Unravelling PTGS: SDE3 – an RNA helicase involved in RNA silencing in Arabidopsis

Unravelling PTGS: SDE3 – an RNA helicase involved in RNA silencing in Arabidopsis

News & Comment TRENDS in Plant Science Vol.6 No.8 August 2001 and HCf-2, either individually or in the double-knockout strain, had no obvious effect...

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News & Comment

TRENDS in Plant Science Vol.6 No.8 August 2001

and HCf-2, either individually or in the double-knockout strain, had no obvious effect on the ability of the fungus to grow on agar plates or in liquid culture: there was no difference in the frequency and speed of conidium germination, and HCf-1 and HCf-2 genes were not necessary for the development of hyphae or conidia. A study of conidium wettability showed that whereas deletion of HCf-2 alone had no effect, deletion of both hydrophobin genes caused a strong reduction in hydrophobicity, and rodlets were missing

from the surfaces of mutant strains of HCf1 and HCf and HCf2 strains but not HCf2 mutants. Whiteford and Spanu concluded that HCf-1 is one of the main determinants of conidium hydrophobicity, but that other Class I hydrophobins (HCf-3 and HCf-4) also play a significant role. The HCf-1 hydrophobin appears to be a principal component of rodlets on the conidium surface, probably the main structure in the outer conidial wall that contributes to surface hydrophobicity and the ability of conidia to be collected by


water. Because conidia of C. fulvum are not disseminated efficiently by air currents, HCf-1 has an important function in aiding the spread of the fungus by water droplets from infected plants to new hosts. 1 Whiteford, J.R. and Spanu, P.D. (2001) The hydrophobin HCf-1 of Cladosporium fulvum is required for efficient water-mediated dispersal of conidia. Fungal Genet. Biol. 32, 159–168

Richard C. Staples [email protected]

Luc-imaging the cold signal transduction pathway Plants adapt to ambient temperatures by adjusting membrane fluidity, metabolism and gene expression profiles. Low temperature is one of the most important environmental factors influencing plant distribution, growth development and survival. Exposure to low temperatures can increase freezing tolerance in plants from temperate regions, a process known as cold acclimation. Low temperature-induced gene expression is key to cold acclimation. Hojoung Lee and colleagues have been studying the cold transduction pathway in Arabidopsis. In previous work, the group constructed Arabidopsis mutant lines displaying bioluminescence in response to low temperature, drought, salinity and the phytohormone abscisic acid. This was achieved by transfecting Arabidopsis plants with a chimeric gene construct consisting of the firefly luciferase coding sequence (LUC) under the control of the stress-responsive RD29A promoter. Many mutants were identified using a high-throughput luminescence imaging system. One of

these mutants, HOS1 (high expression of osmotically responsive genes) is described in a new study by this group1. ‘HOS1 is the first example of cold-induced nucleo-cytoplasmic partitioning of proteins…’ The hos1 gene encodes a novel protein with a RING-finger motif near the amino terminus. The HOS1 protein is ubiquitously expressed in all plant tissues. The current research has identified HOS1 as a key negative regulator of low temperature-responsive gene transcription. HOS1 can be regulated by activating hos1 using a cold cascade. HOS1–green fluorescent protein translational fusion studies reveal that the HOS1 protein localizes to the cytoplasm at normal growth temperatures. However, in response to low temperature, HOS1 accumulates in the nucleus. HOS1 is the first example of cold-induced nucleo-cytoplasmic partitioning of proteins, providing a connection between cold-triggered

cytoplasmic signalling and nuclear gene transcription. These data suggest a new scenario where the stability of crucial regulatory proteins in the cells is dynamically controlled in response to environmental or developmental stimuli. Arabidopsis has multiple cold signalling pathways and therefore the range of responses involving HOS1 needs to be determined. Can HOS1 serve as E3 ubiquitin ligase? What type of proteins might be specifically targeted by HOS1 for degradation? The identification of a new key negative regulator that has the potential to modulate many of the coldresponsive genes of Arabidopsis promises a step forward in our understanding of how plants integrate stress responses. 1 Lee, H. et al. (2001) The Arabidopsis hos1 gene negatively regulates cold signal transduction and encodes a RING finger protein that displays cold-regulated nucleo-cytoplasmic partitioning. Genes Dev. 15, 912–924

Joaquin Medina [email protected]

Unravelling PTGS: SDE3 – an RNA helicase involved in RNA silencing in Arabidopsis Although post-transcriptional gene silencing (PTGS) in plants was first recognized in transgenically engineered lines, it is now believed that PTGS evolved as a defense response against viral attack. Work by David Baulcombe’s laboratory has recently focused on the difference between the genetic requirements for transgene-mediated and viral-mediated RNA silencing in Arabidopsis. The two processes can lead to the formation http://plants.trends.com

of double-stranded RNA (dsRNA) via distinct pathways, whereas the downstream events require a common pathway. Tamus Dalmay et al.1 suggest that sde3 is necessary for the initiation of transgenemediated silencing, but only for maintenance of silencing by tobacco rattle virus (TRV). However, based on the phenotypes displayed, sde3 might have a more general role in the maintenance of PTGS.

sde3 is one of four silencing-defective Arabidopsis loci identified by this group in a screen for transgene-mediated silencing mutants. sde3 mutants display a delayed loss of transgene silencing, with full silencing in the cotyledons and in the hypocotyl, contrasting with a loss of silencing in true leaves. sde3 did not affect the initiation of PTGS or the accumulation of TRV RNA in planta, but PTGS returned in younger leaves,

1360-1385/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.

News & Comment

suggesting that SDE3 is required for the maintenance but not the initiation of TRV silencing. In contrast with cucumber mosaic virus (CMV), there was an increase in viral RNA and an overall reduction in PTGS in sde3 plants, suggesting an effect on initiation. Dalmay et al. suggest that the sde3 mutation affects the silencing by the two viruses in different ways because they have different strategies for defense against PTGS. TRV might not produce an RNA species acted on by SDE3 during the initiation of gene silencing, whereas CMV does.

‘…sde3 is necessary for the initiation of transgene-mediated silencing, but only for maintenance of silencing by tobacco rattle virus…’ Cloning of the sde3 locus revealed a protein with homology to RNA helicases.

TRENDS in Plant Science Vol.6 No.8 August 2001


According to the current model of RNAbased silencing, an RNA helicase probably contributes to the production of dsRNA by creating additional single stranded RNA templates for an RNA-dependent RNA polymerase (RdRP). Although the RNA helicase domain of SDE3 is highly homologous to previously identified RNA helicases involved in RNA silencing in other species, it lacks other important motifs. This suggests that it might not be the functional homologue of these genes. The milder defect in PTGS in sde3 than in sde1 (an RdRP) mutants, and the fact that C. elegans displays efficient dsRNA silencing and yet lacks an RNA helicase of the sde3 subfamily, suggest that sde3 might be a regulator of PTGS rather than an essential factor. The phenotype of the sde3 mutants suggests some alternative explanations beyond those provided by Dalmay et al.

The effect of sde3 on transgene-mediated silencing and viral-mediated silencing might not be different. In both cases, silencing is initiated and maintained at an original site, but is not found in newer tissues. These observations suggest that the sde3 deficiency might be preventing the systemic spread of silencing. Alternatively, sde3 might be more generally involved in maintenance in both instances. Also, as transgene silencing is only seen in tissues of embryonic origin, a maternal effect might be at play.

use the technology to avoid over application of fertilizers. [Reay, D. (2001) New Sci. 3 June, p. 54] TS

fetuses,’ Tobin told a recent meeting of horse breeders. [McCarthy, D. (2001) The Press (Christchurch, New Zealand), Racing section, p. 30] TS

1 Dalmay, T. et al. (2001) SDE3 encodes an RNA helicase required for post-transcriptional gene silencing in Arabidopsis. EMBO J. 20, 2069–2077

Matthew R. Willmann [email protected]

In Brief

‘T’ is for teeth Thanks to research from a College of Dentistry team at the University of Illinois (USA), drinkers of tea (‘the cup that cheers’) might have even more reason to smile. The group has found that compounds in tea destroy cavity-causing bacteria, and if included as part of a normal oral-hygiene routine, tea drinking might lead to a reduction in dental cavities. Although their work related specifically to black tea (which constitutes ~80% of the world tea consumption), it adds to similar results that had been found previously for green tea, which is favored in Japan. Additionally, tea is one of the few dietary sources of fluoride that helps to strengthen teeth. [Chapman, J. (2001) Daily Mail, 23 May, p. 35] NC

Colorful idea Leaf color might enable scientists to determine how well plants are fed says David Reay (University of Edinburgh, UK). When each of the three main nutrients (nitrogen, phosphorus and potassium) required for plant growth is in short supply, leaves change color in a specific manner. Deficiencies in nitrogen, phosphorus and potassium lead to yellowing, purpling or leaf-edge browning, respectively. A sensor could determine the color of the leaves, which would give scientists an idea of the nutrient supply of the plant. Farmers could http://plants.trends.com

Cherry trees harm horse business

Photograph courtesy of Scott Bauer, USDA.

Cherry trees might be the cause of the worst devastation in the history of horse breeding in the state of Kentucky (USA), which is famous for its equine breeding. With >1200 natural abortions to date, the multi-billion $US crisis has led scientists to search exhaustively for the cause. Tom Tobin, a toxicologist at the Gluck Equine Research Centre, concluded that cyanide has infiltrated Kentucky pastures from the many surrounding black cherry trees (Prunus serotina). An unexpected ‘hot to frozen’ weather pattern in April caused the cherrytree leaves to wilt earlier than usual, which released cyanide into the pastures. ‘There is preliminary evidence of cyanide in aborted

Vegetables need space to grow Recent experiments conducted by Chinese scientists suggest that exposure of seeds to the conditions found in space significantly promote the growth of the resulting plants. Reports suggest that rice (Oryza sativa) and peppers (Capsicum spp.) grown from such seed have higher levels of vitamin C, and that some types of melon grow larger and sweeter than their terrestrial counterparts. These effects have been attributed to high levels of radiation and low gravity experienced by the seeds during their space flight. However, NASA – the US space agency – has not reported such effects in its own experiments. [Daily Mail (2001), 28 May, p. 19] NC

Plankton protects grain stores Scientists at the University of Greenwich (London, UK) want to raise awareness of a grain storage method that Chinese farmers used 4000 years ago. Farmers stored grain along with diatomaceous earth (DE), a chalky rock that came from the fossilized remains of plankton. DE, an alternative to the pesticides currently in use, kills insects

1360-1385/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.