Abstract
To better understand the mechanism of sugar signaling in rice cell, the suspension-cultured rice cells were transferred from sucrose-containing (+S) to sucrose-free (-S) of MS culture medium, we found that ribosomal RNAs (rRNAs) were degraded progressively. This suggests that carbon, nitrogen, and phosphate were recycled in this process and the reduction in cellular rRNAs might lead to decreased translation to save energy in response to sugar starvation. Differential screening revealed that two groups of genes, sugar-starvation-repressed (SSR) and sugar-starvation-activated (SSA) genes, were regulated by sugar in an opposing manner. Northern-blot analysis showed that two major hybridization signals of 0.8 and 1.9 kb were induced strongly under sugar starvation. The two populations of genes corresponded with homologs of α-amylases (1.9 kb) and the glycine-rich proteins (GRPs) gene family (0.8 kb), and all were SSA genes. Expression of GRP genes was strongly induced in sugar-starved cells, which suggests that GRPs may help to protect cells against nutritional stress. Treatment of +S and -S cells with the protein kinase (PK) inhibitor staurosporine (St) and the serine/theronine phosphoprotein phosphatases 1 (PP1) and 2A (PP2A) inhibitor okadaic acid (OA) revealed that PP1 and PP2A (PPs) might be involved in increasing SSR gene expression in +S cells, and that activation of the majority of the SSA genes in -S cells might be due to PKs activity. These results suggested that PKs and PPs might be involved in the sugar regulation of SSR and SSA gene expression. An in-gel PK activity assay demonstrated that the activity of two classes of PKs (50 and 66 kDa) may be induced rapidly after transfer of +S cells to -S medium. Following transfer of -S cells to +S medium, a novel class of 38 kDa PK was induced rapidly and showed high activity. The 38 kDa PK might play a role in sugar sensing, and the 50 and 66 kDa PKs might play roles in signal sensing under sugar starvation in rice cells. These results provide valuable information on three classes of protein kinases that might play key roles in sugar sensing and signaling in rice.
Highlights
Carbohydrates are important nutrients in metabolism and important structural constituents in plants
To better understand the mechanism of sugar signaling in rice cell, the suspension-cultured rice cells were transferred from sucrose-containing (+S) to sucrose-free (−S) of Murashige and Skoog (MS) culture medium, we found that ribosomal RNAs were degraded progressively
These results suggest that reduction in the amount of ribosomal RNAs (rRNAs) might be due to increased degradation rather than decreased synthesis during sucrose starvation
Summary
Carbohydrates are important nutrients in metabolism and important structural constituents in plants. A variety of genes involved in key metabolic processes are negatively regulated by sugars, such as α-amylase in rice [9] [10], and the ribulose 1,5-bisphosphate carboxylase/oxygenase small subunit (rbcS) and chlorophyll a/b binding protein (Cab) in maize protoplasts and suspension-cultured Chenopodium cells [11] [12]. A GeneChip analysis detected transcriptional up-regulation of 343 genes in sugar-starved suspension-cultured Arabidopsis cells [13]. These genes are involved in the recycling of cellular components and nutrient scavenging, in a variety of defense and stress-response pathways, with specific protein kinases and transcription factors regulating these processes
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