Lack of ACTPK1, an STY kinase, enhances ammonium uptake and use, and promotes growth of rice seedlings under sufficient external ammonium.

Ammonium influx into plant roots via the high-affinity transport system (HATS) is down-modulated under elevated external ammonium, preventing ammonium toxicity. In ammonium-fed Arabidopsis, ammonium transporter 1 (AMT1) trimers responsible for HATS activity are allosterically inactivated in a dose-dependent manner via phosphorylation of the conserved threonine at the carboxyl-tail by the calcineurin B-like protein 1-calcineurin B-like protein-interacting protein kinase 23 complex and other yet unidentified protein kinases. Using transcriptome and reverse genetics in ammonium-preferring rice, we revealed the role of the serine/threonine/tyrosine protein kinase gene OsACTPK1 in down-modulation of HATS under sufficient ammonium. In wild-type roots, ACTPK1 mRNA and protein accumulated dose-dependently under sufficient ammonium. To determine the function of ACTPK1, two independent mutants lacking ACTPK1 were produced by retrotransposon Tos17 insertion. Compared with segregants lacking insertions, the two mutants showed decreased root growth and increased shoot growth under 1 mm ammonium due to enhanced ammonium acquisition, via aberrantly high HATS activity, and use. Furthermore, introduction of OsACTPK1 cDNA fused to the synthetic green fluorescence protein under its own promoter complemented growth and the HATS influx, and suggested plasma membrane localization. Root cellular expression of OsACTPK1 also overlapped with that of ammonium-induced OsAMT1;1 and OsAMT1;2. Meanwhile, threonine-phosphorylated AMT1 levels were substantially decreased in roots of ACTPK1-deficient mutants grown under sufficient ammonium. Bimolecular fluorescence complementation assay further confirmed interaction between ACTPK1 and AMT1;2 at the cell plasma membrane. Overall, these findings suggest that ACTPK1 directly phosphorylates and inactivates AMT1;2 in rice seedling roots under sufficient ammonium.

Cytosolic glutamine synthetase1;2 is responsible for the primary assimilation of ammonium in rice roots.

Among three genes for cytosolic glutamine synthetase (OsGS1;1, OsGS1;2 and OsGS1;3) in rice (Oryza sativa L.) plants, the OsGS1;2 gene is known to be mainly expressed in surface cells of roots, but its function was not clearly understood. We characterized knock-out mutants caused by the insertion of an endogenous retrotransposon Tos17 into exon 2 of OsGS1;2. Homozygously inserted mutants showed severe reduction in active tiller number and hence panicle number at harvest. Other yield components, such as spikelet number per panicle, 1,000-spikelet weight and proportion of well ripened grains, were nearly identical between the mutants and wild-type plants. When the contents of free amino acids in roots were compared between the mutants and the wild type, there were marked reductions in contents of glutamine, glutamate, asparagine and aspartate, but a remarkable increase in free ammonium ions in the mutants. Concentrations of amino acids and ammonium ions in xylem sap behaved in a similar fashion. Re-introduction of OsGS1;2 cDNA under the control of its own promoter into the knock-out mutants successfully restored yield components to wild-type levels as well as ammonium concentration in xylem sap. The results indicate that GS1;2 is important in the primary assimilation of ammonium ions taken up by rice roots, with GS1;1 in the roots unable to compensate for GS1;2 functions.

Cytosolic Glutamine Synthetase GS1;3 Is Involved in Rice Grain Ripening and Germination.

Ammonium is combined with glutamate to form glutamine. This reaction is catalyzed by glutamine synthetase (GS or GLN). Plants harbor several isoforms of cytosolic GS (GS1). Rice GS1;3 is highly expressed in seeds during grain filling and germination, suggesting a unique role in these processes. This study aimed to investigate the role of GS1;3 for rice growth and yield. Tos17 insertion lines for GS1;3 were isolated, and the nitrogen (N), amino acid, and ammonium contents of GS1;3 mutant grains were compared to wild-type grains. The spatiotemporal expression of GS1;3 and the growth and yield of rice plants were evaluated in hydroponic culture and the paddy field. Additionally, the stable isotope of N was used to trace the foliar N flux during grain filling. Results showed that the loss of GS1;3 retarded seed germination. Seeds of GS1;3 mutants accumulated glutamate but did not show a marked change in the level of phytohormones. The expression of GS1;3 was detected at the beginning of germination, with limited promoter activity in seeds. GS1;3 mutants showed a considerably decreased ripening ratio and decreased N efflux in the 12th leaf blade under N deficient conditions. The ß-glucuronidase gene expression under control of the GS1;3 promoter was detected in the vascular tissue and aleurone cell layer of developing grains. These data suggest unique physiological roles of GS1;3 in the early stage of seed germination and grain filling under N deficient conditions in rice.

Comparison of the expression, activity, and fecal concentration of intestinal alkaline phosphatase between healthy dogs and dogs with chronic enteropathy.

OBJECTIVE To compare expression, activity, and fecal concentration of intestinal alkaline phosphatase (IAP) between healthy dogs and dogs with chronic enteropathy (CE). ANIMALS 9 healthy university-owned Beagles and 109 healthy client-owned dogs (controls) and 28 dogs with CE (cases). PROCEDURES Cases were defined as dogs with persistent (> 3 weeks) gastrointestinal signs that failed to respond to antimicrobials and anti-inflammatory doses of prednisolone or dietary trials, did not have mechanical gastrointestinal abnormalities as determined by abdominal radiography and ultrasonography, and had a diagnosis of lymphoplasmacytic enteritis or eosinophilic gastroenteritis on histologic examination of biopsy specimens. Duodenal and colonic mucosa biopsy specimens were obtained from the 9 university-owned Beagles and all cases for histologic examination and determination of IAP expression (by real-time quantitative PCR assay) and activity (by enzyme histochemical analysis). Fecal samples were obtained from all dogs for determination of fecal IAP concentration by a quantitative enzyme reaction assay. RESULTS For dogs evaluated, IAP expression and activity were localized at the luminal side of epithelial cells in the mucosa and intestinal crypts, although both were greater in the duodenum than in the colon. Active IAP was detected in the feces of all dogs. Intestinal alkaline phosphatase expression and activity were lower for cases than for controls, and fecal IAP concentration for dogs with moderate and severe CE was lower than that for dogs with mild CE. CONCLUSIONS AND CLINICAL RELEVANCE Results indicated that dogs with CE had impaired IAP expression and activity. Additional research is necessary to elucidate the role of IAP in the pathogenesis of CE.