Mobilization and Cellular Distribution of Phosphate in the Diatom Phaeodactylum tricornutum

Abstract

Unicellular organisms that live in marine environments must cope with considerable fluctuations in the availability of inorganic phosphate (P i). Here, we investigated the extracellular P i concentration-dependent expression, as well as the intracellular or extracellular localization, of phosphatases and phosphate transporters of the diatom Phaeodactylum tricornutum. We identified P i-regulated plasma membrane-localized, ER-localized, and secreted phosphatases, in addition to plasma membrane-localized, vacuolar membrane-localized, and plastid-surrounding membrane-localized phosphate transporters that were also regulated in a P i concentration-dependent manner. These studies not only add further knowledge to already existing transcriptomic data, but also highlight the capacity of the diatom to distribute P i intracellularly and to mobilize P i from extracellular and intracellular resources.

Specific acclimations to phosphorus limitation in the marine diatom Phaeodactylum tricornutum

“…Some VTC proteins (Phatrdraft_48811, 35739, 50019) show a non-vacuolar localization when expressed as eGFP proteins, being integrated most likely in the endomembrane system or in not specified compartments in case of Phatrdraft_48538 (Dell'Aquila et al 2020). Despite this, independently to the result described in Alipanah et al 2018, we identified the afore-mentioned vacuolar transporter Phatrdraft_19586, and localized it in the tonoplast (Dell'Aquila et al 2020).…”

“…A fourth P-regulated putative alkaline phosphatase (Phatrdraft_45757) was shown to be an enzyme of the endoplasmic reticulum (ER) and a membrane surrounding the plastid of diatoms (chloroplast-ER, cER). The latter finding might indicate P scavenging activities in internal compartments as well, especially in the phospholipid (PL) degradation pathway under P-starvation, as proposed by Dell'Aquila et al (2020). P. tricornutum might use further P scavenging strategies.…”

“…Similarly, a consistent downregulation of putative sodium-phosphate and P i -transporters was observed after four days in P-replete conditions after cultivation under P-starvation (Cruz de Carvalho et al 2016), confirming that a high and low-affinity P i uptake machinery could work in an alternate manner according to external P concentration. However, studies on the regulation of eGFP expression driven by the upstream/downstream regions of putative P itransporter genes showed that two of them (Phatr-draft_40433, 47667) still retain a basal level of expression when cultivating the cells in several P-replete conditions, whereas the gene expression of Phatrdraft_23830 was repressed (Dell'Aquila et al 2020). In any case, transcriptional regulation is not the only mechanism to control the expression of a protein.…”

“…Again, alkaline phosphatases induction was shown under P-starvation (Phatrdraft_39432, 49678, 47869, 45959), but also downregulation was observed when P-starved cells were transferred into fresh f/2 in case of Phatrdraft_39432, 49678, 47869. In another report, the expression patterns of some P-responsive genes have been shown to be strictly related to the external P concentration (Dell'Aquila et al 2020). Here the regulatory regions (upstream/downstream to the coding region) of several putative alkaline phosphatase genes were used to express an eGFP reporter.…”

“…In our approach, we showed that the expression of the genes encoding for Phatrdraft_49678, 47612, and 45757 is repressed when the cells were cultivated under P-replete conditions suggesting a strong transcriptional control according to the external P-concentration. However, an additional posttranslational control of these proteins was not excluded (Dell'Aquila et al 2020).…”