Abstract
Adequate phosphorous (P) is essential to plant cells to ensure normal plant growth and development. Therefore, plants employ elegant mechanisms to regulate P abundance across their developmentally distinct tissues. One such mechanism is PHOSPHATE2 (PHO2)-directed ubiquitin-mediated degradation of a cohort of phosphate (PO4) transporters. PHO2 is itself under tight regulation by the PO4 responsive microRNA (miRNA), miR399. The DOUBLE-STRANDED RNA BINDING (DRB) proteins, DRB1, DRB2 and DRB4, have each been assigned a specific functional role in the Arabidopsis thaliana (Arabidopsis) miRNA pathway. Here, we assessed the requirement of DRB1, DRB2 and DRB4 to regulate the miR399/PHO2 expression module under PO4 starvations conditions. Via the phenotypic and molecular assessment of the knockout mutant plant lines, drb1, drb2 and drb4, we show here that; (1) DRB1 and DRB2 are required to maintain P homeostasis in Arabidopsis shoot and root tissues; (2) DRB1 is the primary DRB required for miR399 production; (3) DRB2 and DRB4 play secondary roles in regulating miR399 production, and; (4) miR399 appears to direct expression regulation of the PHO2 transcript via both an mRNA cleavage and translational repression mode of RNA silencing. Together, the hierarchical contribution of DRB1, DRB2 and DRB4 demonstrated here to be required for the appropriate regulation of the miR399/PHO2 expression module identifies the extreme importance of P homeostasis maintenance in Arabidopsis to ensure that numerous vital cellular processes are maintained across Arabidopsis tissues under a changing cellular environment.
Highlights
Phosphorous (P) is one of the most limiting factors for plant growth worldwide [1,2,3], with large quantities of P an essential requirement for numerous processes vital to the plant cell, including energy trafficking, signaling cascades, enzymatic reactions and nucleic acid and phospholipid synthesis [3,4].Inorganic phosphate (Pi), in the form of PO4, is the predominant form of P taken up by a plant from the soil, soil PO4 primarily exists in organic or insoluble forms that are largely inaccessible to plant root uptake mechanisms [1]
To determine the consequence of loss of DOUBLE-STRANDED RNA BINDING (DRB) activity on P homeostasis maintenance in 15-day old Arabidopsis plants post a 7-day period of PO4 starvation, a series of phenotypic and physiological parameters were assessed in Col-0, drb1, drb2 and drb4 shoots
A lack of available P in the soil is a key limitation for plant growth globally [3,45] and as a consequence of P limitation, land plants have evolved highly complex regulatory mechanisms to control both the uptake of external P from the soil, primarily in the form of PO4 (Pi), as well as the remobilization of internal stores of P during periods of low external PO4 availability [46]
Summary
Phosphorous (P) is one of the most limiting factors for plant growth worldwide [1,2,3], with large quantities of P an essential requirement for numerous processes vital to the plant cell, including energy trafficking, signaling cascades, enzymatic reactions and nucleic acid and phospholipid synthesis [3,4].Inorganic phosphate (Pi), in the form of PO4 , is the predominant form of P taken up by a plant from the soil, soil PO4 primarily exists in organic or insoluble forms that are largely inaccessible to plant root uptake mechanisms [1]. Phosphorous (P) is one of the most limiting factors for plant growth worldwide [1,2,3], with large quantities of P an essential requirement for numerous processes vital to the plant cell, including energy trafficking, signaling cascades, enzymatic reactions and nucleic acid and phospholipid synthesis [3,4]. Due to limited soil PO4 availability, combined with the importance of an adequate concentration of P in plant cells to ensure normal growth and development, plants employ elegant mechanisms to spatially regulate P abundance across their developmentally distinct tissues [5,6]. P limitation triggers the release of organic acids from the plant root system into the soil rhizosphere to chelate with metal ions to promote soluble PO4 uptake to maintain or increase intracellular P concentration [1,8]. Enhanced P trafficking is achieved via promoting the expression of genes encoding PO4 transporter proteins, and in turn, elevated PO4 transporter protein abundance generally ensures that the cellular P concentration is maintained irrespective of external PO4 levels [1,7]
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