Genetic analysis of the function of major leaf proteases in barley (Hordeum vulgare L.) nitrogen remobilization

Most of the nitrogen harvested with the seeds of annual crops is remobilized and retranslocated within the plant between anthesis and plant death. While chloroplasts contain most of the reduced nitrogen present in photosynthetically active leaf cells, the (major) pathway(s) involved in the degradation of their proteins prior to the retranslocation of the resulting amino acids are unknown. In this study, a population of 146 recombinant inbred barley lines (RIL), derived from the cross between two varieties with a highly inheritable difference in grain protein concentration, was used to map quantitative trait loci (QTL) for leaf amino-, carboxy- and endopeptidase activities relative to previously determined QTL for grain protein, leaf N storage, and remobilization. The results strongly suggested that major endopeptidases, assayed at both acidic and slightly alkaline pH values (favouring vacuolar and extravacuolar enzymes, respectively) are not instrumental in leaf N remobilization or the control of grain protein accumulation. Similarly, QTL determined for aminopeptidases (relative to QTL for N remobilization) indicated no functional role for the enzyme(s) assayed in plant N recycling. By contrast, careful evaluation of QTL data suggested that one or several carboxypeptidase isoenzymes may be involved in this physiologically and economically important process. As these proteases (in contrast to aminopeptidases) have previously been localized in vacuoles, this result appears intriguing. These data, while shedding new light on an old problem, also indicate that the described approach may prove useful in evaluating the functional roles of additional (not assayed in this study) proteolytic systems in whole-plant nitrogen recycling.