SUMMARYThe N-end rule relates the in vivo half-life of a protein to the identity of its N-terminal amino acid residue. While some N-terminal residues result in metabolically stable proteins, other, so-called destabilizing residues, lead to rapid protein turnover. The N-end rule pathway, which mediates the recognition and degradation of proteins with N-terminal destabilizing residues, is present in all organisms examined, including prokaryotes. This protein degradation pathway has a hierarchical organization in which some N-terminal residues, called primary destabilizing residues, are directly recognized by specific ubiquitin ligases. Other destabilizing residues, termed secondary and tertiary destabilizing residues, require modifications before the corresponding proteins can be targeted for degradation by ubiquitin ligases. In eukaryotes, the N-end rule pathway is a part of the ubiquitin/proteasome system and is known to play essential roles in a broad range of biological processes in fungi, animals and plants. While the structure of the N-end rule pathway has been extensively studied in yeast and mammals, knowledge of its organization in plants is limited. Using both tobacco and Arabidopsis, we identified the complete sets destabilizing and stabilizing N-terminal residues. We also characterized the hierarchical organization of the plant N-end rule by identifying and determining the specificity of two distinct N-terminal amidohydrolases (Nt-amidases) of Arabidopsis that are essential for the destabilizing activity of the tertiary destabilizing residues Asn and Gln. Our results indicate that both the N-end rule itself and mechanistic aspects of the N-end rule pathway in angiosperms are very similar to those of mammals.

Potatoes destined for crisping are normally stored above 8 degrees; below this glucose accumulates leading to very dark fry colors and potential acrylamide build up. Unfortunately, sprouting occurs above 4 degrees and impacts product quality, necessitating the use of sprout suppressant chemicals. Therefore, a goal of breeders is to develop potatoes with excellent fry color, which is maintained under storage below 8 degrees. Genomic or marker-assisted selection offers an opportunity to improve the efficiency of potato breeding and thereby assist breeders in achieving this goal. In this study, we have accumulated fry-color data on a large population of potato lines and combined this with genotypic data to carry out a GWAS and to evaluate accuracy of genomic prediction. We were able to identify a major QTL on chromosome 10 for fry color, and predict fry color with moderate accuracy using genome-wide markers. Furthermore, our results provide evidence that it is possible to identify a small subset of SNPs for processing characteristics that can give moderate predictive ability, albeit lower than that achieved with genome-wide markers.

The lpa1 mutations in maize are caused by lesions in the ZmMRP4 (multidrug resistance-associated proteins 4) gene. In previous studies (Raboy et al. in Plant Physiol 124:355-368, 2000; Pilu et al. in Theor Appl Genet 107:980-987, 2003a; Shi et al. Nat Biotechnol 25:930-937, 2007), several mutations have been isolated in this locus causing a reduction of phytic acid (myo-inositol-1,2,3,4,5,6-hexakisphosphate, or InsP(6)) content and an equivalent increasing of free phosphate. In particular, the lpa1-241 mutation causes a reduction of up to 90% of phytic acid, associated with strong pleiotropic effects on the whole plant. In this work, we show, for the first time to our knowledge, an interaction between the accumulation of anthocyanin pigments in the kernel and the lpa mutations. In fact the lpa1-241 mutant accumulates a higher level of anthocyanins as compared to wild type either in the embryo (about 3.8-fold) or in the aleurone layer (about 0.3-fold) in a genotype able to accumulate anthocyanin. Furthermore, we demonstrate that these pigments are mislocalised in the cytoplasm, conferring a blue pigmentation of the scutellum, because of the neutral/basic pH of this cellular compartment. As a matter of fact, the propionate treatment, causing a specific acidification of the cytoplasm, restored the red pigmentation of the scutellum in the mutant and expression analysis showed a reduction of ZmMRP3 anthocyanins' transporter gene expression. On the whole, these data strongly suggest a possible interaction between the lpa mutation and anthocyanin accumulation and compartmentalisation in the kernel.

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