Low And High Temperature

There are several lines of evidence demonstrating the interaction between light perception by phytochromes and temperature in the regulation of some SAS-related responses, such as flowering and germination. However, data on the interaction between phytochrome and temperature in the regulation of the hypocotyl elongation induced by simulated shade are scarce. In a study on the annual weed Abutilon theophrasti, it was shown that hypocotyl elongation in response to low R:FR is increased under high-temperature conditions, indicating that plant responsiveness to low R:FR depends on ambient temperature (Weinig, 2000). Although there are no similar studies in Arabidopsis, exposure of seedlings growing under constant W (high R:FR) to high temperature (28-29°C) results in an auxin-dependent induction of the hypocotyl elongation, compared to seedlings growing at 20-22°C (Gray et al., 1998; Koini et al., 2009; Stavang et al., 2009). In addition, loss-of-function of SHADE AVOIDANCE 3 (SAV3) (see the next section), that results in shorter hypocotyls when mutants are grown under simulated shade (Table I) and partially suppress the constitutive SAS phenotype of a phyB mutant, is also defective in high-temperature-induced hypocotyl elongation (Tao et al., 2008). These observations suggest that high temperature and low R:FR share regulatory mechanisms in the control of hypocotyl elongation. This possibility led to test the role of PIF4 and PIF5, whose stability is increased by low R:FR light (Lorrain et al., 2008), in mediating plant responses to high temperature. The hypocotyl elongation response was completely abolished in pif4 seedlings transferred to 28-29°C, whereas an attenuated response was observed in pif5 mutants, suggesting a role for these PIFs in regulating this process, with PIF4 playing an essential role (Koini et al., 2009; Stavang et al., 2009). It is unknown whether the shade-induced hypocotyl elongation in Arabidopsis is affected by high temperature and whether PIF4 and PIF5 also have a major role in regulating this response at high temperature. However, high temperature induces PIF4 expression without apparently increasing the stability of the protein (Stavang et al., 2009), suggesting that the mechanisms by which the increase in ambient temperature and the decrease in the R:FR light enhance the PIF4-mediated hypocotyl elongation are different.

A connection between simulated shade and low-temperature treatments has been uncovered by performing transcriptomic analysis of Arabidopsis seedlings grown at two different temperatures (16 and 22°C) and differentially treated with high or low R:FR for 24 h. These experiments revealed that low R:FR perception activates the expression of genes of the CBF regulon in Arabidopsis in a temperature-dependent manner. The CBF regulon refers to a set of genes whose expression is induced by cold and osmotic stress through the CBF family of AP2 domain transcription factors (Gilmour et al., 2004). As mentioned previously, the induction of CBF1, CBF2 and CBF3 transcription by simulated shade was gated by the circadian clock and it occurred during the peaks of its endogenous rhythm (Franklin and Whitelam, 2007). Transgenic overexpression of CBF genes in Arabidopsis induces elevated expression of the CBF regulon in the absence of a low-temperature stimulus, leading to enhanced freezing tolerance (Gilmour et al., 2004). Similarly, cold acclimation at 4-5°C induces CBF genes and the CBF regulon, conferring freezing tolerance (Gilmour et al., 2004). The biological significance of the observed interaction between cold- and shade-regulated transcriptional networks was elegantly demonstrated because pretreatments of a few hours with simulated shade of plants grown at 16°C increased survival rate and reduced leaf damage in response to freezing compared to controls plants grown at the same temperature only in high R:FR (with no simulated shade pre-treatment). No notable differences in freezing tolerance were produced by simulated shade pre-treatments when plants were grown at 22°C (Franklin and Whitelam, 2007). Although either decreased ambient temperature or simulated shade alone is unable to protect plants from freezing, the combination of these two signals results in an increased tolerance to protects plants from freezing. This led to suggest that these mechanisms may reflect an adaptation by which plants are able to sense the encroaching winter during autumn, when twilight increases (i.e. the average R:FR decreases) and the temperature starts to drop (Franklin and Whitelam, 2007).

In addition to the CBF regulon, transcriptomic analyses of Arabidopsis seedlings grown at 16 and 22°C and differentially treated with high or low R: FR for 24 h (Franklin and Whitelam, 2007) showed hundreds of genes whose shade-induced expression is affected by the temperature, many of which encode proteins belonging to several families of transcription factors (Fig. 6). Although their function in plant development has not been analysed yet, their role as transcriptional regulators suggests that they might encode key players in the dynamics of the transcriptional networks affected. The extensive connections between transcriptional networks modulated by different external signals, such as simulated shade and cold, represent a potential molecular mechanism to integrate novel combinations of environmental

Induced by low R:FR

Plants growing Plants growing at 16 °C at 22 °C

Induced by low R:FR

Plants growing Plants growing at 16 °C at 22 °C

Fig. 6. Venn diagrams illustrating the subgroup of differentially expressed genes induced (at least twofold induction) by low R:FR for 24 h of 5-day-old phyA-deficient seedlings germinated and grown at 16 or 22°C. Based on the available identity of the Affymetrix probes, the microarrays used by the authors contained a total of 22,591 genes (AGI codes). From those 1774 were classified as encoding transcription factors. Brackets indicate the sub-group of expressed genes tentatively identified as encoding transcription. Data are extracted from the microarray data deposited in the National Center for Biotechnology Information Gene Expression Omnibus, GEO Accession code GSE8745 (Franklin and Whitelam, 2007).

Fig. 6. Venn diagrams illustrating the subgroup of differentially expressed genes induced (at least twofold induction) by low R:FR for 24 h of 5-day-old phyA-deficient seedlings germinated and grown at 16 or 22°C. Based on the available identity of the Affymetrix probes, the microarrays used by the authors contained a total of 22,591 genes (AGI codes). From those 1774 were classified as encoding transcription factors. Brackets indicate the sub-group of expressed genes tentatively identified as encoding transcription. Data are extracted from the microarray data deposited in the National Center for Biotechnology Information Gene Expression Omnibus, GEO Accession code GSE8745 (Franklin and Whitelam, 2007).

cues and to produce novel responses that confer the adaptive phenotypic plasticity displayed by the plants.

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