Wei Chi, Jing Li, Baoye He, Xin Chai, Xiumei Xu, Xuwu Sun, Jingjing Jiang, Peiqiang Feng, Jianru Zuo, Rongcheng Lin, Jean-David Rochaix, and Lixin Zhang

Significance

Selective protein proteolysis is essential for many plant signal transduction pathways and regulates developmental stages of a plant. In addition to the well-characterized ubiquitin-proteasome system, other factors appear to be involved in the degradation of plant signaling components. Here we describe the function of the serine protease degradation of periplasmic protein 9 (DEG9) in plant signaling. We found that DEG9 mediates the degradation of ARABIDOPSIS RESPONSE REGULATOR 4, which is critical for regulating the cross-talk between cytokinin and light-signaling pathways. This study adds to our knowledge about the function of DEG proteases, which are common in the plant kingdom, and emphasizes their importance in plant development.

Abstract

Cytokinin is an essential phytohormone that controls various biological processes in plants. A number of response regulators are known to be important for cytokinin signal transduction. ARABIDOPSIS RESPONSE REGULATOR 4 (ARR4) mediates the cross-talk between light and cytokinin signaling through modulation of the activity of phytochrome B. However, the mechanism that regulates the activity and stability of ARR4 is unknown. Here we identify an ATP-independent serine protease, degradation of periplasmic proteins 9 (DEG9), which localizes to the nucleus and regulates the stability of ARR4. Biochemical evidence shows that DEG9 interacts with ARR4, thereby targeting ARR4 for degradation, which suggests that DEG9 regulates the stability of ARR4. Moreover, genetic evidence shows that DEG9 acts upstream of ARR4 and regulates the activity of ARR4 in cytokinin and light-signaling pathways. This study thus identifies a role for a ubiquitin-independent selective protein proteolysis in the regulation of the stability of plant signaling components.

See: http://www.pnas.org/content/113/25/E3568.full

PNAS June 21 2016; vol.113; no.25: E3568–E3576

Fig. 1.

Involvement of DEG9 in cytokinin signaling. (A) Expression of DEG9 assayed by qRT-PCR. Ten-day-old WT seedlings were treated with 20 µM t-zeatin, aminocyclopropane-1-carboxylic acid (ACC), gibberellin 3 (GA₃), or brassinolide (BL) for 0–6 h and the accumulation of DEG9 transcripts was analyzed by qRT-PCR at different times. Means ± SD (n = 6) are shown. (B) Accumulation of DEG9 protein under different phytohormone treatments. The histone H3 protein was used as a control. The treatment was as in A. (C) Accumulations of ARR4, ARR5, ARR7, and ARR15 mRNAs were analyzed by qRT-PCR in WT, deg9, and DEG9-OX plants treated with 100 nM t-zeatin for 1 h. Data are means ± SD (n = 10).