Nucleo-cytoplasmic transport of proteins and RNA in plants

Merkle T. Nucleo-cytoplasmic transport of proteins and RNA in plants. Plant Cell Reports. 2011;30(2):153-176.Transport of macromolecules between the nucleus and the cytoplasm is an essential necessity in eukaryotic cells, since the nuclear envelope separates transcription from translation. In the past few years, an increasing number of components of the plant nuclear transport machinery have been characterised. This progress, although far from being completed, confirmed that the general characteristics of nuclear transport are conserved between plants and other organisms. However, plant-specific components were also identified. Interestingly, several mutants in genes encoding components of the plant nuclear transport machinery were investigated, revealing differential sensitivity of plant-specific pathways to impaired nuclear transport. These findings attracted attention towards plant-specific cargoes that are transported over the nuclear envelope, unravelling connections between nuclear transport and components of signalling and developmental pathways. The current state of research in plants is summarised in comparison to yeast and vertebrate systems, and special emphasis is given to plant nuclear transport mutants

The DELLA motif is essential for gibberellin-induced degradation of RGA

RGA and GAI are homologous genes that encode putative transcriptional regulators that repress gibberellin (GA) signaling in Arabidopsis. Previously we showed that the green fluorescent protein (GFP)-RGA fusion protein is localized to the nucleus in transgenic Arabidopsis, and expression of this fusion protein rescues the rga null mutation. The GA signal seems to derepress the GA response pathway by degrading the repressor protein RGA. The GA-insensitive, semidominant, semidwarf gai-1 mutant encodes a mutant protein with a 17-amino acid deletion within the DELLA domain of GAI. It was hypothesized that this mutation turns the gai protein into a constitutive repressor of GA signaling. Because the sequences missing in gai-1 are identical between GAI and RGA, we tested whether an identical mutation (rga-Δ17) in the RGA gene would confer a phenotype similar to gai-1. We demonstrated that expression of rga-Δ17 or GFP-(rga-Δ17) under the control of the RGA promoter caused a GA-unresponsive severe dwarf phenotype in transgenic Arabidopsis. Analysis of the mRNA levels of a GA biosynthetic gene, GA4, showed that the feedback control of GA biosynthesis in these transgenic plants was less responsive to GA than that in wild type. Immunoblot and confocal microscopy analyses indicated that rga-Δ17 and GFP-(rga-Δ17) proteins were resistant to degradation after GA application. Our results illustrate that the DELLA domain in RGA plays a regulatory role in GA-induced degradation of RGA. Deletion of this region stabilizes the rga-Δ17 mutant protein, and regardless of the endogenous GA status rga-Δ17 becomes a constitutively active repressor of GA signaling

Auxin-induced fruit-set in tomato is mediated at least in part by gibberellins

13 pages, 6 figures, 1 table.-- PMID: 18702668 [PubMed].Tomato (Solanum lycopersicum L.) fruit-set and growth depend on gibberellins (GA). Auxins, another kind of hormone, can also induce parthenocarpic fruit growth in tomato, although their possible interaction with GA is unknown. We showed that fruit development induced by the auxins indole-3-acetic acid and 2,4-dichlorophenoxyacetic acid (2,4-D) were significantly reduced by simultaneous application of inhibitors of GA biosynthesis (Paclobutrazol and LAB 198999), and that this effect was reversed by applied GA3. This suggested that the effect of auxin was mediated by GA. Parthenocarpic fruits induced by 2,4-D had higher contents of the active GA1, its precursors and metabolite, than unpollinated non-treated ovaries, but similar to pollinated ovaries. Application experiments of radioactive-labelled GAs to unpollinated ovaries showed than 2,4-D altered in vivo GA metabolism (both biosynthesis and catabolism). Transcript levels of genes encoding copalyldiphosphate synthase (SlCPS), SlGA20ox1, -2 and -3, and SlGA3ox1 were higher in unpollinated ovaries treated with 2,4-D. In contrast, transcript levels of SlGA2ox2 (out of the five SlGA2ox genes known to encode this kind of GA inactivating enzymes) were lower in 2,4-D treated ovaries. Our results support the idea that auxins induce fruit-set and growth in tomato, at least partially, by enhancing GA biosynthesis (GA 20-oxidase, GA 3-oxidase and CPS), and decreasing GA inactivation (GA2ox) activities, leading to higher GA1 content. The expression of diverse Aux/IAA and auxin response factors, which may be involved in this effect of auxin, was also altered in 2,4-D-induced ovaries.This work was supported by a grant from the Spanish Ministerio de Educación y Ciencia (BIO2006-13437).Peer reviewe