RESUMO
Genes induced by a progressive 3 week salt stress (final NaCl concentration 300 mM) were identified in the phloem of celery (Apium graveolens L., cv Vert d'Elne). A subtractive library was constructed and screened for salt-induced, phloem-specific genes. Work was focused on phloem due to its central role in inter-organ exchanges. Three genes were studied in more details, 2 coding for metallothioneins (AgMT2 and AgMT3) and one for a new mannitol transporter (AgMaT3). Expression of a reporter gene in transgenic Arabidopsis under control of promoter of each gene was located in the phloem. pAgMT2 has a typical phloem pattern with slight induction by salt stress. pAgMT3 and pAgMaT3 expression was induced by salt stress, except in minor veins. pAgMaT3 was highly active in stressed roots. The promoters described here could be regarded as new tools for engineering salt-resistant plants.
Assuntos
Apium/genética , Regulação da Expressão Gênica de Plantas , Genes de Plantas , Floema/genética , Regiões Promotoras Genéticas , Tolerância ao Sal/genética , Plantas Tolerantes a Sal/genética , Apium/metabolismo , Arabidopsis/genética , Arabidopsis/metabolismo , Proteínas de Transporte/genética , Proteínas de Transporte/metabolismo , Expressão Gênica , Genes Reporter , Proteínas de Membrana/genética , Proteínas de Membrana/metabolismo , Proteínas de Membrana Transportadoras , Metalotioneína/genética , Metalotioneína/metabolismo , Floema/metabolismo , Proteínas de Plantas/genética , Proteínas de Plantas/metabolismo , Raízes de Plantas , Plantas Geneticamente Modificadas/metabolismo , Plantas Tolerantes a Sal/metabolismoRESUMO
A second mannitol transporter, AgMaT2, was identified in celery (Apium graveolens L. var. dulce), a species that synthesizes and transports mannitol. This transporter was successfully expressed in two different heterologous expression systems: baker's yeast (Saccharomyces cerevisiae) cells and tobacco (Nicotiana tabacum) plants (a non-mannitol-producing species). Data indicated that AgMaT2 works as an H(+)/mannitol cotransporter with a weak selectivity toward other polyol molecules. When expressed in tobacco, AgMaT2 decreased the sensitivity to the mannitol-secreting pathogenic fungi Alternaria longipes, suggesting a role for polyol transporters in defense mechanisms. In celery, in situ hybridization showed that AgMaT2 was expressed in the phloem of leaflets, petioles from young and mature leaves, floral stems, and roots. In the phloem of petioles and leaflets, AgMaT2, as localized with specific antibodies, was present in the plasma membrane of three ontologically related cell types: sieve elements, companion cells, and phloem parenchyma cells. These new data are discussed in relation to the physiological role of AgMaT2 in regulating mannitol fluxes in celery petioles.