Extensive studies have been undertaken on senescence processes in barley and wheat and their importance for the nitrogen use efficiency of these crop plants. During the senescence processes, proteins are degraded and nutrients are re‐mobilised from senescing leaves to other organs, especially the developing grain. Most of the proteins degraded reside in the chloroplasts, with Rubisco constituting the most dominant protein fraction. Despite intensive studies, the proteases responsible for Rubisco degradation have not yet been identified. Evidence for degradation of stromal proteins outside of chloroplasts is summarised. Rubisco is thought to be released from chloroplasts into vesicles containing stroma material (RCB = Rubisco‐containing bodies). These vesicles may then take different routes for their degradation. Transcriptome analyses on barley and wheat senescence have identified genes involved in degradative, metabolic and regulatory processes that could be used in future strategies aimed at modifying the senescence process. The breeding of crops for characters related to senescence processes, e.g. higher yields and better nutrient use efficiency, is complex. Such breeding has to cope with the dilemma that delayed senescence, which could lead to higher yields, is correlated with a decrease in nutrient use efficiency. Pinpointing regulatory genes involved in senescence might lead to tools that could effectively overcome this dilemma.

Summary: The senescence process in wheat flag leaves was investigated over a time course from ear emergence until 50% yellowing of harvested leaf samples using an in‐house fabricated cDNA microarray based on a 9K wheat unigene set. The top 1000 ranked differentially expressed probes were subjected to a cluster analysis and, from these, we selected 140 up‐regulated genes with informative annotations. There was a considerable overlap between this list of genes and genes previously observed to be associated with senescence in other species, covering several functional categories involved in the degradation of macromolecules and nutrient remobilization, notably of nitrogen via the metabolism of carboxylic and amino acids. The up‐regulation of a number of genes in this metabolism was confirmed by real‐time polymerase chain reaction experiments. The data suggest a role for cytosolic/peroxisomal routes in the integration of the degradation of carbohydrates, fatty acids and proteins, leading to the remobilization of nitrogen. Illustrative examples of up‐regulated genes comprise cytoplasmic aconitate hydratase and peroxisomal citrate synthase. The data support a protective role of the mitochondria towards oxidative cell damage via the up‐regulation of the alternative oxidase, and possibly also involving the up‐regulated succinate dehydrogenase. A number of up‐regulated regulatory genes were also identified, notably NAC‐domain and WRKY transcription factors. These factors have previously been identified as being associated with senescence in other species. The data support the notion that a generic senescence programme exists across monocot and dicot plant species. However, notable differences can also be recognized. We thus found transcriptional up‐regulation of the biosynthetic pathway for benzoxazinoids, a group of graminaceous‐specific secondary metabolites.

New Breeding Techniques (NBTs) represent the newest technology for introduction of new variation into crop plants for plant breeding, in particular the methods that aim to make targeted mutagenesis at specific sites in the plant genome (NBT mutagenesis). In a decision from 2018, the EU Court of Justice deemed plants modified by these techniques to be GMO. The decision was based in a context of conventional plant breeding using mutagenesis of crop plants by physical or chemical treatments. These plants are explicitly exempted from the EU GMO regulation, based on the long-termed use of mutagenesis. In its decision, the EU Court claims that the NBTs operate “at a rate out of all proportion to those resulting from the application of conventional methods of mutagenesis”. In this paper, we argue that in fact this is not the case anymore; instead, a convergence has taken place between conventional mutagenesis and NBTs, in particular due to the possibilities of TILLING methods that allow the fast detection of mutations in any gene of a genome. Thus, by both strategies mutations in any gene across the genome can be obtained at a rather high speed. However, the differences between the strategies are 1) the precision of the exact site of mutation in a target gene, and 2) the number of off-target mutations affecting other genes than the target gene. Both aspects favour the NBT methods, which provide more precision and fewer off-target mutations. This is in stark contrast to the different status of the two technologies with respect to EU GMO regulation. In the future, this situation is not sustainable for the European plant breeding industry, since it is expected that restrictions on the use of NBTs will be weaker outside Europe. This calls for reconsiderations of the EU regulation of plants generated via NBTs.