Semi-viviparous embryo development and dehydrin expression in the mangrove Rhizophora mucronata Lam.

Rhizophora mucronata Lam. is a tropical mangrove with semi-viviparous (cotyledon body protrusion before shedding), non-quiescent and non-desiccating (recalcitrant) seeds. As recalcitrance has been thought to relate to the absence of desiccation-related proteins such as dehydrins, we for the first time systematically described and classified embryogenesis in R. mucronata and assessed the presence of dehydrin-like proteins. Embryogenesis largely follows the classic pattern till stage eight, the torpedo stage, with the formation of a cotyledonary body. Ovule and embryo express radical adaptations to semi-vivipary in the saline environment: (1) A large, highly vacuolated and persistent endosperm without noticeable food reserves that envelopes the developing embryo. (2) Absence of vascular tissue connections between embryo and maternal tissue, but, instead, transfer layers in between endosperm and integument and endosperm and embryo. Dehydrin-like proteins (55-65 kDa) were detected by the Western analysis, in the ovules till stage 10 when the integuments are dehisced. An additional 50 kDa band was detected at stages 6-8. Together these results suggest a continuous flow of water with nutrients from the integument via the endosperm to the embryo, circumventing the vascular route and probably suppressing the initially induced dehydrin expression.

Characterization of an iodothyronine 5'-deiodinase in gilthead seabream (Sparus auratus) that is inhibited by dithiothreitol.

Iodothyronine deiodinases catalyze the conversion of the thyroid prohormone T(4) to T(3) by outer ring deiodination (ORD) of the iodothyronine molecule. The catalytic cycle of deiodinases is considered to be critically dependent on a reducing thiol cosubstrate that regenerates the selenoenzyme to its native state. The endogenous cosubstrate has still not been firmly identified; in studies in vitro the sulfhydryl reagent dithiothreitol (DTT) is commonly used to activate ORD. We now have characterized an ORD activity in the teleost gilthead seabream (Sparus auratus) that is inhibited by DTT. DTT inhibited reverse T(3) (rT(3)) ORD by 70 and 100% in kidney homogenates (IC(50) 0.4 mmol/liter) and microsomes (IC(50) 0.1 mmol/liter), respectively. The omission of DTT from the incubation medium restored renal ORD Michaelis-Menten kinetics with a Michaelis constant value of 5 mumol/liter rT(3) and unmasked the inhibition by 6-n-propyl-2-thiouracil. A putative seabream deiodinase type 1 (saD1), derived from kidney mRNA, showed high homology (> or = 41% amino acid identity) with vertebrate deiodinases type 1. Features of this putative saD1 include a selenocysteine encoded by an in-frame UGA codon, consensus sequences, and a predicted secondary structure for a selenocysteine insertion sequence and an amino acid composition of the catalytic center that is identical with reported consensus sequences for deiodinase type 1. Remarkably, three of six cysteines that are present in the deduced saD1 protein occur in the predicted amino terminal hydrophobic region. We suggest that the effects of DTT on rT(3) ORD can be explained by interactions with the cysteines unique to the putative saD1 protein.