SF21 is a Protein which Exhibits a Dual Nuclear and Cytoplasmic Localization in Developing Pistils of Sunflower and Tobacco.

SF21 was originally described as a pollen- and pistil-expressed protein from sunflower and tobacco. In pistils excised from these species, transcripts were detected in the stigma and in the transmitting tissue where they accumulated in an ovary-oriented increasing concentration gradient. We studied the cellular localization of the SF21 protein during various stages of pistil development as well as in pollen grains of tobacco and sunflower. Here we demonstrate that in young tobacco pistils (from stage 2 onwards) this protein is expressed exclusively in the papillae and secretory cells of the stigma where it is located first in the nucleus and subsequently also in the cytoplasm. Only several stages later (stage 10) does it appear in the transmitting tissue cells of the style where it exhibits a similar, but temporally-delayed, dual nuclear and cytoplasmic localization pattern. SF21 is no longer present in either the stigma or style at the time of pollination, indicating that it is not directly involved in the pollination process. In tobacco pollen grains, SF21 appears just prior to pollen germination and localizes to the apical region of growing pollen tubes, suggesting a possible role in pollen tube growth. This temporal and spatial expression pattern as well as the dual nuclear/cytoplasmic localization suggest that SF21 could be involved in several molecular functions during pistil development and pollen tube growth.

CYP94A1, a plant cytochrome P450-catalyzing fatty acid omega-hydroxylase, is selectively induced by chemical stress in Vicia sativa seedlings.

CYP94A1 is a cytochrome P450 (P450) catalyzing fatty acid (FA) omega-hydroxylation in Vicia sativa seedlings. To study the physiological role of this FA monooxygenase, we report here on its regulation at the transcriptional level (Northern blot). Transcripts of CYP94A1, as those of two other P450-dependent FA hydroxylases (CYP94A2 and CYP94A3) from V. sativa, are barely detectable during the early development of the seedlings. CYP94A1 transcripts, in contrast to those of the two other isoforms, are rapidly (less than 20 min) and strongly (more than 100 times) enhanced after treatment by clofibrate, an hypolipidemic drug in animals and an antiauxin (p-chlorophenoxyisobutyric acid) in plants, by auxins (2,4-dichlorophenoxyacetic acid and indole-3-acetic acid), by an inactive auxin analog (2,3-dichlorophenoxyacetic acid), and also by salicylic acid. All these compounds activate CYP94A1 transcription only at high concentrations (50-500 microM range). In parallel, these high levels of clofibrate and auxins modify seedling growth and development. Therefore, the expression of CYP94A1 under these conditions and the concomitant morphological and cytological modifications would suggest the implication of this P450 in a process of plant defense against chemical injury.

Immuno-cytochemical localization of indole-3-acetic acid during induction of somatic embryogenesis in cultured sunflower embryos.

Immature zygotic embryos of sunflower (Helianthus annuus L.) produce somatic embryos when cultured on medium supplemented with a cytokinin as the sole source of exogenous growth regulators. The timing of the induction phase and subsequent morphogenic events have been well characterized in previous work. We address here the question of the role of endogenous indole-3-acetic acid (IAA), since auxins are known to have a crucial role in the induction of somatic embryogenesis in many other culture and regeneration systems. The fact that in the sunflower system no exogenous auxin is required for the induction of somatic embryos makes this system very suitable for the study of the internal dynamics of IAA. We used an immuno-cytochemical approach to visualize IAA distribution within the explants before, during and after the induction phase. IAA accumulated transiently throughout cultured embryos during the induction phase. The detected signal was not uniform but certain tissues, such as the root cap and the root meristem, accumulated IAA in a more pronounced manner. IAA accumulation was not restricted to the reactive zone but the kinetics of endogenous variations strikingly mimic the pulse of IAA that is usually provoked by exogenous IAA application. The direct evidence presented here indicates that an endogenous auxin pulse is indeed among the first signals leading to the induction of somatic embryogenesis.

Formation of plant cuticle: evidence for the occurrence of the peroxygenase pathway.

Cuticle plays a major role as a protective barrier in plants. Despite its physiological importance, the mode of formation of this complex structure remains poorly understood. In particular, none of the putative enzymes involved in the biosynthesis of the cutin, the matrix of cuticle, have been cloned. We have shown previously that peroxygenase is able to catalyze in vitro the epoxidation step required for the biosynthesis of C18 cutin monomers. In the present work, we have confirmed in planta that this oxidase is indeed a key enzyme involved in the formation of cutin. Thus, in maize leaves, the specific inactivation of peroxygenase by organophosphorothioates resulted in a dramatic decrease of cuticular epoxide content, as visualized by a specific histochemical technique that was accompanied by a reduced thickness of the cuticle. A strict correlation could also be established between the extent of inhibition of the peroxygenase and the modification of the cuticle triggered by a family of structurally related inhibitors. Importantly, these effects were restricted to plants that contain a cutin originating from C18 monomers. The altered cuticle of maize, treated with the peroxygenase inhibitor, was characterized by an increased permeability to pesticides. In addition, such plants became largely susceptible to infection by fungi, implying that the cuticle represents a crucial target for the modulation of the response in plant-pathogen interactions.