The protein kinase TOUSLED facilitates RNAi in Arabidopsis

RNA silencing is an evolutionarily conserved mechanism triggered by double-stranded RNA that is processed into 21- to 24-nt small interfering (si)RNA or micro (mi)RNA by RNaseIII-like enzymes called Dicers. Gene regulations by RNA silencing have fundamental implications in a large number of biological processes that include antiviral defense, maintenance of genome integrity and the orchestration of cell fates. Although most generic or core components of the various plant small RNA pathways have been likely identified over the past 15 years, factors involved in RNAi regulation through post-translational modifications are just starting to emerge, mostly through forward genetic studies. A genetic screen designed to identify factors required for RNAi in Arabidopsis identified the serine/threonine protein kinase, TOUSLED (TSL). Mutations in TSL affect exogenous and virus-derived siRNA activity in a manner dependent upon its kinase activity. By contrast, despite their pleiotropic developmental phenotype, tsl mutants show no defect in biogenesis or activity of miRNA or endogenous trans-acting siRNA. These data suggest a possible role for TSL phosphorylation in the specific regulation of exogenous and antiviral RNA silencing in Arabidopsis and identify TSL as an intrinsic regulator of RNA interference.     

ABA-activated SnRK2 protein kinase is required for dehydration stress signaling in Arabidopsis

Protein phosphorylation has pivotal roles in ABA and osmotic stress signaling in higher plants. Two protein phosphatase genes, ABI1 and ABI2, are known to regulate these signaling pathways in Arabidopsis: The identity of ABA-activated protein kinases required for the ABA signaling, however, remains to be elucidated. Here we demonstrate that two protein kinases, p44 and p42, were activated by ABA in Arabidopsis T87 cultured cells, and at least one protein kinase, p44, was activated not only by ABA but also by low humidity in Arabidopsis plants. Analysis of T-DNA knockout mutants and biochemical analysis using a specific antibody revealed that the p44 is encoded by a SnRK2-type protein kinase gene, SRK2E. The srk2e mutation resulted in a wilty phenotype mainly due to loss of stomatal closure in response to a rapid humidity decrease. ABA-inducible gene expression of rd22 and rd29B was suppressed in srk2e. These results show that SRK2E plays an important role in ABA signaling in response to water stress.     

An argonaute-like protein is required for meiotic silencing

We demonstrate the involvement of suppressor of meiotic silencing-2 (sms-2(+)), a Neurospora gene coding for an Argonaute-like protein, in meiotic silencing and normal sexual development.     

The Arabidopsis Ethylene-Insensitive 2 gene is required for lead resistance

The Arabidopsis Ethylene-Insensitive 2 (EIN2) gene has been shown to be involved in the regulation of abiotic and biotic stresses, including ozone stress, high salt, oxidative stress and disease resistance. However, little is known about the role of EIN2 gene in lead (Pb) resistance in Arabidopsis. In this study, we showed that EIN2 gene is required for Pb(II) resistance in Arabidopsis. EIN2 gene was induced by Pb(II) treatment, and the ein2-1 mutant showed enhanced sensitivity to Pb(II). A higher Pb content was detected in ein2-1 plants than in wild-type plants when subjected to Pb(II) treatment, which was associated, at least in part, with reduction in expression of AtPDR12 gene, a pump excluding Pb(II) and/or Pb(II)-containing toxic compounds from the cytoplasm. Moreover, the ein2-1 mutation also impaired glutathione (GSH)-dependent Pb(II) resistance, which was related to constitutive reduction of express of GSH1 gene involved in GSH synthesis and consequently reduced GSH content. Taken together, all these results suggest that EIN2 gene mediates Pb(II) resistance, at least in part, through two distinct mechanisms, a GSH-dependent mechanism and a GSH-independent AtPDR12-mediated mechanism.     

The BIG gene is required for auxin-mediated organ growth in Arabidopsis

Control of organ size by cell expansion and cell proliferation is a fundamental process during development, but the importance of BIG in this process is still poorly understood. Here, we report the isolation and characterization of a new allele mutant of BIG in Arabidopsis: big-j588. The mutant displayed small aerial organs that were characterized by reduced cell size in the epidermis and short roots with decreased cell numbers. The big-j588 axr1 double and big-j588 arf7 arf19 triple mutants displayed more severe defects in leaf expansion and root elongation than their parents, implying BIG is involved in auxin-dependent organ growth. Genetic analysis suggests that BIG may act synergistically with PIN1 to affect leaf growth. The PIN1 protein level decreased in both the root cells and the tips of leaf pavement cell lobes of big-j588. Further analysis showed that the auxin maxima in the roots and the leaves of big-j588 decreased. Therefore, we concluded that the small leaves and the short roots of big-j588 were associated with reduction of auxin maxima. Overall, our study suggested that BIG is required for Arabidopsis organ growth via auxin action.     

Arabidopsis MSI1 is required for epigenetic maintenance of reproductive development

WD40 repeat proteins similar to yeast MSI1 are conserved in animals and plants, in which they participate in complexes involved in chromatin metabolism. Although MSI1-like proteins are well characterised biochemically, their function in the development of multicellular eukaryotes is not well understood. We constructed Arabidopsis plants in which the AtMSI1 protein level was altered. Strong ectopic expression of AtMSI1 produced no visible altered phenotype, but reduction of AtMSI1 dramatically affected development. The primary shoot apical meristem was unable to develop organs after the transition to flowering. Flowers that developed on floral shoots from axillary meristems experienced a progressive loss of floral morphology, including a reduction in size of the petals and stamens and the development of carpel-like sepals. Ovule development was disrupted in all flowers, resulting in complete female sterility. Molecular analysis of the mutant plants revealed that AtMSI1 is required to maintain the correct temporal and organ-specific expression of homeotic genes, including AGAMOUS and APETALA2. In contrast, FAS1 and FAS2, which together with AtMSI1 form the chromatin assembly complex CAF-1, are not required for repression of these genes. Therefore, AtMSI1 has specific functions in addition to CAF-1-mediated chromatin assembly. Efficient formation of heterochromatin, but not methylation of centromeric DNA repeats, depends on AtMSI1 presence demonstrating a key role of AtMSI1 in maintenance of chromatin structure.