Nucleus- and plastid-targeted annexin 5 promotes reproductive development in Arabidopsis and is essential for pollen and embryo formation.

BACKGROUND: Pollen development is a strictly controlled post-meiotic process during which microspores differentiate into microgametophytes and profound structural and functional changes occur in organelles. Annexin 5 is a calcium- and lipid-binding protein that is highly expressed in pollen grains and regulates pollen development and physiology. To gain further insights into the role of ANN5 in Arabidopsis development, we performed detailed phenotypic characterization of Arabidopsis plants with modified ANN5 levels. In addition, interaction partners and subcellular localization of ANN5 were analyzed to investigate potential functions of ANN5 at cellular level. RESULTS: Here, we report that RNAi-mediated suppression of ANN5 results in formation of smaller pollen grains, enhanced pollen lethality, and delayed pollen tube growth. ANN5 RNAi knockdown plants also displayed aberrant development during the transition from the vegetative to generative phase and during embryogenesis, reflected by delayed bolting time and reduced embryo size, respectively. At the subcellular level, ANN5 was delivered to the nucleus, nucleolus, and cytoplasm, and was frequently localized in plastid nucleoids, suggesting a likely role in interorganellar communication. Furthermore, ANN5-YFP co-immunoprecipitated with RABE1b, a putative GTPase, and interaction in planta was confirmed in plastidial nucleoids using FLIM-FRET analysis. CONCLUSIONS: Our findings let us to propose that ANN5 influences basal cell homeostasis via modulation of plastid activity during pollen maturation. We hypothesize that the role of ANN5 is to orchestrate the plastidial and nuclear genome activities via protein-protein interactions however not only in maturing pollen but also during the transition from the vegetative to the generative growth and seed development.

Differential defense reactions in leaf tissues of barley in response to infection by Rhynchosporium secalis and to treatment with a fungal avirulence gene product.

Expression of defense-associated genes was analyzed in leaf tissues of near-isogenic resistant and susceptible barley cultivars upon infection by Rhynchosporium secalis. The genes encoding pathogenesis-related (PR) proteins PR-1, PR-5, and PR-9 are specifically expressed in the mesophyll of resistant plants, whereas a germin-like protein (OxOLP) is synthesized in the epidermis irrespective of the resistance genotype. Restriction-mediated differential display was employed to identify additional epidermis-specific genes. This resulted in the detection of another PR gene, PR-10, along with a lipoxygenase gene, LoxA, and a gene of unknown function, pI2-4, which are specifically induced in the epidermis of resistant plants. The gene encoding a putative protease inhibitor, SD10, is preferentially but not exclusively expressed in the epidermis. The fungal avirulence gene product NIP1 triggers the induction of the four PR genes only. At least two additional elicitors, therefore, must be postulated, one for the unspecific induction of OxOLP and one for the resistance-specific induction of LoxA, pI2-4, and SD10. PR-10 expression can be assumed to be the consequence of NIP1 perception by epidermis cells. In contrast, gene expression in the mesophyll is likely to be triggered by an as yet unknown signal that appears to originate in the epidermis and that is strongly amplified in the mesophyll.