The Plant Journal (2009) 57, 690–705
doi: 10.1111/j.1365-313X.2008.03722.x
A molecular and structural characterization of senescing Arabidopsis siliques and comparison of transcriptional profiles with senescing petals and leaves Carol Wagstaff1,*, Thomas J.W. Yang2, Anthony D. Stead3, Vicky Buchanan-Wollaston4 and Jeremy A. Roberts2 School of Food Biosciences, University of Reading, Whiteknights, PO Box 226, Reading RG6 6AP, UK, 2 Plant Sciences Division, School of Biosciences, University of Nottingham, Sutton Bonington Campus, Loughborough LE12 5RD, UK, 3 School of Biological Sciences, Royal Holloway, University of London, Egham TW20 0EX, UK, and 4 Warwick HRI, University of Warwick, Wellesbourne CV35 9EF, UK
1
Received 31 July 2008; revised 26 September 2008; accepted 2 October 2008; published online 21 November 2008. * For correspondence (fax +44 118 931 0080; e-mail
[email protected]). Present address: Thomas J.W. Jang, Institute of Plant and Microbial Biology (IPMB), Academica Sinica Nanking, Taipei 11529, Taiwan, China.
Summary Senescence of plant organs is a genetically controlled process that regulates cell death to facilitate nutrient recovery and recycling, and frequently precedes, or is concomitant with, ripening of reproductive structures. In Arabidopsis thaliana, the seeds are contained within a silique, which is itself a photosynthetic organ in the early stages of development and undergoes a programme of senescence prior to dehiscence. A transcriptional analysis of the silique wall was undertaken to identify changes in gene expression during senescence and to correlate these events with ultrastructural changes. The study revealed that the most highly up-regulated genes in senescing silique wall tissues encoded seed storage proteins, and the significance of this finding is discussed. Global transcription profiles of senescing siliques were compared with those from senescing Arabidopsis leaf or petal tissues using microarray datasets and metabolic pathway analysis software (MapMan). In all three tissues, members of NAC and WRKY transcription factor families were up-regulated, but components of the shikimate and cell-wall biosynthetic pathways were down-regulated during senescence. Expression of genes encoding ethylene biosynthesis and action showed more similarity between senescing siliques and petals than between senescing siliques and leaves. Genes involved in autophagy were highly expressed in the late stages of death of all plant tissues studied, but not always during the preceding remobilization phase of senescence. Analyses showed that, during senescence, silique wall tissues exhibited more transcriptional features in common with petals than with leaves. The shared and distinct regulatory events associated with senescence in the three organs are evaluated and discussed. Keywords: silique, senescence, leaf, petal, transcriptomic, comparative.
Introduction The terminal events in the life cycle of a plant organ initially provide a mechanism for mobilization of nutrients from the ageing tissue to support the development of younger tissues or seeds. This is followed by a cell-death phase during which unwanted structures are discarded. Senescence is a highly organized process regulating tissue ageing and death to enable nutrient recycling to occur. Autumn leaf senescence is a classical example of this, and shows efficient mobilization of many cellular components (Keskitalo et al., 2005). There have been many studies identifying genes that are 690
differentially expressed during leaf senescence (BuchananWollaston et al., 2003, 2005; Gepstein et al., 2003; Guo and Gan, 2006), and there is evidence to suggest that the leaf does not take the quickest route to death, but remains viable for as long as possible to allow nutrients to be mobilized from the dying tissues (Hortensteiner and Feller, 2002; Smart, 1994; Soudry et al., 2005). Fewer studies have taken place on other senescing organs, such as petals, but preliminary studies indicate that the two processes do have common features such as proteolytic (Xu and Hanson, 2000) ª 2008 The Authors Journal compilation ª 2008 Blackwell Publishing Ltd
Regulation of silique wall senescence in Arabidopsis 691 silique has used intact pods with the aim of following embryo development of the seeds (de Folter et al., 2004; Louvet et al., 2006; To et al., 2006), inevitably masking differential regulation of genes expressed specifically in the senescing silique wall. The aim of the present study was to characterize the transcriptional and structural changes that occur during silique development in Arabidopsis, followed by a comparative analysis with genes expressed during leaf and petal senescence. Investigation of large sets of differentially regulated genes from microarray experiments has enabled us to identify key metabolic and transcriptional pathways during senescence that are shared, and those that are specific to only one or two of the organs under study. Results and discussion Anatomy of silique wall development Silique senescence in Arabidopsis is developmentally regulated and is far less responsive to environmentally induced variability than leaves, and, unlike petals, neither organ undergoes abscission in Arabidopsis. The Arabidopsis silique elongates immediately after pollination and reaches its final length approximately 10 days after anthesis (DAA), at which point the chlorophyll content of the valves or fruit walls is maximal (Figure 1). Thereafter, chlorophyll levels decline, even before visible yellowing is apparent, such that the chlorophyll content (per mg FW) at approximately 20 DAA is
