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Universidade do Minho (2009)

Physiological and molecular studies on the invader Hakea sericea : a contribution for its control

Sousa, M. F.

Titre : Physiological and molecular studies on the invader Hakea sericea : a contribution for its control

Auteur : Sousa, M. F.

Université de soutenance : Universidade do Minho

Grade : Tese de doutoramento em Ciências Biológicas 2009

Résumé
Hakea sericea Schrad. (needlebush or silky hakea) is an Australian Proteaceae that can be found naturalized in South Africa, New Zealand and southern Europe. It presents xeromorphic adaptations in leaves, produces high amounts of long-lived, fire resistant seeds that are easily dispersed by both air and water, and relys on proteoid roots for enhanced nutrient uptake. Such unique abilities allowed H. sericea to proliferate and spread throughout its new habitats, being currently considered by regulatory agencies as one of the most agressive invaders established in Portuguese territory. Although mechanical and biological control have already been extensively tested in South Africa to constrain the expansion of H. sericea, in Portugal no large scale attempts have been made to control its spreading. Here we report the identification of a naturally occuring pathogenic fungus able to infect H. sericea that could, in a nearby future, be used as part of an integrated control strategy for this invader. The pathogenic agent was isolated from leaf spots of infected wild-growing H. sericea found in “Serra de Arga”, in north-western Portugal. Microscopical observation revealed typical Pestalotiopsis sp. 5-celled spores, whose members have been described as pathogenic for Proteaceae species. Thermocyclic amplification of the internal transcribed spacer region of the fungal rDNA, using ITS4 and ITS5 universal primers, identified the pathogenic agent as Pestalotiopsis funerea. Pathogenicity of P. funerea on H. sericea was confirmed with the infection of leaf-wounded and non-wounded 6-week-old in vitro-grown plants. Only the leafwounded plants sprayed with P. funerea developed lesions identical to those observed in wild-grown specimens. A major feature of H. sericea is the ability to produce proteoid roots in response to mineral scarcity, mainly N and P. Although the enhanced capacity of proteoid roots to absorb inorganic phosphate (Pi) has been mainly attributed to an increased root surface area and higher exudation of organic acids and phosphatases, here we focused on the Pi uptake system of proteoid roots by itself. Uptake experiments with 32Pi suggested the involvement of H+/Pi co- transport systems with Km values of 0.225 and 40.8 μM Pi, both prone to competitive inhibition with the analogs phosphite and arsenate, but not vanadate. This biphasic Pi uptake system with the highest affinity at submicromolar range is likely to confer H. sericea the ability to invade and proliferate throughout vast areas of nutrient-poor soils, where survival for most plant species seems harder. In plants, Pi uptake, translocation through tissues and interorganellar allocation is carried out by members of the phosphate transporter (PhT) family. Here we report the identification of four PhT genes (PiT2, PiT6, PH5 and PH7) from H. sericea genome. Through phylogenetic analysis and transmembrane domain (TMD) prediction, all identified genes were placed in the PhT1 family, meaning PiT2, PiT6, PH5 and PH7 are probably high- or low-affinity H+/Pi symporters involved in Pi transport into the cell. Phylogenetic analysis confirmed the homology between PhT1 members from different plant species, while PhT2 members (chloroplastidial H+/Pi symporters) were placed in a cluster of their own. Based on phylogeny, we also report the possible existence of new Na+/Pi symporter family in plants. The correlation between the expression site of each PhT1 with its deduced amino acid sequence has also been studied. Phylogenetic studies suggest that the amino acid structure of each PhT1 is insufficient to predict the tissue where the protein is likely to be expressed. Expression studies of PiT2, PiT6, PH5 and PH7 in plants grown in high- and low-Pi availabity shall provide insights on the Pi transport regulation mechanism of H. sericea.

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Page publiée le 28 mars 2011, mise à jour le 12 juin 2017