The ultraviolet-B (UV-B) portion of the solar radiation functions as an environmental signal for which plants have evolved specific and sensitive UV-B perception systems. The UV-B-specific UV RESPONSE LOCUS 8 (UVR8) and the multifunctional E3 ubiquitin ligase CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) are key regulators of the UV-B response. We show here that uvr8-null mutants are deficient in UV-B-induced photomorphogenesis and hypersensitive to UV-B stress, whereas overexpression of UVR8 results in enhanced UV-B photomorphogenesis, acclimation and tolerance to UV-B stress. By using sun simulators, we provide evidence at the physiological level that UV-B acclimation mediated by the UV-B-specific photoregulatory pathway is indeed required for survival in sunlight. At the molecular level, we demonstrate that the wild type but not the mutant UVR8 and COP1 proteins directly interact in a UV-B-dependent, rapid manner in planta. These data collectively suggest that UV-B-specific interaction of COP1 and UVR8 in the nucleus is a very early step in signalling and responsible for the plant's coordinated response to UV-B ensuring UV-B acclimation and protection in the natural environment.

Plants fend off potentially damaging ultraviolet (UV)-

Methylations at position N of internal adenosines (mAs) are the most abundant and widespread mRNA modifications. These modifications play crucial roles in reproduction, growth, and development by controlling gene expression patterns at the posttranscriptional level. Their function is decoded by readers that share the YTH domain, which forms a hydrophobic pocket that directly accommodates the mA residues. While the physiological and molecular functions of YTH readers have been extensively studied in animals, little is known about plant readers, even though mAs are crucial for plant survival and development. Viridiplantae contains high numbers of YTH domain proteins. Here, we performed comprehensive evolutionary analysis of YTH domain proteins and demonstrated that they are highly likely to be actual readers with redundant as well as specific functions. We also show that the ECT2 protein from binds to mA-containing RNAs in vivo and that this property relies on the mA binding pocket carried by its YTH domain. ECT2 is cytoplasmic and relocates to stress granules upon heat exposure, suggesting that it controls mRNA fate in the cytosol. Finally, we demonstrate that ECT2 acts to decode the mA signal in the trichome and is required for their normal branching through controlling their ploidy levels.

To survive adverse and ever-changing environmental conditions, an organism must be able to adapt. It has long been established that the cellular reaction to stress includes the upregulation of genes coding for specific stress-responsive factors. In the present study, we demonstrate that during the early steps of the heat stress response, 25% of the Arabidopsis seedling transcriptome is targeted for rapid degradation. Our findings demonstrate that this process is catalyzed from 5' to 3' by the cytoplasmic exoribonuclease XRN4, whose function is seemingly reprogrammed by the heat-sensing pathway. The bulk of mRNAs subject to heat-dependent degradation are likely to include both the ribosome-released and polysome associated polyadenylated pools. The cotranslational decay process is facilitated at least in part by LARP1, a heat-specific cofactor of XRN4 required for its targeting to polysomes. Commensurate with their respective involvement at the molecular level, LARP1 and XRN4 are necessary for the thermotolerance of plants to long exposure to moderately high temperature, with xrn4 null mutants being almost unable to survive. These findings provide mechanistic insights regarding a massive stress-induced posttranscriptional downregulation and outline a potentially crucial pathway for plant survival and acclimation to heat stress.

The reprogramming of gene expression in heat stress is a key determinant to organism survival. Gene expression is downregulated through translation initiation inhibition and release of free mRNPs that are rapidly degraded or stored. In mammals, heat also triggers 5′-ribosome pausing preferentially on transcripts coding for HSC/HSP70 chaperone targets, but the impact of such phenomenon on mRNA fate remains unknown. Here, we provide evidence that, in Arabidopsis thaliana, heat provokes 5′-ribosome pausing leading to the XRN4-mediated 5′-directed decay of translating mRNAs. We also show that hindering HSC/HSP70 activity at 20°C recapitulates heat effects by inducing ribosome pausing and co-translational mRNA turnover. Strikingly, co-translational decay targets encode proteins with high HSC/HSP70 binding scores and hydrophobic N-termini, two characteristics that were previously observed for transcripts most prone to pausing in animals. This work suggests for the first time that stress-induced variation of translation elongation rate is an evolutionarily conserved process leading to the polysomal degradation of thousands of ‘non-aberrant’ mRNAs.

Heat shock (HS) is known to have a profound impact on gene expression at different levels, such as inhibition of protein synthesis, in which HS blocks translation initiation and induces the sequestration of mRNAs into stress granules (SGs) or P-bodies for storage and/or decay. SGs prevent the degradation of the stored mRNAs, which can be reengaged into translation in the recovery period. However, little is known on the mRNAs stored during the stress, how these mRNAs are released from SGs afterward, and what the functional importance is of this process. In this work, we report that Arabidopsis HEAT SHOCK PROTEIN101 (HSP101) knockout mutant (hsp101) presented a defect in translation recovery and SG dissociation after HS. Using RNA sequencing and RNA immunoprecipitation approaches, we show that mRNAs encoding ribosomal proteins (RPs) were preferentially stored during HS and that these mRNAs were released and translated in an HSP101-dependent manner during recovery. By 15 N incorporation and polysome profile analyses, we observed that these released mRNAs contributed to the production of new ribosomes to enhance translation. We propose that, after HS, HSP101 is required for the efficient release of RP mRNAs from SGs resulting in a rapid restoration of the translation machinery by producing new RPs.

The complexity of the plastid transcriptional apparatus (two or three different RNA polymerases and numerous regulatory proteins) makes it very difficult to attribute specific function(s) to its individual components. We have characterized an Arabidopsis T-DNA insertion line disrupting the nuclear gene coding for one of the six plastid sigma factors (SIG4) that regulate the activity of the plastid-encoded RNA polymerase PEP. This mutant shows a specific diminution of transcription of the plastid ndhF gene, coding for a subunit of the plastid NDH [NAD(P)H dehydrogenase] complex. The absence of another NDH subunit, i.e. NDHH, and the absence of a chlorophyll fluorescence transient previously attributed to the activity of the plastid NDH complex indicate a strong down-regulation of NDH activity in the mutant plants. Results suggest that plastid NDH activity is regulated on the transcriptional level by an ndhF-specific plastid sigma factor, SIG4.

The plastid genome is transcribed by nucleus-encoded (NEP) and plastid-encoded (PEP) RNA polymerases. PEP is a prokaryotic-type enzyme whose activity is regulated by sigma-like transcription initiation factors that are nucleus-encoded. cDNAs coding for six different potential a-like factors have been cloned and sequenced recently. However, functional analyses of these factors are still limited. We have used an anti-sense approach in order to study the function of SIG1, SIG2 and SIG3. Only SIG2 anti-sense plants show a visible phenotype characterized by chlorophyll deficiency. Surprisingly, this phenotype is different from the phenotype of SIG2 knockout plants in that the chlorophyll deficiency is limited to cotyledons. In later developmental stages, the SIG2 anti-sense plants can overcome SIG2 mRNA under-expression by adjusting SIG2 protein levels to that of wild-type plants, suggesting that SIG2 expression is also regulated at the post-transcriptional level. The efficient recovery of the wild-type phenotype could also be supported by partial take-over of SIG2 function by one of the six other sigma factors. A good candidate for such substitution of SIG2 function represents SIG3. SIG3 is constitutively expressed during plant development and its specificity in promoter discrimination is less pronounced than that of SIG1 and SIG2. Finally, SIG3 protein is enhanced in SIG2 anti-sense plants when compared to wild-type plants. SIG2 is present as a soluble factor while SIG3 is partly attached to the plastid membranes. We suggest that membrane localization is necessary for efficient SIG3 function. Therefore, SIG3 cannot substitute for SIG2 function in early chloroplast biogenesis, when plastid membranes are not yet made up.