Protein turnover is a key component in cellular homeostasis; however, there is little quantitative information on degradation kinetics for individual plant proteins. We have used 15 N labeling of barley (Hordeum vulgare) plants and gas chromatography-mass spectrometry analysis of free amino acids and liquid chromatography-mass spectrometry analysis of proteins to track the enrichment of 15 N into the amino acid pools in barley leaves and then into tryptic peptides derived from newly synthesized proteins. Using information on the rate of growth of barley leaves combined with the rate of degradation of 14 N-labeled proteins, we calculate the turnover rates of 508 different proteins in barley and show that they vary by more than 100-fold. There was approximately a 9-h lag from label application until 15 N incorporation could be reliably quantified in extracted peptides. Using this information and assuming constant translation rates for proteins during the time course, we were able to quantify degradation rates for several proteins that exhibit half-lives on the order of hours. Our workflow, involving a stringent series of mass spectrometry filtering steps, demonstrates that 15 N labeling can be used for large-scale liquid chromatography-mass spectrometry studies of protein turnover in plants. We identify a series of abundant proteins in photosynthesis, photorespiration, and specific subunits of chlorophyll biosynthesis that turn over significantly more rapidly than the average protein involved in these processes. We also highlight a series of proteins that turn over as rapidly as the well-known D1 subunit of photosystem II. While these proteins need further verification for rapid degradation in vivo, they cluster in chlorophyll and thiamine biosynthesis.

Toxin production during cyanobacterial blooms poses a significant public health threat in water bodies globally and requires the development of effective bloom management strategies. Previously, synthesis of the hepatotoxin microcystin has been proposed to be regulated by iron availability, but the contribution of the toxin to the adaptation of cyanobacteria to environmental stresses, such as changing light intensity and nutrient limitation, remains unclear. The aim of this study was to compare the iron stress response in toxic and non-toxic strains of Microcystis aeruginosa subjected to moderate and severe iron limitation. The transcription of a number of genes involved in iron uptake, oxidative stress response, toxin synthesis and transcriptional control of these processes was accessed by quantitative real-time PCR (qRT-PCR). The process of adaptation of M. aeruginosa to iron stress was found to be highly dynamic and strain-specific. Toxin production in PCC 7806 increased in an iron-dependent manner and appeared to be regulated by FurA. The inability to produce microcystin, either due to natural mutations in the mcy gene cluster or due to insertional inactivation of mcyH, affected the remodelling of the photosynthetic machinery in iron-stressed cells, the transport of Fe(II) and transcription of the Fur family of transcriptional regulators. The presence of the toxin appears to give an advantage to microcystin-producing cyanobacteria in the early stages of exposure to severe iron stress and may protect the cell from reactive oxygen species-induced damage.

Toxin production in algal blooms presents a significant problem for the water industry. Of particular concern is microcystin, a potent hepatotoxin produced by the unicellular freshwater species Microcystis aeruginosa. In this study, the proteomes of six toxic and nontoxic strains of M. aeruginosa were analyzed to gain further knowledge in elucidating the role of microcystin production in this microorganism. This represents the first comparative proteomic study in a cyanobacterial species. A large diversity in the protein expression profiles of each strain was observed, with a significant proportion of the identified proteins appearing to be strain-specific. In total, 475 proteins were identified reproducibly and of these, 82 comprised the core proteome of M. aeruginosa. The expression of several hypothetical and unknown proteins, including four possible operons was confirmed. Surprisingly, no proteins were found to be produced only by toxic or nontoxic strains. Quantitative proteome analysis using the labelfree normalized spectrum abundance factor approach revealed nine proteins that were differentially expressed between toxic and nontoxic strains. These proteins participate in carbon-nitrogen metabolism and redox balance maintenance and point to an involvement of the global nitrogen regulator NtcA in toxicity. In addition, the switching of a previously inactive toxin-producing strain to microcystin synthesis is reported. Molecular & Cellular

Phosphate is an essential element for plants and is involved in the composition of sugar phosphates, nucleic acids, membrane lipids, and energy metabolism via the generation of ATP. Crop farming requires the application of large amounts of phosphate fertilizer, but the fossilized rock deposits used as a commercial source of phosphate fertilizer are a nonrenewable resource that is predicted to reach a peak within the next century and drive plant production costs up as the global demand for food increases. Recent progress in the identification of key molecular regulators of the plant response to phosphate deprivation has highlighted differences in the response of the model plant Arabidopsis compared to economically important crops. This review focuses on the potential of proteomics to unravel the common and specific biochemical changes that contribute to phosphate use efficiency in cultivars of rice, maize, and oilseed rape. Proteome studies reveal a wide scope of species-specific metabolic strategies that lead to changes in root morphology and metabolism, driven by secretion of specific proteins and alteration of energy metabolism, carbon, and nitrogen assimilation inside the root cells. Understanding of the mechanisms underlying plant phosphate use efficiency in crops is critical for developing sustainable agriculture practices.