RESUMO
Hevea brasiliensis is an important industrial crop for natural rubber production. Latex biosynthesis occurs in the cytoplasm of highly specialized latex cells and requires sucrose as the unique precursor. Ethylene stimulation of latex production results in high sugar flow from the surrounding cells of inner bark towards the latex cells. The aim of this work was to understand the role of seven sucrose transporters (HbSUTs) and one hexose transporter (HbHXT1) in this process. Two Hevea clones were used: PB217 and PB260, respectively described as high and low yielding clones. The expression pattern of these sugar transporters (HbSUTs and HbHXT1) was monitored under different physiological conditions and found to be maximal in latex cells. HbSUT1, one of the most abundant isoforms, displayed the greatest response to ethylene treatment. In clone PB217, ethylene treatment led to a higher accumulation of HbSUT1B in latex cells than in the inner bark tissues. Conversely, stronger expression of HbSUT1B was observed in inner bark tissues than in latex cells of PB260. A positive correlation with HbSUT1B transcript accumulation and increased latex production was further supported by its lower expression in latex cells of the virgin clone PB217.
Assuntos
Etilenos/farmacologia , Regulação da Expressão Gênica de Plantas/efeitos dos fármacos , Hevea/efeitos dos fármacos , Látex/metabolismo , Proteínas de Membrana Transportadoras/metabolismo , Compostos Organofosforados/farmacologia , Proteínas de Plantas/metabolismo , Clonagem Molecular , DNA Complementar/genética , DNA Complementar/metabolismo , DNA de Plantas/genética , Hevea/genética , Hevea/metabolismo , Proteínas de Membrana Transportadoras/genética , Filogenia , Casca de Planta , Reguladores de Crescimento de Plantas/farmacologia , Proteínas de Plantas/genética , Caules de Planta , Transporte Proteico , Fatores de TempoRESUMO
Quebrachitol is a cyclic polyol and, along with sucrose, is one of the main sugars in Hevea latex. However, in contrast to sucrose, the mechanism and regulation of quebrachitol absorption is still unknown. Screening a latex-derived cDNA library using polyol transporter-specific probes, two full-length cDNAs were isolated, and named HbPLT1 and HbPLT2 (for Hevea brasiliensis polyol transporter 1 and 2, respectively). Their respective sequences exhibited close similarity with the previously cloned acyclic sugar polyol transporters, and shared the main features of the major facilitative superfamily. The functional activity of one of the cDNAs was determined by using an HbPLT2-complemented yeast strain. These strains displayed a marginal absorption of cyclic (inositol) and acyclic (mannitol and sorbitol) polyol but no absorption of sucrose, hexose and glycerol. Active absorption for xylitol was detected, and was competitively inhibited by quebrachitol. HbPLT1 and HbPLT2 expression patterns varied in response to different stimuli. Bark treatment with ethylene resulted in an early and significant up-regulation of HbPLT2 transcripts in laticifers as well as in inner bark cells, when compared with HbPLT1. Other treatments, especially mechanical wounding, strongly induced HbPLT2 transcripts. These data were consistent with the presence of ethylene and a wound-responsive regulatory cis-element on the sequence of the HbPLT2 promoter. All these findings together with those recently obtained for sucrose transporters and aquaporins are discussed in relation to the different roles for quebrachitol in Hevea brasiliensis.
Assuntos
Euphorbiaceae/genética , Proteínas de Plantas/metabolismo , Polímeros/metabolismo , Sequência de Aminoácidos , Transporte Biológico , Clonagem Molecular , DNA Complementar/genética , Genes de Plantas , Hibridização In Situ , Dados de Sequência Molecular , Filogenia , Proteínas de Plantas/classificação , Proteínas de Plantas/genética , Saccharomyces cerevisiae/genética , Homologia de Sequência de AminoácidosRESUMO
BACKGROUND AND AIMS: The major economic product of Hevea brasiliensis is a rubber-containing cytoplasm (latex), which flows out of laticifers (latex cells) when the bark is tapped. The latex yield is stimulated by ethylene. Sucrose, the unique precursor of rubber synthesis, must cross the plasma membrane through specific sucrose transporters before being metabolized in the laticifers. The relative importance of sucrose transporters in determining latex yield is unknown. Here, the effects of ethylene (by application of Ethrel on sucrose transporter gene expression in the inner bark tissues and latex cells of H. brasiliensis are described. METHODS: Experiments, including cloning sucrose transporters, real time RT-PCR and in situ hybridization, were carried out on virgin (untapped) trees, treated or untreated with the latex yield stimulant Ethrel. KEY RESULTS: Seven putative full-length cDNAs of sucrose transporters were cloned from a latex-specific cDNA library. These transporters belong to all SUT (sucrose transporter) groups and differ by their basal gene expression in latex and inner soft bark, with a predominance of HbSUT1A and HbSUT1B. Of these sucrose transporters, only HbSUT1A and HbSUT2A were distinctly increased by ethylene. Moreover, this increase was shown to be specific to laticifers and to ethylene application. CONCLUSION: The data and all previous information on sucrose transport show that HbSUT1A and HbSUT2A are related to the increase in sucrose import into laticifers, required for the stimulation of latex yield by ethylene in virgin trees.