Regulatory Network of MicroRNA399 andPHO2by Systemic Signaling    

Abstract

Recently, we showed that microRNA399s (miR399s) control inorganic phosphate (Pi) homeostasis by regulating the expression of PHO2 encoding a ubiquitin-conjugating E2 enzyme 24. Arabidopsis (Arabidopsis thaliana) plants overexpressing miR399 or the pho2 mutant overaccumulate Pi in shoots. The association of Pi translocation and coexpression of miR399s and PHO2 in vascular tissues suggests their involvement in long-distance signaling. In this study, we used reciprocal grafting between wild-type and miR399-overexpressing transgenic plants to dissect the systemic roles of miR399 and PHO2. Arabidopsis rootstocks overexpressing miR399 showed high accumulation of Pi in the wild-type scions because of reduced PHO2 expression in the rootstocks. Although miR399 precursors or expression was not detected, we found a small but substantial amount of mature miR399 in the wild-type rootstocks grafted with transgenic scions, which indicates the movement of miR399 from shoots to roots. Suppression of PHO2 with miR399b or c was less efficient than that with miR399f. Of note, findings in grafted Arabidopsis were also discovered in grafted tobacco (Nicotiana benthamiana) plants. The analysis of the pho1 mutant provides additional support for systemic suppression of PHO2 by the movement of miR399 from Pi-depleted shoots to Pi-sufficient roots. We propose that the long-distance movement of miR399s from shoots to roots is crucial to enhance Pi uptake and translocation during the onset of Pi deficiency. Moreover, PHO2 small interfering RNAs mediated by the cleavage of miR399s may function to refine the suppression of PHO2. The regulation of miR399 and PHO2 via long-distance communication in response to Pi deficiency is discussed.

Physiological and molecular alterations in plants exposed to high [CO2] under phosphorus stress

“…Moreover, transcription factors such as PHR1 (OsPHR1, OsPHR2, PvPHR1, ZmPHR1, TaPHR1), PTF1 (OsPTF1, ZmPTF1), MYB2P-1 (OsMYB2P1), MYB62, WRKY (WRKY75, WRKY6), bHLH32 and ZAT6 are involved in the signalling network to regulate plant adaptation to P stress (Yi et al, 2005;Devaiah et al, 2007a, b;Chen et al, 2007;Valdes-Lopez et al, 2008;Zhou et al, 2008;Devaiah et al, 2009;Chen et al, 2009;Li et al, 2011;Dai et al, 2012;Wang et al, 2013). The posttranscriptional regulation as well as long-distance signal is carried out by microRNAs, for instance miR399 maintain P homeostasis by regulating P transporter PHO2 (Bari et al, 2006;Aung et al, 2006;Lin et al, 2008;Pant et al, 2008). Although the molecular components of P stress signalling in plants has been identified, the overall pathway is still poorly understood and requires further investigation.…”

Vacuolar Ca2+/H+ Transport Activity Is Required for Systemic Phosphate Homeostasis Involving Shoot-to-Root Signaling in Arabidopsis

“…The miR399 generated in shoots after the onset of P i starvation serves as a long-distance signal to activate P i transport systems by suppressing PHO2 expression in roots (Lin et al, 2008;Pant et al, 2008). Our results obtained in split-root experiments indicate that CAX1 and CAX3 do not mediate the generation and movement of systemic suppressors from the +P i root half to the 2P i root half.…”

“…In accordance with the inverse correlation between miR399 and PHO2 mRNA levels, transgenic plants overexpressing miR399 phenocopied the pho2 mutant and PHO2 T-DNA knockout lines Chiou et al, 2006). Reciprocal micrografting experiments further demonstrated that a pho2 root genotype is sufficient and necessary for P i accumulation in the shoot, whereas the shoot-to-root movement of mature miR399 is responsible for the degradation of PHO2 mRNA in roots (Bari et al, 2006;Lin et al, 2008;Pant et al, 2008). Moreover, upregulation of miR399 by P i deprivation is mediated by the PHOSPHATE STARVATION RESPONSE1 (PHR1) transcription factor, a key positive regulator of multiple P i starvation-induced (PSi) genes (Rubio et al, 2001;Bari et al, 2006).…”

“…4C). As the pho2 rootstock genotype has been shown sufficient for P i accumulation in the scion (Bari et al, 2006;Lin et al, 2008), the role of PHO2 in the shoot and the implication of increased transcript levels of PHO2 in the shoot of cax1/cax3 remain to be resolved. On the other hand, the level of PHO2 mRNA in the root of cax1/cax3 was reduced by 36% under +P i conditions as compared with wild-type plants but was not as low as that detected in the root of wild-type plants under P i deficiency (Fig.…”

“…Because both pho2 and cax1/cax3 mutants displayed an increased level of shoot P i , it is tempting to know whether PHO2 and CAX1/CAX3 act in the same P i signaling pathway. Down-regulation of PHO2 in the roots results in increased uptake and root-to-shoot translocation of P i ; however, the role of PHO2 in the shoots is unclear (Lin et al, 2008). Here, we observed that the PHO2 transcript level was greatly increased in the shoot of cax1/cax3 under +P i and 2P i conditions, whereas the level in the root of cax1/cax3 under +P i conditions was slightly reduced but still higher than that in the 2P i root of wild-type plants (Fig.…”