Plants take up and translocate nutrients through transporters. In Arabidopsis thaliana, the borate exporter BOR1 acts as a key transporter under boron (B) limitation in the soil. Upon sufficient-B supply, BOR1 undergoes ubiquitination and is transported to the vacuole for degradation, to avoid overaccumulation of B. However, the mechanisms underlying B-sensing and ubiquitination of BOR1 are unknown. In this study, we confirmed the lysine-590 residue in the C-terminal cytosolic region of BOR1 as the direct ubiquitination site and showed that BOR1 undergoes K63-linked polyubiquitination. A forward genetic screen identified that amino acid residues located in vicinity of the substrate-binding pocket of BOR1 are essential for the vacuolar sorting. BOR1 variants that lack B-transport activity showed a significant reduction of polyubiquitination and subsequent vacuolar sorting. Coexpression of wild-type (WT) and a transport-defective variant of BOR1 in the same cells showed degradation of the WT but not the variant upon sufficient-B supply. These findings suggest that polyubiquitination of BOR1 relies on its conformational transition during the transport cycle. We propose a model in which BOR1, as a B transceptor, directly senses the B concentration and promotes its own polyubiquitination and vacuolar sorting for quick and precise maintenance of B homeostasis.

The Large scale distributed system is constructed over long period of time step by step. Even under the construction, on-line operation is started in its system. Afer starting on-line operation, it continuously has to be tested and maintained in various area without stopping on-line operation. On the conventional on-line test technique on Autonomous Decentralized System, the on-line and the test data exist concurrently in the same system. The real-time response of on-line system is not assured.In this Paper, by clustering testlon-line subsystems and controlling data flow among each cluster autonomously, each different type of subsystem is isolated and protects perturbation from the other subsystem. The reliability and real time response of on-line system is assured.

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