2000; Bossi et al. 2009), and root development (Signora et al. 2001; Shkolnik-Inbar and Bar-Zvi 2011). There are hundreds of loci whose expression is altered in the ABI4 mutant (Kerchev et al. 2011). Given that it is a transcription factor, this is not surprising, but does illustrate the challenge of functional annotation of

CH5424802 mouse such pleiotropic loci. abi4 had higher SLA and LWC than wildtype, revealing a novel effect of this TF on leaf anatomy. In addition, abi4 had increased g m and more negative δ13C, consistent with the idea that SLA causes variation in δ13C via effects on g m (Fig. 7). The correlation of SLA, A, and g s with LWC helps to explain why LWC is strongly correlated with leaf gas exchange, i.e., LWC appears to be an inverse proxy for cell wall thickness. When taken together, our data show that Arabidopsis leaves trade-off high WUE for low A, by

trading off leaf anatomy based diffusional CO2 limitation with water loss through stomata. Essentially, plants with the highest A achieve this via the combination of high g s and thin leaves (high SLA). High g s keeps C i high and the thin leaves have cells with thin walls. Thin walls increase g m and keeps CO2 concentration at the sites of carboxylation (C c) high (Evans et al. 1994). Conversely, when photosynthesis is directly limited by the combination of cool winter temperatures and high light BIRB 796 chemical structure through effects on electron transport, then low g s would be selected for to improve WUE. We hypothesize that thicker leaves would CUDC-907 price provide more internal shading and more efficient light use, further decreasing g m and C c explaining the winter annual phenotype. Fig. 7 Comparison of specific leaf area (SLA), leaf water content (LWC), mesophyll conductance (g m), and leaf carbon isotope composition (δ13C) between abi4-1 and Columbia (Col) wildtype. Each bar represents Nitroxoline the mean ± SE (n = 7) for each genetic line. P < 0.05 for g m, SLA, LWC, and δ13C Although, a few of the AP2/ERF transcription factors in Arabidopsis have been the subject of detailed study, there are 122 of these loci in Arabidopsis (Nakano

et al. 2006) and much remains unknown about their function. Recent studies have revealed increasingly complex roles for members of this transcription factor family. For example, a recent study identified eight AP2/ERFs induced by photorespiration (Foyer et al. 2012). This, combined with the known roles of ABI4 in sugar signaling to photosynthesis including repression of RBCS (Van Oosten et al. 1997; Teng et al. 2008), and our results showing effects on leaf density and g m, are expanding this picture. Conclusions Detailed measurements on a diverse set of accessions detail the traits underlying natural variation in intrinsic WUE and carbon isotope composition. Previous studies have shown that spring accessions have lower intrinsic WUE than accessions with winter life histories.