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1 Title: Variation in heat-shock proteins and photosynthetic ...

1 Title: Variation in heat-shock proteins and photosynthetic ...

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 greater thermal damage. Alternately, other growth-temperature induced changes, e.g. greater membrane thermostabililty, and increased cellular protectants (Hochachka and Somero, 2002), could decrease damage at higher temperatures and thereby reduce the requirement for more Hsps. Lastly, it is also possible that no single kind of Hsp-habitat temperature relationship exists, and instead, that positive, negative, and no relationships occur, depending on the organisms in question. To explore some of these possibilities, in this study, we examined variation in Hsp content and thermotolerance in five naturally occurring populations of Chenopodium album from contrasting thermal environments. Specifically, we asked: (a) whether Hsp content and physiological thermotolerance varies between the populations of C. album from contrasting thermal environments; (b) how variation in Hsp content and thermotolerance relate to the thermal environment from which the populations originated; (c) if variation in Hsps is related to mean habitat temperatures, or to temperature variability; and (d) how do differences in growth temperature affect Hsp content and thermotolerance. We examined Hsp content and thermotolerance in naturally occurring populations of a cosmopolitan weed, Chenopodium album. C. album is widely distributed in North America, grows in a variety of habitats, and demonstrates considerable phenotypic plasticity for growth, morphology, and other traits (Basset and Crompton 1978). In addition, genetic variation for growth, enzymes, herbicide resistance, thermotolerance, and protein content has been reported for C. album (Warwick and Marriage 1982, Al Mouemar and Gasquez 1983, Mukherjee 1986, Bera and Mukherjee 1992, Fuks et al.1992, Barua et al. 2003), and some of this variation is related to geographical (Warwick and Marriage 1982), and environmental (Al Mouemar and Gasquez 1983) correlates. Most comparative work in Hsps has been undertaken in model organisms, and naturally occurring populations have received less attention (Barua and Heckathorn 2006). This is especially true for plants, where the vast majority of work has focused on commercial cultivars (Blumenthal et al. 1990, Fender and O’Connell 1990, Frova and Gorla 1993, Ristic et al. 1996, Preczewski et al. 2000). The five study sites were chosen to represent a north-south gradient in habitat temperatures. In addition to mean habitat temperature, selection of sites was also based on differences in temperature variability, to represent stable climates with small seasonal and diurnal ranges, as well as more extreme climates with larger temperature fluctuations. To 4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 assess the variability of the thermal environment, we analysed climate data over the last three decades to calculate the growing season diurnal range of temperatures and the frequency of exposure to extreme temperatures. Given that induction of Hsps is a response to extreme temperatures, we expected differences in the relationship of Hsp content to central tendencies of the thermal environment (mean and maximum temperatures), and measures of variability in the thermal environment (diurnal range and frequency of exposure to heat stress). Current studies often examine the former, but none have addressed the latter, in relation to Hsps. We examined the relative abundance of chloroplast Hsp70, Hsp60, total small Hsps, and chloroplast small Hsps, which are representative of the major Hsps in plants (Vierling 1991; Wang et al. 2004). Chloroplast Hsp70, a member of the diverse Hsp70 family, is involved in a range of functions within the chloroplast that include protein folding, assembly, and translocation. Hsp60 is an abundant molecular chaperone found in chloroplasts and mitochondria. The abundance and diversity of small Hsps (sHsps) are unique to plants (Waters et al. 1996). Total sHsps reported here include members of the family that localize to different cellular compartments, but are dominated by the cytosolic forms. The chloroplastspecific sHsps have been shown to protect PSII during heat stress (Heckathorn et al. 1998, Heckathorn et al. 2002). We assayed PSII thermotolerance as an index of physiological thermotolerance. PSII is particularly thermolabile, and a sensitive indicator of photosynthetic and organismal thermotolerance (Weiss and Berry 1988). Since Hsp production is typically an induced response to heat stress, we partitioned total thermotolerance into its basal and induced components by assaying it with and without a previous Hsp-inducing pretreatment. As predicted global climate change is expected to include both increases in mean temperatures, as well as increases in the frequency, severity, and duration of acute heat stress (or heat waves) (IPCC 2001), plants will experience increases in heat stress in the near future, and this will have an impact on plant productivity and diversity (Thomas et al. 2004; Ciais et al. 2005). If there is ecotypic variation in Hsp production with acute heat stress in plants, and ecotypes vary in their acclimation (or lack thereof) of the Hsp response to acute heat stress with concurrent increase in background mean temperatures, then ecotypic variation in Hsps will be important in determining species-level responses to global warming. Results 5

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