Acid phosphatases (APases, EC 3.1.3.2) are abundant in plants and their role in P metabolism is to hydrolyse P from orthophosphate monoesters and function by recycling and scavenging P from internal and external sources (Duff et al., 1994).
The hydrolysis of inorganic P (Pi) to organic P (Po) by acid phosphatases in the roots
assists the plant to acquire chemically bound forms and have either specific or non- specific substrate specificity, which is important for recycling P (Duff et al., 1994). Plants contain a number of intracellular and extracellular acid phosphatases, and increased activity is one of the signature P-stress responses (Duff et al., 1994; Tran et al., 2010a).
The intracellular phosphatases are localised in the vacuole and cytoplasm (located in the soluble fraction, (SF), and the extracellular acid phosphatases are located in the cell wall (CW). The extracellular acid phosphatases are secreted by the roots under phosphate stress to increase the hydrolysis of esterified P in the rhizosphere. The large family of acid phosphatases function in the recycling and remobilising of P as part of normal P homeostasis (Duff et al., 1994; Tran et al., 2010a). Other acid phosphatases are specifically induced in response to P starvation, which are regulated by transcription factors such as PHR1, WKY75 and ZAT6 (Rubio et al., 2001; Devaiah et al., 2007a; Devaiah et al., 2007b; Tran et al., 2010a), and ethylene (Lei et al., 2011).
One of the subgroups of induced acid phosphatases, are the purple acid phosphatases (PAPs), which are distinguishable by their pink or purple colour in solution and contain a conserved motif with seven metal ligating residues (Tran et al., 2010a). The PAP family contains many members in different plants. For example, there are 29 members in Arabidopsis (Li et al., 2002), 35 in soybean (Glycine max) (Li et al., 2012a) and 26 in rice (Oryza sativa) with some that contain PB1S elements in the promoter region (Zhang et al., 2011). The different members of the PAP gene family perform different functions in recycling and recovering P, and are a biological target for improving P efficiency (Tran et al., 2010a; Wang et al., 2010b).
The Arabidopsis PAP genes are divided into three groups, and eight subgroups according to the sequences; and characterisation of the different genes is an on-going process (Li et al., 2002; Tran et al., 2010a). Three (AtPAP10, AtPAP12 and AtPAP26) of the 29 members of PAPs in Arabidopsis that are classified into the same group (group 1, subgroup Ia-2) all play an important role in scavenging and recycling P during
AtPAP26 is also found in the vacuole, and AtPAP10 and AtPAP12 are located in the
cell wall (Hurley et al., 2010; Tran et al., 2010a).
A recent study with Arabidopsis demonstrated that growth of P-starved plants could be
restored with ADP supply (Wang et al., 2011). Here, wild-type Arabidopsis (ecotype
Colombia) seedlings and five mutants with no acid phosphatases activity (nop1-1, nop1-
2, nop1-3, nop1-6 and nop1-10) were grown for 14 days in agar medium containing a
full nutrient supply, or no P-supply which was supplemented with 10 µM, 50 µM or
150 µM of ADP. The shoot and root fresh weight increased in all of the plant lines with
increasing ADP, and shoot fresh weight was restored to about 80% of the plants grown
on P-sufficient media. Interestingly, the root fresh weight of the wild-type and nop1-10
plants grown on –P, containing 150 µM of ADP in the media was exceeded the P-
sufficient plants. The supply of ADP failed to restore the primary root length of the P-
stressed plants. However, the addition of 150 µM ADP to the low P media increased the
number of lateral roots in the wild-type plants compared with the P-sufficient plants.
The mutations were located in the AtPAP10 gene, which is an acid phosphatase that is
located at the root surface, and is activated specifically in response to P-stress.
The plants were stained with 5-bromo-4-chloro-3-indolyl-phosphate (BCIP), which is cleaved by acid phosphatase to produce a blue colour. There was an increased level of staining in the plants with no P-supply and higher levels of ADP, which confirmed the
role of AtPAP10 in the tolerance to P starvation and the role of ADP in cellular
processes (Wang et al., 2011).
The significance of the dual role of the AtPAP26 (At5g34850) gene and its functions in
the vacuole are becoming more evident in the adaptation of low P environments, with
well-conserved homologs in tomato, soybean, rice and onion (Veljanovski et al., 2006;
Hurley et al., 2010). The AtPAP26 protein is a 55 kDa monomer with two isoforms that
are constitutively expressed irrespective of the external nutrient status and further
upregulated during P-stress through post-translational glycosylation (Veljanovski et al.,
The identification of an atpap26 mutant recently confirmed the specific role in P-stress induced homeostasis by scavenging external P from the environment (Hurley et al., 2010). The mutation was the result of a T-DNA insert in the seventh intron of the AtPAP26 and affected the secretory region of the protein. The atpap26 mutant had less AtPAP26 activity in the roots and shoots, no expression in suspension cells and
impaired development compared to the wild-type plants with no difference in AtPAP12, and AtPAP17 activity (Hurley et al., 2010). The shoot fresh weight of the mutant was compared with Colombia under P, N, K and paraquat-mediated oxidative stress, but no differences were observed in any of the other treatments expect for P starvation. The primary root length of the wild-type and atpap26 mutants was lower under P starvation as expected, with a further reduction in the atpap26 mutant. It was concluded that the extra reduction in primary root length was the result of the malfunction of the AtPAP26 gene to scavenge external P, which makes it a potential target for improving P efficiency in plants (Hurley et al., 2010).
The role of P-stress induced PAPs has in scavenging and recycling P is well established, and until recently, there was no link between the changes in root morphology and PAP activity. However, a recent study observed an increase in biomass in tobacco plants transformed with an Arabidopsis AtPAP18 (Zamani et al., 2012). The transgenic lines also had an increased primary root length and more lateral roots than the control plants, and were not affected by low P medium. This suggested that AtPAP18 plays an indirect role in root modifications. P-acquisition was also enhanced in the transgenic lines, and although the internal levels of P were lower between treatments, they were higher than the wild-type tobacco under P-sufficient and P-deficient conditions (Zamani et al., 2012).