Roles of plastid-located phosphate transporters in carotenoid accumulation

Abstract

Enhanced carotenoid accumulation in plants is crucial for the nutritional and health demands of the human body since these beneficial substances are acquired through dietary intake. Plastids are the major organelles to accumulate carotenoids in plants and it is reported that manipulation of a single plastid phosphate transporter gene enhances carotenoid accumulation. Amongst all phosphate transport proteins including phosphate transporters (PHTs), plastidial phosphate translocators (pPTs), PHOSPHATE1 (PHO1), vacuolar phosphate efflux transporter (VPE), and Sulfate transporter [SULTR]-like phosphorus distribution transporter (SPDT) in plants, plastidic PHTs (PHT2 & PHT4) are found as the only clade that is plastid located, and manipulation of which affects carotenoid accumulation. Manipulation of a single chromoplast PHT (PHT4;2) enhances carotenoid accumulation, whereas manipulation of a single chloroplast PHT has no impact on carotenoid accumulation. The underlying mechanism is mainly attributed to their different effects on plastid orthophosphate (Pi) concentration. PHT4;2 is the only chromoplast Pi efflux transporter, and manipulating this single chromoplast PHT significantly regulates chromoplast Pi concentration. This variation subsequently modulates the carotenoid accumulation by affecting the supply of glyceraldehyde 3-phosphate, a substrate for carotenoid biosynthesis, by modulating the transcript abundances of carotenoid biosynthesis limited enzyme genes, and by regulating chromoplast biogenesis (facilitating carotenoid storage). However, at least five orthophosphate influx PHTs are identified in the chloroplast, and manipulating one of the five does not substantially modulate the chloroplast Pi concentration in a long term due to their functional redundancy. This stable chloroplast Pi concentration upon one chloroplast PHT absence, therefore, is unable to modulate Pi-involved carotenoid accumulation processes and finally does affect carotenoid accumulation in photosynthetic tissues. Despite these advances, several cases including the precise location of plastid PHTs, the phosphate transport direction mediated by these plastid PHTs, the plastid PHTs participating in carotenoid accumulation signal pathway, the potential roles of these plastid PHTs in leaf carotenoid accumulation, and the roles of these plastid PHTs in other secondary metabolites are waiting for further research. The clarification of the above-mentioned cases is beneficial for breeding high-carotenoid accumulation plants (either in photosynthetic or non-photosynthetic edible parts of plants) through the gene engineering of these transporters.

Keywords: carotenoid accumulation; chloroplast; chromoplast; phosphate transporter; plastid.

Figure 1

Roles of plastid-located PHTs in carotenoid accumulation. (A) Underlying mechanisms for the observation that overexpression of single chromoplast PHT enhances carotenoid accumulation in non-photosynthetic tissues. ① Overexpression of a major chromoplast phosphate exporter (PHT4;2) results in accelerated phosphate efflux from the chromoplast. ② The enhanced phosphate efflux from chromoplast facilitates ATP hydrolysis, promoting the energy supply required for the process of substrate synthesis (GA3P) required for carotenoid biosynthesis. ③ The lower phosphate concentration in chromoplast upregulates the transcriptional abundances of carotenoid biosynthesis key genes (PSY, PDS). ④ Accelerated phosphate efflux facilitates chromoplast development, favoring carotenoid storage. The chromoplast location of PHT2;1, PHT4;3, PHT4;4, PHT4;5 is obtained by speculation, which case needs further experimental pieces of evidence. (B) Underlying mechanisms for the observation that manipulation of single chloroplast PHT does not affect the carotenoid accumulation in photosynthetic tissues. (1) The functional redundancy amongst at least five chloroplast PHTs causes the result that manipulation of a single chloroplast PHT does not substantially affect the phosphate concentration in the chloroplast. This case consequently leads to the inability to affect processes (2) to (4). (2) As the basis for the photosynthesis of plants, photosynthetic pigments (chlorophylls and carotenoids) are crucial for the generation of ATP and NADPH at the light reaction stage. (3) Synthesized ATP and NADPH then participate Calvin cycle to generate sugar. Sugar catabolism (glycolysis and TCA cycle) provides the ATP and substrate (GA3P) required for carotenoid biosynthesis. (4) In addition, chloroplast Pi concentration changes regulate transcript abundances of carotenoid biosynthesis-limited genes PSY, PDS. Note: G6P, Glucose 6-phosphate; Xul-5p, xylulose 5-phosphate; PEP, phosphoenolpyruvic acid; TP, triose-phosphate; GA3P, glyceraldehyde 3-phosphate; IPP, isopentenyl diphosphate; DMAPP, dimethylallyl diphosphate; GGPP, geranylgeranyl diphosphate; PSY, phytoene synthase; PDS, phytoene desaturase; ABA, abscisic acids; ATP, adenosine triphosphate; ADP, adenosine diphosphate; TCA cycle, tricarboxylic acid cycle; BASS2, plastid-localized pyruvate transporter bile acid: sodium symporter family protein 2.

Figure 2

Subcellular locations of phosphate transport proteins. Plasma membrane located members: PHT1, PHO1, and SPDT. Golgi located members: PHT4;6 and PHO1. Mitochondrion located members: PHT3;1, PHT3;2, and PHT3;3. Vacuole located members: PHT5;1, PHT5;2, PHT5;3, and OsVPE1/2. Chloroplast located members: TPT, XPT, PPT, GPT, PHT2;1, PHT4;1, PHT4;2, PHT4;3, PHT4;4, PHT4;5. Chromoplast located members: XPT, PPT, GPT, PHT2;1, PHT4;2, PHT4;3, PHT4;4, PHT4;5. Chromoplast locations of PHT4;3, PHT4;4, PHT4;5 are proposed by speculation. Substrates transported are indicated by arrows. Note: PHT, phosphate transporter; PHO1, PHOSPHATE1; SPDT, sulfate transporter [SULTR]-like phosphorus distribution transporter; VPE, vacuolar phosphate efflux transporter; TPT, triose-phosphate/phosphate translocator; PPT, phosphoenolpyruvate/phosphate translocator; XPT, xylulose 5-phosphate/phosphate translocator; GPT, glucose 6-phosphate/phosphate translocator.