Senescence in plant evolutionScaling; molecular phylogeny; WRKY transcription factors; allometry; life-history; vegetation strategy Senescence is apparent at every level in plant structure, from the molecular to the organelle scale to the cellular, tissue, organ, whole plant and on to the population and beyond (Leopold 1975). To get the complete picture, the evolution of senescence needs to be studied at a corresponding range of scales. If the appropriate DNA sequence information is available, molecular phylogeny is the approach of choice for tracing the origins and evolution of particular biological traits, including senescence. Genomics is rapidly establishing which genes are necessary and/or sufficient for trait expression (though caution is necessary when dealing with complex environmental interactions – Jansson and Thomas 2008). An example of an exercise in molecular phylogeny relevant to plant senescence is the study of WRKY transcription factors carried out by Zhang and Wang (2005). WRKY53 is among the genes activated early in Arabidopsis leaf senescence (Hinderhofer and Zentgraf 2001), is induced by H2O2, negatively regulates its own expression and is part of a complex transcription factor signalling network regulating senescence-specific gene expression (Miao et al. 2004). WRKYs of Group 3, which include AtWRKY53, are found only in angiosperms. Molecular phylogeny estimates their appearance to be at a point before the divergence of monocots and dicots but later than diversification of the bryophytes – which puts their origin between 160 and 330 million years ago (Zhang and Wang 2005). This suggests that a significant change in the regulation of senescence happened around this time, perhaps related to increasing complexity in regulatory circuitry. Model of the origin and duplications of WRKY gene family. The solid lines correspond to branches where WRKY homologues are identified, while the thickness of the line represents the relative size of WRKY family for the branch, from the thinnest for one copy in Giardia, the slime mold and the green alga to the thickest for over 100 copies in rice. The broken lines represent branches where WRKY genes are not present or have not been identified. The WRKY gene is symbolized by the box for the WRKY domain and the lines for sequences around the domain. The text in the box indicates the group the WRKY domain belongs to (1, Group 1; 1N and 1C, N- and C-terminal domains of Group 1 proteins; a + b: Group 2_a + 2_b; c: Group 2_c; d + e, Group 2_d + 2_e; 3: Group 3). The major gene duplications and diversifications are shown above the branch. The number shown below the branch is the divergence time (million years ago) of its children branches. The branch length is not scaled to the evolutionary distance. Image source: Figure 3 from Zhang and Wang (2005) At the whole plant level, allometry and metabolic scaling have played a major role in the evolution and ecology of plant form, function, and diversity. Developing the capacity to deploy programmed senescence for regulating the scaling relationship between the areas of resource exchange surfaces on the one hand, and transport times and resistances on the other, has been important for the emergence of functional types and vegetation strategies (Enquist et al. 2007). Similarly, the cost-benefit calculations that influence the expression of life history characteristics such as annuality versus perenniality are a reflection of the evolution of selective and progressive patterns of senescence, growth and reproduction (Charnov and Schaffer 1973, Thomas et al. 2000). References
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