Localization of eIF4A-III in the nucleolus and splicing speckles is an indicator of plant stress

Abstract

The mechanisms of long-term adaptation to low oxygen environment are quite well studied, but little is known about the sensing of oxygen shortage, the signal transduction and the shortterm effects of hypoxia in plant cells. We have found that an RNA helicase eIF4A-III, a putative component of the Exon Junction Complex, rapidly changes its pattern of localisation in the plant nucleus under hypoxic conditions. In normal cell growth conditions GFPeIF4A-III was mainly nucleoplasmic, but in hypoxia stress conditions it moved to the nucleolus and splicing speckles. This transition occurred within 15-20 min in Arabidopsis culture cells and seedling root cells, but took more than 2 h in tobacco BY-2 culture cells. Inhibition of respiration, transcription or phosphorylation in cells and ethanol treatment had similar effects to hypoxia. The most likely consequence is that a certain mRNA population will remain bound to the eIF4A-III and other mRNA processing proteins, rather than being transported from the nucleus to the cytoplasm, and thus its translation will be suspended.

Keywords: eIF4A-III; exon junction complex; hypoxia; nucleolus; splicing speckles.

Figure 1

Patterns of localisation of GFPeIF4A-III observed in cells treated with inhibitors. Single confocal sections and corresponding bright-field images are shown. (A) diffuse staining of nucleoplasm; (B) nucleolar; (C) nucleolar rim; (D) speckle in the nucleolar cavity and diffuse staining of nucleoplasm; (E) nucleolus and nucleoplasmic speckles; (F) nucleoplasmic speckles. Arrows indicate positions of nucleoli.

Figure 2

Localization GFP-eIF4AIII in BY-2 tobacco cells. (A) control cells, six nuclei with diffuse signal in the nucleoplasm are shown; (B) mitotic control cell in anaphase: more signal is associated with the separating chromosomes than with the mitotic spindle; (C) cells treated by 5% ethanol for 5 h show localization of signal in nucleoli and speckles. (D) cells treated by MG132 for 5 h show uniform localization in nucleoli only. Bar, 10 µm. Arrows indicate position of nucleoli, arrowheads indicate splicing speckles.