Why are plants unable to take up Phosphorus directly in their organic form like Phytic Acid?

Question

I am researching acquisition strategies of phosphorus by decidious trees. I am reading a lot that plants take up nutrients as their inorganic form. In the case of P according to literature this is mainly $H_2PO_4^{-}$ (Schachtmann et al. (1998), Hinsinger (2001), Buchanan (2015)). So far so good...

In forest soils for instance, Phosphorus is much more concentrated in the form of phytic acids (derivates of inositol hexaphosphate), which are organic acids. Not in any paper which I have read before, it is stated why organic P is not taken up directly, only that it is enzymatically cleaved before it is taken up as inorganic P. The best, which I have found so far is that

Although organic P ($P_o$) is present in soil solution at higher concentrations than inorganic phosphate (Ron Vaz et al., 1993; Seeling and Jungk, 1996), direct uptake of ($P_o$) compounds by plants is considered unlikely. --Hayes et al. 2000

My problem - or rather limited understanding - is that I wonder, whether plants in fact do have the ability to take up nutrients directly as organic forms. I would prefer not to take the presence of enzymes as a proof that organic P cannot be taken up directly by plants. Also phytate seems to be used in plant tissue for storage of P.

Unfortunately my understanding of biochemistry is limited, that's why I'm pretty sure that I am missing some basic facts, which could clarify the situation. My best guess so far is, that organic sources of P are simply too big to pass through the cell wall.

So my question is: are there certain structural or biochemical barriers or obstancles, which would prevent plant roots from absorbing organic P directly, or is it for some reason better to cleave phytates "outside" the cell?

I would be very happy if you could point me in the right direction. Thanks in advance

Sources:

Buchanan, Bob B.; Gruissem, Wilhelm; Jones, Russell L. (Hg.) (2015): Biochemistry & molecular biology of plants. 2. ed. Chichester, Rockville, Md.: Wiley Blackwell; American Society of Plant Biologists.

Hayes, Julie E.; Simpson, Richard J.; Richardson, Alan E. (2000): The growth and phosphorus utilisation of plants in sterile media when supplied with inositol hexaphosphate, glucose 1-phosphate or inorganic phosphate. In: Plant Soil 220 (1/2), S. 165–174. DOI: 10.1023/A:1004782324030.

Hinsinger, Philippe (2001): Bioavailability of soil inorganic P in the rhizosphere as affected by root-induced chemical changes: a review. In: Plant Soil (237), S. 173–195.

Schachtman, Daniel P.; Reid, Robert J.; Ayling, S. M. (1998): Phosphorous Uptake by Plants: From Soil to Cell. In: Plant Physiology (116), S. 447–453.

https://en.wikipedia.org/wiki/Phytic_acid

Answer 1

I am not a plant biochemist but would make a couple of points that may or may not be relevant or helpful.

1. The living organisms that preceded plants had evolved a wide repertoire of systems for transporting ions across membranes, involving variations on a few general designs. Given the importance of inorganic phosphate in living organisms, phosphate transporters would have evolved early on.
2. Phosphate may have been more abundant in soil before sophisticated evolution of plants led to secondary metabolites like phytic acid. If this were the case it would explain why roots developed phosphate transporters.

This is not to say that it is impossible for organisms to develop transporters for more sophisticated molecules — there are many such examples — but in this case such a development would only be expected if there was such a lack and intense competition for phosphate that it would convey an evolutionary advantage.

This sort of question is always a matter of conjecture. One imagines there is a tipping point when the difficulty of developing a new system is outweighed by the advantage it would bring. This clearly hasn’t happened, but which of these factors is most important is difficult to say (for me, at any rate).