Salinity promotes opposite patterns of carbonylation and nitrosylation of C4 phosphoenolpyruvate carboxylase in sorghum leaves.

MAIN CONCLUSION: Carbonylation inactivates sorghum C 4 PEPCase while nitrosylation has little impact on its activity but holds back carbonylation. This interplay could be important to preserve photosynthetic C4 PEPCase activity in salinity. Previous work had shown that nitric acid (NO) increased phosphoenolpyruvate carboxylase kinase (PEPCase-k) activity, promoting the phosphorylation of phosphoenolpyruvate carboxylase (PEPCase) in sorghum leaves (Monreal et al. in Planta 238:859-869, 2013b). The present work investigates the effect of NO on C4 PEPCase in sorghum leaves and its interplay with carbonylation, an oxidative modification frequently observed under salt stress. The PEPCase of sorghum leaves could be carbonylated in vitro and in vivo, and this post-translational modification (PTM) was accompanied by a loss of its activity. Similarly, PEPCase could be S-nitrosylated in vitro and in vivo, and this PTM had little impact on its activity. The S-nitrosylated PEPCase showed increased resistance towards subsequent carbonylation, both in vitro and in vivo. Under salt shock, carbonylation of PEPCase increased in parallel with decreased S-nitrosylation of the enzyme. Subsequent increase of S-nitrosylation was accompanied by decreased carbonylation. Taken together, the results suggest that S-nitrosylation could contribute to maintain C4 PEPCase activity in stressed sorghum plants. Thus, salt-induced NO synthesis would be protecting photosynthetic PEPCase activity from oxidative inactivation while promoting its phosphorylation, which will guarantee its optimal functioning in suboptimal conditions.

Phosphoenolpyruvate carboxylase (PEPC) and PEPC-kinase (PEPC-k) isoenzymes in Arabidopsis thaliana: role in control and abiotic stress conditions.

MAIN CONCLUSION: Arabidopsis ppc3 mutant has a growth-arrest phenotype and is affected in phosphate- and salt-stress responses, showing that this protein is crucial under control or stress conditions. Phosphoenolpyruvate carboxylase (PEPC) and its dedicated kinase (PEPC-k) are ubiquitous plant proteins implicated in many physiological processes. This work investigates specific roles for the three plant-type PEPC (PTPC) and the two PEPC-k isoenzymes in Arabidopsis thaliana. The lack of any of the PEPC isoenzymes reduced growth parameters under optimal growth conditions. PEPC activity was decreased in shoots and roots of ppc2 and ppc3 mutants, respectively. Phosphate starvation increased the expression of all PTPC and PPCK genes in shoots, but only PPC3 and PPCK2 in roots. The absence of any of these two proteins was not compensated by other isoforms in roots. The effect of salt stress on PTPC and PPCK expression was modest in shoots, but PPC3 was markedly increased in roots. Interestingly, both stresses decreased root growth in each of the mutants except for ppc3. This mutant had a stressed phenotype in control conditions (reduced root growth and high level of stress molecular markers), but was unaffected in their response to high salinity. Salt stress increased PEPC activity, its phosphorylation state, and L-malate content in roots, all these responses were abolished in the ppc3 mutant. Our results highlight the importance of the PPC3 isoenzyme for the normal development of plants and for root responses to stress.

Nitric oxide regulation of leaf phosphoenolpyruvate carboxylase-kinase activity: implication in sorghum responses to salinity.

Nitric oxide (NO) is a signaling molecule that mediates many plant responses to biotic and abiotic stresses, including salt stress. Interestingly, salinity increases NO production selectively in mesophyll cells of sorghum leaves, where photosynthetic C4 phosphoenolpyruvate carboxylase (C4 PEPCase) is located. PEPCase is regulated by a phosphoenolpyruvate carboxylase-kinase (PEPCase-k), which levels are greatly enhanced by salinity in sorghum. This work investigated whether NO is involved in this effect. NO donors (SNP, SNAP), the inhibitor of NO synthesis NNA, and the NO scavenger cPTIO were used for long- and short-term treatments. Long-term treatments had multifaceted consequences on both PPCK gene expression and PEPCase-k activity, and they also decreased photosynthetic gas-exchange parameters and plant growth. Nonetheless, it could be observed that SNP increased PEPCase-k activity, resembling salinity effect. Short-term treatments with NO donors, which did not change photosynthetic gas-exchange parameters and PPCK gene expression, increased PEPCase-k activity both in illuminated leaves and in leaves kept at dark. At least in part, these effects were independent on protein synthesis. PEPCase-k activity was not decreased by short-term treatment with cycloheximide in NaCl-treated plants; on the contrary, it was decreased by cPTIO. In summary, NO donors mimicked salt effect on PEPCase-k activity, and scavenging of NO abolished it. Collectively, these results indicate that NO is involved in the complex control of PEPCase-k activity, and it may mediate some of the plant responses to salinity.

Silencing of SbPPCK1-3 Negatively Affects Development, Stress Responses and Productivity in Sorghum.

Phosphoenolpyruvate carboxylase (PEPC) plays central roles in photosynthesis, respiration, amino acid synthesis, and seed development. PEPC is regulated by different post-translational modifications. Between them, the phosphorylation by PEPC-kinase (PEPCk) is widely documented. In this work, we simultaneously silenced the three sorghum genes encoding PEPCk (SbPPCK1-3) by RNAi interference, obtaining 12 independent transgenic lines (Ppck1-12 lines), showing different degrees of SbPPCK1-3 silencing. Among them, two T2 homozygous lines (Ppck-2 and Ppck-4) were selected for further evaluation. Expression of SbPPCK1 was reduced by 65% and 83% in Ppck-2 and Ppck-4 illuminated leaves, respectively. Expression of SbPPCK2 was higher in roots and decreased by 50% in Ppck-2 and Ppck-4 in this tissue. Expression of SbPPCK3 was low and highly variable. Despite the incomplete gene silencing, it decreased the degree of phosphorylation of PEPC in illuminated leaves, P-deficient plants, and NaCl-treated plants. Both leaves and seeds of Ppck lines had altered metabolic profiles and a general decrease in amino acid content. In addition, Ppck lines showed delayed flowering, and 20% of Ppck-4 plants did not produce flowers at all. The total amount of seeds was lowered by 50% and 36% in Ppck-2 and Ppck-4 lines, respectively. The quality of seeds was lower in Ppck lines: lower amino acid content, including Lys, and higher phytate content. These data confirm the relevance of the phosphorylation of PEPC in sorghum development, stress responses, yield, and quality of seeds.

Phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxylase kinase isoenzymes play an important role in the filling and quality of Arabidopsis thaliana seed.

Three plant-type phosphoenolpyruvate carboxylase (PPC1 to PPC3) and two phosphoenolpyruvate carboxylase kinase (PPCKs: PPCK1 and 2) genes are present in the Arabidopsis thaliana genome. In seeds, all PPC genes were found to be expressed. Examination of individual ppc mutants showed little reduction of PEPC protein and global activity, with the notable exception of PPC2 which represent the most abundant PEPC in dry seeds. Ppc mutants exhibited moderately lower seed parameters (weight, area, yield, germination kinetics) than wild type. In contrast, ppck1-had much altered (decreased) yield. At the molecular level, ppc3-was found to be significantly deficient in global seed nitrogen (nitrate, amino-acids, and soluble protein pools). Also, N-deficiency was much more marked in ppck1-, which exhibited a tremendous loss of 95% and 90% in nitrate and proteins, respectively. The line ppck2-had accumulated amino-acids but lower levels of soluble proteins. Regarding carboxylic acid pools, Krebs cycle intermediates were found to be diminished in all mutants; this was accompanied by a consistent decrease in ATP. Lipids were stable in ppc mutants, however ppck1-seeds accumulated more lipids while ppck2-seeds showed high level of polyunsaturated fatty acid oleic and linolenic (omega 3). Altogether, the results indicate that the complete PEPC and PPCK family are needed for normal C/N metabolism ratio, growth, development, yield and quality of the seed.

Genetic and Pharmacological Inhibition of Autophagy increases the Monoubiquitination of Non-Photosynthetic Phosphoenolpyruvate Carboxylase.

Phosphoenolpyruvate carboxylase (PEPC) is an enzyme with key roles in carbon and nitrogen metabolisms. The mechanisms that control enzyme stability and turnover are not well known. This paper investigates the degradation of PEPC via selective autophagy, including the role of the monoubiquitination of the enzyme in this process. In Arabidopsis, the genetic inhibition of autophagy increases the amount of monoubiquitinated PEPC in the atg2, atg5, and atg18a lines. The same is observed in nbr1, which is deficient in a protein that recruits monoubiquitinated substrates for selective autophagy. In cultured tobacco cells, the chemical inhibition of the degradation of autophagic substrates increases the quantity of PEPC proteins. When the formation of the autophagosome is blocked with 3-methyladenine (3-MA), monoubiquitinated PEPC accumulates as a result. Finally, pull-down experiments with a truncated version of NBR1 demonstrate the recovery of intact and/or fragmented PEPC in Arabidopsis leaves and roots, as well as cultured tobacco cells. Taken together, the results show that a fraction of PEPC is cleaved via selective autophagy and that the monoubiquitination of the enzyme has a role in its recruitment towards this pathway. Although autophagy seems to be a minor pathway, the results presented here increase the knowledge about the role of monoubiquitination and the regulation of PEPC degradation.

Proline synthesis in barley under iron deficiency and salinity.

This work investigates proline synthesis in six barley varieties subjected to iron deficiency, salinity or both stresses. The highest growth under Fe sufficiency corresponded to Belgrano and Shakira. A moderate augment of leaf phosphoenolpyruvate carboxylase (PEPC) activity was observed in all six varieties in response to Fe deficiency, consistently in leaves and sporadically in roots. All six varieties accumulated proline under Fe deficiency, to a higher extent in leaves than in roots. The decrease of Fe supply from 100 µM NaFe(III)-EDTA to 0.5 µM NaFe(III)-EDTA reduced growth and photosynthetic pigments similarly in the six barley varieties. On the contrary, differences between varieties could be observed with respect to increased or, conversely, decreased proline content as a function of the amount of NaFe(III)-EDTA supplied. These two opposite types were represented by Belgrano (higher proline under Fe deficiency) and Shakira (higher proline under Fe sufficiency). Time-course experiments suggested that leaf PEPC activity was not directly responsible for supplying C for proline synthesis under Fe deficiency. High proline levels in the leaves of Fe-deficient Belgrano plants in salinity were associated to a better performance of this variety under these combined stresses.