Absence of SICKLE triggers programed cell death by disturbing alternative splicing and decay of mRNAs.

Programed cell death (PCD) plays fundamental roles in plant development and responses to environmental stresses. Here, we report a protein, SICKLE (SIC), which represses PCD. In Arabidopsis (Arabidopsis thaliana), the loss-of-function mutant of SIC, sic-4, hyperaccumulated lariat intronic RNAs (lariRNAs) and exhibited PCD. The gene encoding an RNA debranching enzyme 1 (DBR1), a rate-limiting enzyme for lariRNAs decay, was overexpressed to reduce the level of lariRNAs in the sic-4 mutant, which led to suppression of PCD. Meanwhile, another lariRNAs hyper-accumulating mutant, dbr1-2, also exhibited PCD, further indicating that sic-4 PCD is caused by hyper-accumulation of lariRNAs. Transcriptional profiling analyses revealed that the sic-4 mutation disturbed alternative splicing and decay of mRNAs associated with salicylic acid (SA) homeostasis, a well-known molecule functioning in PCD regulation. Moreover, SA is dramatically increased in sic-4 and the disruption of SA biosynthesis and signaling suppressed PCD in the mutant, demonstrating that SA functions downstream of sic-4. Taken together, our results demonstrate that SIC is involved in regulating SA-triggered PCD.

SICKLE modulates lateral root development by promoting degradation of lariat intronic RNA.

Plant lateral roots (LRs) play vital roles in anchorage and uptake of water and nutrients. Here, we reveal that degradation of lariat intronic RNAs (lariRNAs) modulated by SICKLE (SIC) is required for LR development in Arabidopsis (Arabidopsis thaliana). Loss of SIC results in hyper-accumulation of lariRNAs and restricts the outgrowth of LR primordia, thereby reducing the number of emerged LRs. Decreasing accumulation of lariRNAs by over-expressing RNA debranching enzyme 1 (DBR1), a rate-limiting enzyme of lariRNA decay, restored LR defects in SIC-deficient plants. Mechanistically, SIC interacts with DBR1 and facilitates its nuclear accumulation, which is achieved through two functionally redundant regions (SIC1-244 and SIC252-319) for nuclear localization. Of the remaining amino acids in this region, six (SIC245-251) comprise a DBR1-interacting region while two (SICM246 and SICW251) are essential for DBR1-SIC interaction. Reducing lariRNAs restored microRNA (miRNA) levels and LR development in lariRNA hyper-accumulating plants, suggesting that these well-known regulators of LR development mainly function downstream of lariRNAs. Taken together, we propose that SIC acts as an enhancer of DBR1 nuclear accumulation by driving nuclear localization through direct interaction, thereby promoting lariRNA decay to fine-tune miRNA biogenesis and modulating LR development.

Overexpression of AHL9 accelerates leaf senescence in Arabidopsis thaliana.

BACKGROUND: Leaf senescence, the final stage of leaf growth and development, is regulated by numerous internal factors and environmental cues. Ethylene is one of the key senescence related hormones, but the underlying molecular mechanism of ethylene-induced leaf senescence remains poorly understood. RESULTS: In this study, we identified one AT-hook like (AHL) protein, AHL9, as a positive regulator of leaf senescence in Arabidopsis thaliana. Overexpression of AHL9 significantly accelerates age-related leaf senescence and promotes dark-induced leaf chlorosis. The early senescence phenotype observed in AHL9 overexpressing lines is inhibited by the ethylene biosynthesis inhibitor aminooxyacetic acid suggesting the involvement of ethylene in the AHL9-associated senescence. RNA-seq and quantitative reverse transcription PCR (qRT-PCR) data identified numerous senescence-associated genes differentially expressed in leaves of AHL9 overexpressing transgenic plants. CONCLUSIONS: Our investigation demonstrates that AHL9 functions in accelerating the leaf senescence process via ethylene synthesis or signalling.

HY5 Contributes to Light-Regulated Root System Architecture Under a Root-Covered Culture System.

Light is essential for plant organogenesis and development. Light-regulated shoot morphogenesis has been extensively studied; however, the mechanisms by which plant roots perceive and respond to aboveground light are largely unknown, particularly because the roots of most terrestrial plants are usually located underground in darkness. To mimic natural root growth conditions, we developed a root-covered system (RCS) in which the shoots were illuminated and the plant roots could be either exposed to light or cultivated in darkness. Using the RCS, we observed that root growth of wild-type plants was significantly promoted when the roots were in darkness, whereas it was inhibited by direct light exposure. This growth change seems to be regulated by ELONGATED HYPOCOTYL 5 (HY5), a master regulator of photomorphogenesis. Light was found to regulate HY5 expression in the roots, while a HY5 deficiency partially abolished the inhibition of growth in roots directly exposed to light, suggesting that HY5 expression is induced by direct light exposure and inhibits root growth. However, no differences in HY5 expression were observed between illuminated and dark-grown cop1 roots, indicating that HY5 may be regulated by COP1-mediated proteasome degradation. We confirmed the crucial role of HY5 in regulating root development in response to light under soil-grown conditions. A transcriptomic analysis revealed that light controls the expression of numerous genes involved in phytohormone signaling, stress adaptation, and metabolic processes in a HY5-dependent manner. In combination with the results of the flavonol quantification and exogenous quercetin application, these findings suggested that HY5 regulates the root response to light through a complex network that integrates flavonol biosynthesis and reactive oxygen species signaling. Collectively, our results indicate that HY5 is a master regulator of root photomorphogenesis.

Jacalin domain in wheat jasmonate-regulated protein Ta-JA1 confers agglutinating activity and pathogen resistance.

Ta-JA1 is a jacalin-like lectin from wheat (Triticum aestivum) plants. To date, its homologs are only observed in the Gramineae family. Our previous experiments have demonstrated that Ta-JA1 contains a modular structure consisting of an N-terminal dirigent domain and a C-terminal jacalin-related lectin domain (JRL) and this protein exhibits a mannose-specific lectin activity. The over-expression of Ta-JA1 gene provides transgenic plants a broad-spectrum resistance to diseases. Here, we report the differential activities of the dirigent and JRL domains of Ta-JA1. In vitro assay showed that the recombinant JRL domain could effectively agglutinate rabbit erythrocytes and pathogen bacteria Pseudomonas syringe pv tabaci. These hemagglutination activities could be inhibited by mannose but not by galactose. In contrast, the recombinant dirigent domain did not show agglutination activity. Corresponding to these differentiations of activities, similar to full-length of Ta-JA1, the over-expression of JRL domain in transgenic plants also increased resistance to the infection of P. syringe. Unlike JRL, the over-expression of dirigent domain in transgenic plants led to alteration of the seedling sensitivity to salts. In addition, a d(N)/d(S) ratio analysis of Ta-JA1 and its related proteins showed that this protein family functionally limited to a few crop plants, such as maize, rice and wheat.