Emerging seedlings respond to environmental conditions such as light and temperature to optimize their establishment. Seedlings grow initially through elongation of the hypocotyl, which is regulated by signaling pathways that integrate environmental information to regulate seedling development. The hypocotyls of Arabidopsis (Arabidopsis thaliana) also elongate in response to sucrose. Here, we investigated the role of cellular sugar-sensing mechanisms in the elongation of hypocotyls in response to Suc. We focused upon the role of SnRK1, which is a sugar-signaling hub that regulates metabolism and transcription in response to cellular energy status. We also investigated the role of TPS1, which synthesizes the signaling sugar trehalose-6-P that is proposed to regulate SnRK1 activity. Under light/dark cycles, we found that Suc-induced hypocotyl elongation did not occur in tps1 mutants and overexpressors of KIN10 (AKIN10/SnRK1.1), a catalytic subunit of SnRK1. We demonstrate that the magnitude of Suc-induced hypocotyl elongation depends on the day length and light intensity. We identified roles for auxin and gibberellin signaling in Suc-induced hypocotyl elongation under short photoperiods. We found that Suc-induced hypocotyl elongation under light/dark cycles does not involve another proposed sugar sensor, HEXOKINASE1, or the circadian oscillator. Our study identifies novel roles for KIN10 and TPS1 in mediating a signal that underlies Suc-induced hypocotyl elongation in light/dark cycles.

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A mechanism participating in energy sensing and signalling in plants involves the regulation of sucrose non-fermenting1 (Snf1)-related protein kinase 1 (SnRK1) activity in response to sugar availability. SnRK1 is thought to regulate the activity of both metabolic enzymes and transcription factors in response to changes in energy availability, with trehalose-6-phospate functioning as a signalling sugar that suppresses SnRK1 activity under sugar-replete conditions. Sucrose supplementation increases the elongation of hypocotyls of developing Arabidopsis seedlings, and this response to sucrose involves both the SnRK1 subunit KIN10 and also TREHALOSE-6-PHOSPHATE SYNTHASE1 (TPS1). Here, we measured sucrose-induced hypocotyl elongation in two insertional mutants of KIN10 (akin10 and akin10-2). Under short photoperiods, sucrose supplementation caused great proportional hypocotyl elongation in these KIN10 mutants compared with the wild type, and these mutants had shorter hypocotyls than the wild type in the absence of sucrose supplementation. One interpretation is that SnRK1 activity might suppress hypocotyl elongation in the presence of sucrose, because KIN10 overexpression inhibits sucrose-induced hypocotyl elongation and akin10 mutants enhance sucrose-induced hypocotyl elongation.

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In plants, water use efficiency is a complex trait derived from numerous physiological and developmental characteristics. Here, we investigated the involvement of circadian regulation in long-term water use efficiency. Circadian rhythms are generated by the circadian oscillator, which provides a cellular measure of the time of day. In plants, the circadian oscillator contributes to the regulation of many aspects of physiology, including stomatal opening, the rate of photosynthesis, carbohydrate metabolism and developmental processes. We investigated in Arabidopsis the impact of the misregulation of genes encoding a large number of components of the circadian oscillator upon whole plant, long-term water use efficiency. From this, we identified a role for the circadian oscillator in water use efficiency. This appears to be due to contributions of the circadian clock to the control of transpiration and biomass accumulation. We also identified that the circadian oscillator within guard cells can contribute to long-term water use efficiency. Our experiments indicate that knowledge of circadian regulation will be important for developing future crops that use water more efficiently.One-sentence summaryThe circadian clock in Arabidopsis makes an important contribution to long-term water use efficiency.

17Trichomes are large epidermal cells on the surface of leaves that are thought to deter 18 herbivores, yet the presence of trichomes can also negatively impact plant growth and 19 reproduction. Stomatal guard cells and trichomes have shared developmental origins, and 20 experimental manipulation of trichome formation can lead to changes in stomatal density. 21The influence of trichome formation upon stomatal development in natural populations of 22 plants is currently unknown. Here, we show that a natural population of Arabidopsis halleri 23 that includes hairy (trichome-bearing) and glabrous (no trichomes) morphs has differences in 24 stomatal density that are associated with this trichome dimorphism. We found that glabrous 25 morphs had significantly greater stomatal density and stomatal index than hairy morphs. 26One interpretation is that this arises from a trade-off between the proportions of cells that 27 have trichome and guard cell fates during leaf development. The differences in stomatal 28 density between the two morphs might have impacts upon environmental adaptation, in 29 addition to herbivory deterrence caused by trichome development. 30 31