Long-distance transport of sucrose in source leaves promotes sink root growth by the EIN3-SUC2 module.

In most plants, sucrose, a major storage sugar, is transported into sink organs to support their growth. This key physiological process is dependent on the function of sucrose transporters. Sucrose export from source tissues is predominantly controlled through the activity of SUCROSE TRANSPORTER 2 (SUC2), required for the loading of sucrose into the phloem of Arabidopsis plants. However, how SUC2 activity is controlled to support root growth remains unclear. Glucose is perceived via the function of HEXOKINASE 1 (HXK1), the only known nuclear glucose sensor. HXK1 negatively regulates the stability of ETHYLENE-INSENSITIVE3 (EIN3), a key ethylene/glucose interaction component. Here we show that HXK1 functions upstream of EIN3 in the regulation of root sink growth mediated by glucose signaling. Furthermore, the transcription factor EIN3 directly inhibits SUC2 activity by binding to the SUC2 promoter, regulating glucose signaling linked to root sink growth. We demonstrate that these molecular components form a HXK1-EIN3-SUC2 module integral to the control of root sink growth. Also, we demonstrate that with increasing age, the HXK1-EIN3-SUC2 module promotes sucrose phloem loading in source tissues thereby elevating sucrose levels in sink roots. As a result, glucose signaling mediated-sink root growth is facilitated. Our findings thus establish a direct molecular link between the HXK1-EIN3-SUC2 module, the source-to sink transport of sucrose and root growth.

Plant casein kinases phosphorylate and destabilize a cyclin-dependent kinase inhibitor to promote cell division.

Cell cycle is one of the most fundamentally conserved biological processes of plants and mammals. Casein kinase1s (CK1s) are critical for cell proliferation in mammalian cells; however, how CK1s coordinate cell division in plants remains unknown. Through genetic and biochemical studies, here we demonstrated that plant CK1, Arabidopsis (Arabidopsis thaliana) EL1-like (AELs), regulate cell cycle/division by modulating the stability and inhibitory effects of Kip-related protein6 (KRP6) through phosphorylation. Cytological analysis showed that AELs deficiency results in suppressed cell-cycle progression mainly due to the decreased DNA replication rate at S phase and increased period of G2 phase. AELs interact with and phosphorylate KRP6 at serines 75 and 109 to stimulate KRP6's interaction with E3 ligases, thus facilitating the KRP6 degradation through the proteasome. These results demonstrate the crucial roles of CK1s/AELs in regulating cell division through modulating cell-cycle rates and elucidate how CK1s/AELs regulate cell division by destabilizing the stability of cyclin-dependent kinase inhibitor KRP6 through phosphorylation, providing insights into the plant cell-cycle regulation through CK1s-mediated posttranslational modification.

EL1-like Casein Kinases Suppress ABA Signaling and Responses by Phosphorylating and Destabilizing the ABA Receptors PYR/PYLs in Arabidopsis.

Unveiling the signal transduction of phytohormone abscisic acid (ABA) and its regulatory mechanisms is critical for developing the strategies toward improving plant responses to stressful environments. ABA signaling is perceived and mediated by multiple PYR/PYL receptors, whose post-translational modifications, especially phosphorylation, remain largely unknown. In this study, we demonstrate that Arabidopsis EL1-like (AEL) protein, a casein kinase that regulates various physiological processes, phosphorylate PYR/PYLs to promote their ubiquitination and degradation, resulting in suppressed ABA responses. Arabidopsis ael triple mutants display hypersensitive responses to ABA treatment, which is consistent with the suppressed degradation of PYR/PYL proteins. PYR/PYLs are phosphorylated in vivo and mutation of the conserved AEL phosphorylation sites results in reduced phosphorylation, ubiquitination, and degradation of PYR/PYLs, and hence enhanced ABA responses. Taken together, these results demonstrate that AEL-mediated phosphorylation plays crucial roles in regulating the stability and function of PYR/PYLs, providing significant insights into the post-translational regulation of PYR/PYL receptors and ABA signaling.

Ectopic expression of NnPER1, a Nelumbo nucifera 1-cysteine peroxiredoxin antioxidant, enhances seed longevity and stress tolerance in Arabidopsis.

Seed longevity, the maintenance of viability during storage, is a major factor for conservation of genetic resources and biodiversity. Seed longevity is an important trait of agriculture crop and is impaired by reactive oxygen species (ROS) during seed desiccation, storage and germination (C. R. Biol., 331, 2008 and 796). Seeds possess a wide range of systems (protection, detoxification, repair) allowing them to survive during storage and to preserve a high germination ability. In many plants, 1-cys peroxiredoxin (1-Cys Prx, also named PER1) is a seed-specific antioxidant which eliminates ROS with cysteine residues. Here we identified and characterized a seed-specific PER1 protein from seeds of sacred lotus (Nelumbo nucifera Gaertn.). Purified NnPER1 protein protects DNA against the cleavage by ROS in the mixed-function oxidation system. The transcription and protein accumulation of NnPER1 increased during seed desiccation and imbibition and under abiotic stress treatment. Ectopic expression of NnPER1 in Arabidopsis enhanced the seed germination ability after controlled deterioration treatment (CDT), indicating that NnPER1 improves seed longevity of transgenic plants. Consistent with the function of NnPER1 on detoxifying ROS, we found that the level of ROS release and lipid peroxidation was strikingly lower in transgenic seeds compared to wild-type with or without CDT. Furthermore, transgenic Arabidopsis seeds ectopic-expressing NnPER1 displayed enhanced tolerance to high temperature and abscisic acid (ABA), indicating that NnPER1 may participate in the thermotolerance and ABA signaling pathway.