of EX2 function reinforces EX1-dependent retrograde signaling by accelerating EX1-Trp643 oxidation and subsequent EX1 proteolysis, whereas overexpression of EX2 produces molecular phenotypes opposite to those observed in the loss–of- function mutants of EX2. Intriguingly, phylogenetic analysis suggests that EX2 may have emerged evolutionarily to attenuate the sensitivity of EX1 toward 1O 2. Collectively, these results suggest that EX2 functions as a negative regulator of the EX1 signalosome through its own 1O 2 -dependent oxidation, providing a new mechanistic insight into the regulation of EX1-mediated 1O 2 signaling. EXECUTER1 (EX1) and EX2 proteins have long been considered major players in mediating singlet oxygen (1O 2)-triggered chloroplast-to-nucleus retrograde signaling, but the specific mechanisms by which they regulate 1O 2 signaling have been unclear. This study demonstrates that the 1O 2 -driven oxidative modification of EX2 modulates EX1-mediated 1O 2 signaling by hindering the oxidation of EX1 that is essential for the initiation of 1O 2 signaling. [ABSTRACT FROM AUTHOR]

Apple is known to be susceptible to various virus and viroid pathogens. Symptomatic apple cultivars and rootstocks were collected and analyzed by ELISA and then through RT-PCR. The study reports the presence of Apple mosaic virus (ApMV), Apple stem grooving virus (ASGV), Apple stem pitting virus (ASPV), Apple chlorotic leaf spot virus (ACLSV), the major apple viruses and Prunus necrotic ringspot virus (PNRSV), a minor apple virus, at the molecular level in India. Apple scar skin viroid (ASSVd) infection was also confirmed at the molecular level. Sporadic incidences of Tomato ringspot virus and Arabis mosaic virus infections were also detected by ELISA in nursery plants. [ABSTRACT FROM AUTHOR]

Lignin biosynthesis enzymes form complexes for metabolic channelling during lignification and these enzymes also play an essential role in biotic and abiotic stress response. Cinnamyl alcohol dehydrogenase (CAD) is a vital enzyme that catalyses the reduction of aldehydes to alcohols, which is the final step in the lignin biosynthesis pathway. In the present study, we identified 49 CAD enzymes in five Bambusoideae species and analysed their phylogenetic relationships and conserved domains. Expression analysis of Moso bamboo PheCAD genes in several developmental tissues and stages revealed that among the PheCAD genes, PheCAD2 has the highest expression level and is expressed in many tissues and PheCAD1, PheCAD6, PheCAD8 and PheCAD12 were also expressed in most of the tissues studied. Co-expression analysis identified that the PheCAD2 positively correlates with most lignin biosynthesis enzymes, indicating that PheCAD2 might be the key enzyme involved in lignin biosynthesis. Further, more than 35% of the co-expressed genes with PheCADs were involved in biotic or abiotic stress responses. Abiotic stress transcriptomic data (SA, ABA, drought, and salt) analysis identified that PheCAD2, PheCAD3 and PheCAD5 genes were highly upregulated, confirming their involvement in abiotic stress response. Through yeast two-hybrid analysis, we found that PheCAD1, PheCAD2 and PheCAD8 form homo-dimers. Interestingly, BiFC and pull-down experiments identified that these enzymes form both homo- and hetero- dimers. These data suggest that PheCAD genes are involved in abiotic stress response and PheCAD2 might be a key lignin biosynthesis pathway enzyme. Moreover, this is the first report to show that three PheCAD enzymes form complexes and that the formation of PheCAD homo- and hetero- dimers might be tissue specific. [ABSTRACT FROM AUTHOR]