A cucumber protein, Phloem Phosphate Stress Repressed 1, rapidly degrades in response to a phosphate stress condition

Under depleted external phosphate (Pi), many plant species adapt to this stress by initiating downstream signalling cascades. In plants, the vascular system delivers nutrients and signalling agents to control physiological and developmental processes. Currently, limited information is available regarding the direct role of phloem-borne long-distance signals in plant growth and development under Pi-stress conditions. Here, we report on the identification and characterization of a cucumber protein, Cucumis sativus Phloem Phosphate-Stress-Repressed 1 (CsPPSR1), whose level in the phloem translocation stream rapidly responds to imposed Pi-limiting conditions. CsPPSR1 degradation is mediated by the 26S proteasome; under Pi-sufficient conditions, CsPPSR1 is stabilized by its phosphorylation, within the sieve tube system, through the action of CsPPSR1 Kinase. Further, we discovered that CsPPSR1 Kinase was susceptible to Pi-starvation-induced degradation, in the sieve tube system. Our findings offer insight into a molecular mechanism underlying the response of phloem-borne proteins to Pi-limited stress conditions.

Methods
In-gel kinase assays were performed as follows: Briefly, E. coli-purified GFP and CsPPSR1-4M8H were used as substrates. Fast protein liquid chromatography (FPLC) fractionated cucumber phloem sap proteins were separated on a substrate-containing SDS-PAGE (200 µg/ml of each substrate). Conditioned gels were incubated with 50 µM ATP and [γ32P]-ATP (20 Ci/mL) for in-gel phosphorylation. The reaction was terminated by incubating the gels with gel wash solution (5% TCA [V/V] and 1% [W/V] sodium pyrophosphate) and phosphorimaging using a Storm 860 system (GE Healthcare Lifesciences). To identify the kinase that phosphorylated CsPPSR1, named CsPPSRK, fractions with CsPPSR1-specific phosphorylation activity were separated on a 13% SDS-PAGE gel. Then the approximate 40 kDa protein band with phosphorylation activity was excised for mass spectrometry analysis. These mass spectral data were interrogated to identify sequences associated with a protein kinase of the appropriate mass.