The M flax rust resistance pre-mRNA is alternatively spliced and contains a complex upstream untranslated region

Transcript-level expression control of plant NLR genes

Plant NLR genes encode sensitive immune receptors that can mediate the specific recognition of pathogen avirulence effectors and activate a strong defence response, termed effector-triggered immunity. The expression of NLRs requires strict regulation, as their ability to trigger immunity is dependent on their dose, and overexpression of NLRs results in autoimmunity and massive fitness costs. An elaborate interplay of different mechanisms controlling NLR transcript levels allows plants to maximize their defence capacity, whilst limiting negative impact on their fitness. Global suppression of NLR transcripts may be a prerequisite for the fast evolution of new NLR variants and the expansion of this gene family. Here, we summarize recent progress made towards a comprehensive understanding of NLR transcript-level expression control. Multiple mechanistic steps, including transcription as well as co-/post-transcriptional processing and transcript turn-over, contribute to balanced base levels of NLR transcripts and allow for dynamic adjustments to defence situations.

Molecular characterization of the CRa gene conferring clubroot resistance in Brassica rapa

Clubroot disease is one of the major diseases affecting Brassicaceae crops, and a number of these crops grown commercially, such as Chinese cabbage (Brassica rapa L. ssp. pekinensis), are known to be highly susceptible to clubroot disease. To provide protection from this disease, plant breeders have introduced genes for resistance to clubroot from the European turnip into susceptible lines. The CRa gene confers specific resistance to the clubroot pathogen Plasmodiophora brassicae isolate M85. Fine mapping of the CRa locus using synteny to the Arabidopsis thaliana genome and partial genome sequences of B. rapa revealed a candidate gene encoding a TIR-NBS-LRR protein. Several structural differences in this candidate gene were found between susceptible and resistant lines, and CRa expression was observed only in the resistant line. Four mutant lines lacking clubroot resistance were obtained by the UV irradiation of pollen from a resistant line, and all of these mutant lines carried independent mutations in the candidate TIR-NBS-LRR gene. This genetic and molecular evidence strongly suggests that the identified gene is CRa. This is the first report on the molecular characterization of a clubroot Resistance gene in Brassicaceae and of the disease resistance gene in B. rapa.

The cyclin L homolog MOS12 and the MOS4-associated complex are required for the proper splicing of plant resistance genes

Plant resistance (R) proteins protect cells from infections through recognizing effector molecules produced by pathogens and initiating downstream defense cascades. To mount proper immune responses, the expression of R genes has to be tightly controlled transcriptionally and post-transcriptionally. Intriguingly, alternative splicing of the R genes of the nucleotide binding leucine-rich repeat (NB-LRR) type was observed in different plant species, but its regulatory mechanism remains elusive. Here, we report the positional cloning and functional analysis of modifier of snc1,12 (mos12-1), a partial loss-of-function mutant that can suppress the constitutive defense responses conferred by the gain-of-function R gene mutant suppressor of npr1-1 constitutive 1 (snc1). MOS12 encodes an arginine-rich protein that is homologous to human cyclin L. A null allele of mos12-2 is lethal, suggesting it has a vital role in plant growth and development. MOS12 localizes to the nucleus, and the mos12-1 mutation results in altered splicing patterns of SNC1 and RPS4, indicating that MOS12 is required for the proper splicing of target R genes. MOS12 co-immunoprecipitates with MOS4, indicating that MOS12 associates with the MOS4-associated complex (MAC). Accordingly, splicing patterns of SNC1 and RPS4 are changed in most MAC core mutants. Our study highlights the contribution of MOS12 and the MAC in the alternative splicing of R genes, providing regulatory details on how alternative splicing is used to fine-tune R gene expression in plant immunity.

Organ-specific alternatively spliced transcript isoforms of the sunflower SF21C gene

The sunflower sf21C gene is a member of a small plant gene family related to the human N-myc downstream-regulated gene family (NDRG) involved in stress and hormone responses, cell growth and differentiation. We have shown earlier that this sunflower gene is interrupted by nine introns and generates multiple mRNAs by alternative splicing of its primary transcript (Lazarescu et al. in Gene 374:77-86, 2006). In the present study, we have characterized 20 splice isoforms of this gene encoding 14 different protein variants. We show that these isoforms occur in distinct combinations in the various plant organs. Five identified variants were generated by splicing at novel sites, different from those classically recognized by U2- or U12-type spliceosomes. Twelve transcript variants contain premature termination codons (PTCs), which make them potential candidates for nonsense-mediated RNA degradation (NMD). Interestingly, all 12 transcripts are relatively abundant in at least one organ, suggesting that NMD is not a "default" mechanism for PTC-containing RNA transcripts generated by alternative splicing. These data indicate that alternative splicing of the sunflower sf21C primary transcript as well as the mRNA quality control mechanism by NMD are complex and regulated spatially and developmentally.

Functional identification and regulation of the PtDrl02 gene promoter from triploid white poplar

The PtDrl02 gene belongs to the TIR-NBS gene family in triploid white poplar (Populus tomentosa x P. bolleana) x P. tomentosa. Its expression pattern displays tissue-specificity, and the transcript level can be induced by wounding, methyl jasmonate (MeJA), and salicylic acid (SA). To understand the regulatory mechanism controlling PtDrl02 gene expression, we functionally characterized the PtDrl02 promoter region. Using the beta-glucuronidase as a reporter, we found that the PtDrl02 promoter directed gene expression mainly in the aerial parts of the plants and was confined to the cortex tissues of leaf veins, petioles, stems, and stem piths, showing a typical tissue-specific expression pattern. Deletion analysis revealed two positive regulatory regions (-985 to -669 and -669 to -467) responsible for the basal activity of the PtDrl02 promoter. Impressively, the sequence from -669 to -467 was shown to contain cis-element (s) responding to wounding and MeJA, while the promoter region between -244 and 0 could individually display wounding-responsiveness, and the fragment from -467 to -244 was required for SA- and NaCl-inducible expression of the PtDrl02 promoter. Additionally, it was found that the -985 to -669 sequence was the ABA-responding promoter fragment. These results suggested that the PtDrl02 promoter was modulated by multiple cis-regulatory elements in distinct and complex patterns to regulate PtDrl02 gene expression. Our study also suggested that the PtDrl02 gene 5' untranslated region, as well as a Populus WRKY transcription factor, PtWRKY1, was involved in the regulation of PtDrl02 promoter activities.

Functional analysis of 5' untranslated region of a TIR-NBS-encoding gene from triploid white poplar

Genome-wide analyses have identified a set of TIR-NBS-encoding genes in plants. However, the molecular mechanism underlying the expression of these genes is still unknown. In this study, we presented a TIR-NBS-encoding gene, PtDrl02, that displayed a low level of tissue-specific expression in a triploid white poplar [(Populus tomentosa x P. bolleana) x P. tomentosa], and analyzed the effects of the 5' untranslated region (UTR) on gene expression. The 5' UTR sequence repressed the reporter activity of beta-glucuronidase (GUS) gene under PtDrl02 promoter by 113.5-fold with a staining ratio of 2.97% in the transgenic tobacco plants. Quantitative RT-PCR assays revealed that the 5' UTR sequence decreased the transcript level of the GUS reporter gene by 13.3-fold, implying a regulatory role of 5' UTR in transcription and/or mRNA destabilization. The comparison of GUS activity with the transcript abundance indicated that the 5' UTR sequence decreased the translation efficiency of target gene by 88.3%. Additionally, the analysis of the transgenic P-985/UTRDelta/GUS plants showed that both the exon1 sequence and the leading intron within the 5' UTR region were responsible for the regulation of gene expression. Our results suggested a negative effect of the 5' UTR of PtDrl02 gene on gene expression.

Plant NB-LRR immune receptors: from recognition to transcriptional reprogramming

Both plants and animals contain nucleotide-binding domain and leucine-rich repeat (NB-LRR)-type immune receptors that function during defense against pathogens. Unlike animal NB-LRRs that recognize general pathogen or microbe-associated molecular patterns (PAMPs or MAMPs), plant NB-LRR immune receptors have evolved the ability to specifically recognize a wide range of effector proteins from different pathogens. Recent research has revealed that plant NB-LRRs are incredibly adaptive in their ways of pathogen recognition and defense initiation. This review focuses on the remarkable variety of functions, recognition mechanisms, subcellular localizations, and host factors associated with plant NB-LRR immune receptors.