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Calendar Name: seminars
Scheduled for: Thursday, June 23 2011, 11:00 - 12:30
Event text:    Dr Hannele Tuominen
	       
	       Umeå Plant Science Centre
	       Department of Plant Physiology
	       Umeå University
	       
	       Umeå
	       SWEDEN
Details:       “Identification of novel regulators of lignification and
	       programmed cell death of xylem elements”
	       
	       ABSTRACT
	       Xylem maturation involves lignification and programmed
	       cell death (PCD) that are interlinked through the action
	       of the NAC family transcription factors. The downstream
	       regulators and the executioners are poorly known. We
	       have identified two metacaspase genes as candidates for
	       such key regulators in Populus trees on the basis of
	       specific upregulation during the xylem cell death phase
	       of wood formation. The closest Arabidopsis homologue to
	       these two Populus genes is METACASPASE9 (AtMC9,
	       At5g04200). Also AtMC9 shows xylem specific expression
	       pattern throughout the plant, suggesting involvement in
	       xylem cell death. In order to characterise the function
	       of AtMC9 in xylem development, a reverse genetic
	       approach was taken in Arabidopsis. AtMC9 T-DNA knock-out
	       lines showed rather normal progression of protoxylem
	       differentiation and PCD in in vitro grown seedling roots
	       by analysis of genetic crosses to various xylem reporter
	       lines. Interestingly, AtMC9 RNAi lines revealed
	       increased overall growth of the plants, which together
	       with double mutant analyses in the metacaspase family
	       suggest involvement of AtMC9 together with other type II
	       metacaspase genes in growth control. We are currently
	       performing proteomic analyses to identify targets of the
	       AtMC9 action. Upstream regulators have been sought by
	       screening an EMS mutagenised proAtMC9::GFP reporter
	       line. 
	       
	       We have also utilized the Zinnia elegans in vitro
	       tracheary element (TE) differentiation system to
	       identify novel regulators of lignification and PCD. In
	       this system, application of silver thiosulphate (STS)
	       blocked both lignification and PCD of TEs resulting in
	       prolonged lifespan of the TEs, while TE secondary
	       cellulose formation was not affected. This allowed us to
	       identify novel genes specifically involved in
	       lignification and/or PCD by analyzing differential gene
	       expression patterns between cell cultures treated with
	       or without STS. One of the identified genes was a PIRIN
	       gene that was previously implicated in transcriptional
	       regulation in mammals. Expression of the Zinnia PIRIN
	       gene was strongly upregulated during lignification
	       and/or PCD of normally developing TEs, whereas no
	       upregulation was present in non-lignifying TEs after STS
	       treatment. Functional analysis of the PIRIN gene family
	       was undertaken in Arabidopsis thaliana which contains
	       four PIRIN (PRN) homologs: AtPRN1 (At3g59220), AtPRN2
	       (At2g43120), AtPRN3 (At3g59260) and AtPRN4 (At1g50590).
	       Promoter activity assay by using GUS reporter gene
	       showed that, out of the four PIRIN genes, AtPRN2 was
	       most specifically expressed in the vascular tissues.
	       Pyrolysis-GC/MS analysis of the various PIRIN gene
	       family mutants suggests involvement of AtPRN2 in control
	       of lignification. However, in vitro and in vivo analyses
	       demonstrated interaction of AtPRN2 with several
	       different types of proteases, which suggests involvement
	       of AtPRN2 primarily in regulation of hydrolytic
	       processes during xylem cell death. We can therefore link
	       together the two processes of lignification and PCD and
	       provide evidence on a novel player in this interplay.

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