SPX4 Negatively Regulates Phosphate Signaling and Homeostasis through Its Interaction with PHR2 in Rice  

Abstract

PHR2, a central regulator of phosphate signaling in rice, enhanced the expression of phosphate starvation-induced (PSI) genes and resulted in the enhancement of Pi acquisition under Pi deficiency stress. This occurred via PHR2 binding to a cis-element named the PHR1 binding sequences. However, the transcription level of PHR2 was not responsive to Pi starvation. So how is activity of transcription factor PHR2 adjusted to adapt diverse Pi status? Here, we identify an SPX family protein, Os-SPX4 (SPX4 hereafter), involving in Pi starvation signaling and acting as a negative regulator of PHR2. SPX4 is shown to be a fast turnover protein. When Pi is sufficient, through its interaction with PHR2, SPX4 inhibits the binding of PHR2 to its cis-element and reduces the targeting of PHR2 to the nucleus. However, when plants grow under Pi deficiency, the degradation of SPX4 is accelerated through the 26S proteasome pathway, thereby releasing PHR2 into the nucleus and activating the expression of PSI genes. Because the level of SPX4 is responsive to Pi concentration and SPX4 interacts with PHR2 and regulates its activity, this suggests that SPX4 senses the internal Pi concentration under diverse Pi conditions and regulates appropriate responses to maintain Pi homeostasis in plants.

Reducing the Genetic Redundancy of Arabidopsis PHOSPHATE TRANSPORTER1 Transporters to Study Phosphate Uptake and Signaling

“…In such case, phosphate transporters would be distinct from nitrate transporter CHL1, which acts as a transceptor (Ho et al, 2009). This is reinforced by a very recent report on OsSPX4 regulatory proteins in rice (Lv et al, 2014). These results demonstrate that an internal high-Pi concentration promotes interactions between SPX4 and PHR2 (the PHR1 rice homolog).…”

The Rice CK2 Kinase Regulates Trafficking of Phosphate Transporters in Response to Phosphate Levels

“…Thus, CK2, PHO2, and NLA generate a negative feedback loop for the regulation of PT activity according to plant Pi status. Such a negative loop is paralleled by a similar feedback mechanism for transcriptional control generated between the PHR1 master regulator of Pi starvation responses and several SPX proteins (Lv et al, 2014;Puga et al, 2014;Wang et al, 2014).…”

“…For the +P and 2P conditions, the concentration of NaH 2 PO 4 was adjusted to 200 or 0 mM, respectively, unless specified otherwise. Nicotiana benthamiana plants were cultivated in growth chambers as described before (Lv et al, 2014).…”

Integrative Comparison of the Role of the PHOSPHATE RESPONSE1 Subfamily in Phosphate Signaling and Homeostasis in Rice

“…PHR1 directly binds to the promoters of IPS1 (a noncoding RNA; Martín et al, 2000) and miR399 under Pi-deficient conditions (Rubio et al, 2001;Bustos et al, 2010;Lv et al, 2014;Wang et al, 2014). Next, miR399 represses the expression of PHO2, encoding an ubiquitin-conjugating E2 enzyme (UBIQUITIN-CONJUGATING ENZYME24) under Pi starvation conditions (Bari et al, 2006;Hu et al, 2011).…”

“…The responses to Pi and oxygen deficiency stress were recently shown to both be controlled by PHR1 (Klecker et al, 2014 (Miura et al, 2005). Suppressor of yeast (Saccharomyces cerevisiae) guanine nucleotidebinding protein subunit a1, Yeast Phosphatase 81 (cyclin-dependent kinase inhibitor in yeast PHO pathway), and xenotropic and polytropic retrovirus receptor1 family proteins regulate the transcriptional activity and localization of PHRs by directly interacting with these proteins in a Pi-dependent manner (Lv et al, 2014;Puga et al, 2014;Wang et al, 2014).…”