Jeong Eun-Young, Seo Pil Joon, Woo Je Chang, Park Chung-Mo
Department of Chemistry, Seoul National University, Seoul, 151-742, South Korea.
Department of Biological Science, Mokpo National University, Jeonnam, 534-729, South Korea.
BMC Plant Biol. 2015 May 1;15:110. doi: 10.1186/s12870-015-0503-8.
Sugar plays a central role as a source of carbon metabolism and energy production and a signaling molecule in diverse growth and developmental processes and environmental adaptation in plants. It is known that sugar metabolism and allocation between different physiological functions is intimately associated with flowering transition in many plant species. The INDETERMINATE DOMAIN (IDD)-containing transcription factor IDD8 regulates flowering time by modulating sugar metabolism and transport under sugar-limiting conditions in Arabidopsis. Meanwhile, it has been reported that SUCROSE NONFERMENTING-1-RELATED PROTEIN KINASE 1 (SnRK1), which acts as a sensor of cellular energy metabolism, is activated by sugar deprivation. Notably, SnRK1-overexpressing plants and IDD8-deficient mutants exhibit similar phenotypes, including delayed flowering, suggesting that SnRK1 is involved in the IDD8-mediated metabolic control of flowering.
We examined whether the sugar deprivation-sensing SnRK1 is functionally associated with IDD8 in flowering time control through biochemical and molecular genetic approaches. Overproduction of AKIN10, the catalytic subunit of SnRK1, delayed flowering in Arabidopsis, as was observed in IDD8-deficient idd8-3 mutant. We found that AKIN10 interacts with IDD8 in the nucleus. Consequently, AKIN10 phosphorylates IDD8 primarily at two serine (Ser) residues, Ser-178 and Ser-182, which reside in the fourth zinc finger (ZF) domain that mediates DNA binding and protein-protein interactions. AKIN10-mediated phosphorylation did not affect the subcellular localization and DNA-binding property of IDD8. Instead, the transcriptional activation activity of the phosphorylated IDD8 was significantly reduced. Together, these observations indicate that AKIN10 antagonizes the IDD8 function in flowering time control, a notion that is consistent with the delayed flowering phenotypes of AKIN10-overexpressing plants and idd8-3 mutant.
Our data show that SnRK1 and its substrate IDD8 constitute a sugar metabolic pathway that mediates the timing of flowering under sugar deprivation conditions. In this signaling scheme, the SnRK1 signals are directly integrated into the IDD8-mediated gene regulatory network that governs flowering transition in response to fluctuations in sugar metabolism, further supporting the metabolic control of flowering.
糖在植物碳代谢和能量产生中起着核心作用，并且在多种生长发育过程及环境适应中作为信号分子。众所周知，糖代谢以及在不同生理功能间的分配与许多植物物种的开花转变密切相关。在拟南芥中，含不定结构域（IDD）的转录因子IDD8在糖限制条件下通过调节糖代谢和转运来调控开花时间。同时，据报道，作为细胞能量代谢传感器的蔗糖非发酵-1-相关蛋白激酶1（SnRK1）被糖剥夺激活。值得注意的是，过表达SnRK1的植物和缺失IDD8的突变体表现出相似的表型，包括开花延迟，这表明SnRK1参与了IDD8介导的开花代谢调控。
我们通过生化和分子遗传学方法研究了糖剥夺感应蛋白SnRK1在开花时间控制中是否与IDD8功能相关。SnRK1的催化亚基AKIN10过量表达导致拟南芥开花延迟，这与缺失IDD8的idd8 - 3突变体的情况相同。我们发现AKIN10在细胞核中与IDD8相互作用。因此，AKIN10主要在位于介导DNA结合和蛋白质-蛋白质相互作用的第四个锌指（ZF）结构域中的两个丝氨酸（Ser）残基Ser - 178和Ser - 182处使IDD8磷酸化。AKIN10介导的磷酸化不影响IDD8的亚细胞定位和DNA结合特性。相反，磷酸化的IDD8的转录激活活性显著降低。这些观察结果共同表明，AKIN10在开花时间控制中拮抗IDD8的功能，这一观点与过表达AKIN10的植物和idd8 - 3突变体的开花延迟表型一致。
我们的数据表明，SnRK及其底物IDD8构成了一条糖代谢途径，该途径在糖剥夺条件下介导开花时间。在这个信号传导模式中，SnRK1信号直接整合到IDD8介导的基因调控网络中，该网络响应糖代谢波动来控制开花转变，进一步支持了开花的代谢调控。
