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The response of shoots to phosphate (Pi) deficiency implicates long-distance communication between roots and shoots, but the participating components are poorly understood. We have studied the topology of the Arabidopsis (Arabidopsis thaliana) PHOSPHATE1 (PHO1) Pi exporter and defined the functions of its different domains in Pi homeostasis and signaling. The results indicate that the amino and carboxyl termini of PHO1 are both oriented toward the cytosol and that the protein spans the membrane twice in the EXS domain, resulting in a total of six transmembrane a-helices. Using transient expression in Nicotiana benthamiana leaf, we demonstrated that the EXS domain of PHO1 is essential for Pi export activity and proper localization to the Golgi and trans-Golgi network, although the EXS domain by itself cannot mediate Pi export. In contrast, removal of the aminoterminal hydrophilic SPX domain does not affect the Pi export capacity of the truncated PHO1 in N. benthamiana. While the Arabidopsis pho1 mutant has low shoot Pi and shows all the hallmarks associated with Pi deficiency, including poor shoot growth and overexpression of numerous Pi deficiency-responsive genes, expression of only the EXS domain of PHO1 in the roots of the pho1 mutant results in a remarkable improvement of shoot growth despite low shoot Pi. Transcriptomic analysis of pho1 expressing the EXS domain indicates an attenuation of the Pi signaling cascade and the up-regulation of genes involved in cell wall synthesis and the synthesis or response to several phytohormones in leaves as well as an altered expression of genes responsive to abscisic acid in roots.

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Abstract:Seed production requires the transfer of nutrients from the maternal seed coat to the filial endosperm and embryo. Since seed coat and filial tissues are symplasmically isolated, nutrients arriving in the seed coat via the phloem must be exported to the apoplast before reaching the embryo. Proteins implicated in the transfer of inorganic phosphate (Pi) from the seed coat to the embryo are unknown despite seed P content being an important agronomic trait. Here, we show that the Arabidopsis Pi exporter PHO1 and PHOH1 are expressed in the chalazal seed coat (CZSC) of developing seeds. PHO1 is additionally expressed in developing ovules. Phosphorus (P) content and Pi flux between seed coat and embryo was analyzed in seeds from grafts between WT roots and scions from either pho1, phoh1 or pho1 phoh1 double mutant. While P content and distribution between seed coat and embryo in fully mature dry seeds of these mutants are similar to WT, at the mature green stage of seed development the seed coat of the pho1 and pho1 phoh1 mutants, but not of the phoh1 mutant, retains approximately 2-fold more P than its WT control. Expression of PHO1 under a CZSCspecific promoter complemented the seed P distribution phenotype of the pho1 phoh1 double mutant. CZSC-specific down-expression of PHO1 also recapitulated the seed P distribution phenotype of pho1. Together, these experiments show that PHO1 expression in the CZSC is important for the transfer of P from seed coat to the embryo in developing seeds. SummarySeed production requires the transfer of nutrients from the maternal seed coat to the filial endosperm and embryo. Since seed coat and filial tissues are symplasmically isolated, nutrients arriving in the seed coat via the phloem must be exported to the apoplast before reaching the embryo. Proteins implicated in the transfer of inorganic phosphate (Pi) from the seed coat to the embryo are unknown despite seed P content being an important agronomic trait. Here, we show that the Arabidopsis Pi exporter PHO1 and PHOH1 are expressed in the chalazal seed coat (CZSC) of developing seeds. PHO1 is additionally expressed in developing ovules. Phosphorus (P) content and Pi flux between seed coat and embryo was analyzed in seeds from grafts between WT roots and scions from either pho1, phoh1 or pho1 phoh1 double mutant. While P content and distribution between seed coat and embryo in fully mature dry seeds of these mutants are similar to WT, at the mature green stage of seed development the seed coat of the pho1 and pho1 phoh1 mutants, but not of the phoh1 mutant, retains approximately 2-fold more P than its WT control. Expression of PHO1 under a CZSCspecific promoter complemented the seed P distribution phenotype of the pho1 phoh1 double mutant. CZSC-specific down-expression of PHO1 also recapitulated the seed P distribution phenotype of pho1. Together, these experiments show that PHO1 expression in the CZSC is important for the transfer of P from seed coat to the embryo in developing seeds.

Accumulation of incompletely folded proteins in the endoplasmic reticulum (ER) leads to ER stress, activates ER protein degradation pathways, and upregulates genes involved in protein folding (Unfolded Protein Response; UPR). ER stress has been associated with abiotic stress conditions that affect protein folding, including salt stress. However, the role of ER protein folding in plant responses to nutrient deficiencies is unclear. We analyzed several Arabidopsis thaliana mutants affected in ER protein quality control and established that both CALNEXIN (CNX) genes function in the primary root response to phosphate (Pi) deficiency. CNX and calreticulin (CRT) are homologous ER lectins that bind to N-glycosylated proteins to promote their folding. Growth of cnx1-1 and cnx2-2 single mutants was similar to that of the wild type under high and low Pi conditions, but the cnx1-1 cnx2-2 double mutant showed decreased primary root growth under low Pi conditions due to reduced meristematic cell division. This phenotype was specific to Pi deficiency; the double mutant responded normally to osmotic and salt stress. The root growth phenotype was Fe dependent and was associated with Fe accumulation in the root. Two genes involved in Fe-dependent inhibition of root growth under Pi deficiency, the ferroxidase gene LPR1 and P5-type ATPase PDR2, are epistatic to CNX1/CNX2. Overexpressing PDR2 failed to complement the cnx1-1 cnx2-2 root phenotype. cnx1-1 cnx2-2 showed no evidence of UPR activation, indicating a limited effect on ER protein folding. CNX might process a set of N-glycosylated proteins specifically involved in the response to Pi deficiency.