Maize ZmPT7 regulates Pi uptake and redistribution which is modulated by phosphorylation

Abstract

Phosphorus, an essential mineral macronutrient, is a major constituent of fertilizers for maize (Zea mays L.) production. However, the molecular mechanisms of phosphate (Pi) acquisition in maize plants and its redistribution remain unclear. This study presents the functional characterization of ZmPT7 in Pi uptake and redistribution in maize. The ZmPT7 was expressed in roots and leaves, and induced during Pi starvation. The ZmPT7 complemented the Pi-uptake deficiency of yeast mutant phoDnull and Arabidopsis mutant pht1;1D4D, indicating that ZmPT7 functioned as a Pi transporter. We generated zmpt7 mutants by CRISPR/Cas9 and ZmPT7overexpressing lines. The zmpt7 mutants showed reduced, whereas the ZmPT7-overexpressing lines displayed increased Pi-uptake capacity and Pi redistribution from old to young leaves, demonstrating that ZmPT7 played central roles in Pi acquisition and Pi redistribution from old to young leaves. The ZmCK2 kinases phosphorylated ZmPT7 at Ser-521 in old maize leaves, which enhanced transport activity of ZmPT7. The Ser-520 of Arabidopsis AtPHT1;1, a conserved residue of ZmPT7 Ser-521, was also phosphorylated by AtCK2 kinase, and the mutation of Ser-520 to Glu (phosphorylation mimic) yielded enhanced transport activity of AtPHT1;1. Taken together, these results indicate that ZmPT7 plays important roles in Pi acquisition and redistribution, and its transport activity is modulated by phosphorylation.

Potassium and phosphorus transport and signaling in plants

“…The first Pi transporters (PTs) identified in higher plant were Arabidopsis AtPT1 (AtPHT1;1) and AtPT2 (AtPHT1;4), which can complement a yeast (Saccharomyces cerevisiae) mutant deficient in the Pi transporter Pho84 (Muchhal et al, 1996). PHT1 is generally encoded by a multigene family; for example, Arabidopsis has nine PHT1 genes (AtPHT1;1 to AtPHT1;9) (Muchhal et al, 1996;Mudge et al, 2002;, rice has 13 PHT1 genes (Paszkowski et al, 2002;Liu et al, 2011), soybean (Glycine max) has 15 (Qin et al, 2012;Fan et al, 2013), maize has 13 (Nagy et al, 2006;Liu et al, 2016a;Wang et al, 2020a), and tomato (Solanum lycopersicum) has eight (Chen et al, 2014).…”

“…During the late vegetative and reproductive growth stages of plants, senescing leaves serve as a major source of nutrients for sink tissues, including young leaves, storage organs, and developing seeds (Veneklaas et al, 2012). Several PHT1 genes are expressed in the phloem of leaves, including barley (Hordeum vulgare) HvPHT1;6 (Rae et al, 2003), Arabidopsis AtPHT1;5 (Nagarajan et al, 2011), maize ZmPHT1;7 (Wang et al, 2020a), and rice OsPHT1;1 (Sun et al, 2012), OsPHT1;2 (Ai et al, 2009), OsPHT1;3 (Chang et al, 2019), and OsPHT1;8 (Jia et al, 2011). OsPHT1;1, OsPHT1;2, OsPHT1;3, OsPHT1;8, AtPHT1;5, and ZmPHT1;7 participate in the redistribution of shoot Pi from mature to younger organs.…”

“…Most PHT and PHO1 genes are transcriptionally upregulated by Pi deficiency and downregulated by excess Pi supply (Figure 2). Some TFs, mainly PHRs (phosphate starvation response) and WRKYs, are reported to play important roles in the regulation of transcription of PHT and PHO1 genes (Rubio et al, 2001;Devaiah et al, 2007aDevaiah et al, , 2007bZhou et al, 2008;Chen et al, 2009;Bustos et al, 2010;Dai et al, 2012Dai et al, , 2016Castrillo et al, 2013;Liang et al, 2014;Wang et al, 2014bWang et al, , 2020aSu et al, 2015;Gu et al, 2017;Zhang et al, 2020a). Unlike AtPHO1, rice OsPHO1 genes have a cisnatural antisense transcript (cis-NAT) (Secco et al, 2010), and cis-NAT OsPHO1;2 positively regulates OsPHO1;2 expression in response to Pi deficiency (Jabnoune et al, 2013).…”

“…Phosphorylation and dephosphorylation sites have been identified at the C-terminal end of PHT1 in Arabidopsis and rice (Nühse et al, 2004;Hem et al, 2007;Bayle et al, 2011;Chen et al, 2015b;Yang et al, 2020a;Wang et al, 2020a), and phosphorylation/dephosphorylation modification affects AtPHT1;1, OsPHT1;2, and OsPHT1;8 trafficking to the PM (Bayle et al, 2011;Chen et al, 2015b;Yang et al, 2020a) and the activity of the Pi transporters ZmPHT1;7 and AtPHT1;1 (Wang et al, 2020a). The PM localization of AtPHT1;1, OsPHT1;2, and OsPHT1;8 is also regulated by AtPHF1 (phosphate transporter traffic facilitator 1) and OsPHF1, and loss of AtPHF1 and OsPHF1 activity results in the retention of AtPHT1;1, OsPHT1;2, and OsPHT1;8 in the ER and reduces accumulation at the PM (González et al, 2005;Bayle et al, 2011;Chen et al, 2011).…”

“…NH 4 + is another important N source for plants. NH 4 + and K + have many similarities in terms of their hydrated diameters, charge, and influence on membrane potentials, which result in an antagonism between the two ions (Wang et al, 1996; ten Hoopen et al, 2010). External NH 4 + inhibits the high‐affinity K + uptake of plants, especially for the AtHAK5‐mediated K + uptake in Arabidopsis , apparently by competing for K + binding sites on the transporter (Spalding et al, 1999).…”