Iron homeostasis alteration in transgenic tobacco overexpressing ferritin.

Intracellular iron concentration requires tight control and is regulated both at the uptake and storage levels. Our knowledge of the role that the iron-storage protein ferritins play in plants is still very limited. Overexpression of this protein, either in the cytoplasm or the plastids of transgenic tobacco, was obtained by placing soybean ferritin cDNA cassettes under the control of the CAMV 35S promoter. The protein accumulated in 4- and 6-day-old seedlings and in leaves of 3-week-old plants but not in dry seeds or in 2-day-old seedlings, which is consistent with previous reports describing a post-transcriptional control of ferritin amounts during the germination process. Overaccumulated ferritin in leaves was correctly assembled as 24-mers. Transformants were more resistant to methylviologen toxicity, indicating that the transgenic ferritins were functional in vivo. Ferritin overaccumulation in transgenic tobacco leaves leads to an illegitimate iron sequestration. As a consequence, these transgenic plants behave as iron deficient and activate iron transport systems as revealed by an increase in root ferric reductase activity and in leaf iron content.

Regulation of plant ferritin synthesis: how and why.

Plant ferritins are key iron-storage proteins that share important structural and functional similarities with animal ferritins. However, specific features characterize plant ferritins, among which are plastid cellular localization and transcriptional regulation by iron. Ferritin synthesis is developmentally and environmentally controlled, in part through the differential expression of the various members of a small gene family. Furthermore, a strict requirement for plant ferritin synthesis regulation is attested to by alterations of the photosynthetic apparatus and of iron homeostasis in transgenic tobaccos overexpressing these proteins. Plant ferritin gene regulation appears to consist of a complex interplay of transcriptional and posttranscriptional mechanisms, involving cellular relays such as plant hormones, oxidative steps and Ser/Thr phosphatase.

Iron triggers a rapid induction of ascorbate peroxidase gene expression in Brassica napus.

In plants, only ferritin gene expression has been reported to be iron-dependent. Here it is demonstrated that an iron overload of Brassica napus seedlings causes a large and rapid accumulation of ascorbate peroxidase transcripts, a plant-specific hydrogen peroxide-scavenging enzyme. This result documents a novel link between iron metabolism and oxidative stress. The ascorbate peroxidase mRNA abundance was not modified by reducing agents like N-acetyl cysteine, glutathione and ascorbate or by pro-oxidants such as hydrogen peroxide or diamide. Furthermore, the iron-induced ascorbate peroxidase mRNA accumulation was not antagonized by N-acetyl cysteine. Abscisic acid had no effect on the ascorbate peroxidase gene expression. Taken together these results suggest that iron-mediated expression of ascorbate peroxidase gene occurs through a signal transduction pathway apparently different from those already described for plant genes responsive to oxidative stress.

Identification by 2D-page analysis of salt-stress induced proteins in radish (Raphanus sativus).

The response of Raphanus sativus to salinity has been investigated. Salt-responsive proteins were detected by comparative 2D-PAGE analysis. Three polypeptides of 22, 28 and 28.5 kDa were isolated and subjected to microsequencing. The P22 possesses homology with members of the Kunitz family of trypsin inhibitor; the P28.5 is homologous to ascorbate peroxidase of several plant species while P28 did not exhibit significant homology with sequences archived in available databases.

Indoleacetic acid pretreatment of tobacco plants in vivo increases the in vitro sensitivity to auxin of the plasma membrane H(+)-ATPase from leaves and modifies the polypeptide composition of the membrane.

The sensitivity to auxin of the H(+)-ATPase-mediated proton translocation was investigated in vitro using purified plasma membrane vesicles from tobacco leaves. In vivo pretreatment of tobacco plants with auxin promotes a 100-fold increase of the in vitro sensitivity to auxin. This effect is specific for biologically active auxins and is dose- and time-dependent. In addition, pretreatment with auxin induces the accumulation of several polypeptides in the plasma membrane. These polypeptides constitute the first set of hormone-responsive polypeptides evidenced in plant membranes.

rol A modulates the sensitivity to auxin of the proton translocation catalyzed by the plasma membrane H(+)-ATPase in transformed tobacco.

In order to investigate the effect of the rol A gene product on the plasma membrane response to auxin, a clone of rol A-transformed tobacco was prepared. Auxin sensitivity was measured by the auxin concentration which induced the highest stimulation of H(+)-ATPase-mediated proton translocation on isolated plasma membrane vesicles. Both transformed and control plants exhibited identical auxin-sensitivity changes during vegetative and induction to flowering periods. However the sensitivity of flowering-transformed plants was 100-times higher than that of control plants. Consistent observations were also made when using rol A+B+C-transformed plants. These results suggest that the rol A gene product could either participate in or affect the reception/transduction pathway of auxin signal at the plasma membrane.

The changing sensitivity to auxin of the plasma-membrane H(+)-ATPase: Relationship between plant development and ATPase content of membranes.

The auxin sensitivity of the plasma-membrane H(+)-ATPase from tobacco leaves (Nicotiana tabacum L. cv. Xanthi) depends on the physiological state of the plant (Santoni et al., 1990, Plant Sci. 68, 33-38). Results based on the study of auxin sensitivity according to culture conditions which accelerate or delay tobacco development demonstrate that the highest auxin sensitivity is always associated with the end of the period of induction to flowering. Auxin stimulation of H(+)-translocation activity corresponds to an increase of the apparent ATPase affinity for ATP. The plasma-membrane H(+)-ATPase content, measured with an enzyme-linked immunosorbent assay using a specific anti-H(+)-ATPase antibody, varies according to plant development, and was found to increase by 100% during floral induction. The specific molecular ATPase activity also changes according to plant development; more particularly, the decrease in molecular ATPase activity upto and during the floral-induction period parallels the increase of sensitivity to indole-3-acetic acid.