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ABSTRACT FROM PLANT CANADA 2005 - Arabidopsis eIF-5A1 is a positive regulator of xylem development

LIU, Z., DUGUAY, J., MA F., WANG, T.-W.,

TSHIN, R., HOPKINS M.T., NOWACK, L.,

THOMPSON, J.E.

Univ of Waterloo, Dept of Biology, Waterloo,

ON N2L 3G1

Eucaryotic translation initiation factor 5A (eIF-5A) is thought to facilitate protein synthesis by participating in the nuclear export of specific mRNAs. In Arabidopsis, there are three isoforms of eIF -5A, and one of them, AteIF-5A1, is expressed in senescing tissues. More recently, we found that AteIF-5A1 is also expressed in developing xylem. In order to determine whether AteIF -5A1 plays a role in xylem formation, its full-length cDNA was constitutively over-expressed in transgenic Arabidopsis plants. These plants had larger rosette leaves as well as taller and thicker

inflorescence stems in comparison to control plants, reflecting more rapid growth. In addition, their main inflorescence stems exhibited a significant enhancement in xylem development. In both vascular and interfascicular regions, cell layers and tissue thicknesses were increased by up to 80% and 60%, respectively, depending on the line. Increased xylem growth was more pronounced in secondary xylem than in primary xylem. Moreover, constitutive antisense suppression of AteIF-5A1 resulted in stunted statures and delayed leaf senescence as well as ~30% reduction in xylem tissue in comparison with control plants. eIF5A is post-translationally modified by conversion of a conserved lysine to hypusine. To confirm that it is the hypusinated form of eIF5A1 that regulates xylogenesis, we also generated transgenic Arabidopsis plants with constitutive overexpression of a mutant form of AteIF-5A1 that cannot be hypusinated. The mutation was made by changing the codon, AAG, for the conserved lysine to GCG (for alanine), thereby disabling hypusination of AteIF-5A1. In the mutant population, there was no change in xylem development relative to control plants. Thus, AteIF-5A1 appears to positively regulate xylogenesis, possibly by facilitating the programmed death of treachery elements, and such a function requires activation of the protein through hypusination.