Lets look at some real-world examples of plants which already absorb and use metals, namely hyperaccumulators.
Hyeraccumulators are plants which can withstand extremely high concentrations of metals otherwise toxic to non-hyperaccumulating plants.
They extract metals from the soil and store them within leaves, shoots and their roots. Other plants can also extract metals from soils, but hyperaccumulators can do this at a much, much faster rate and are also able to store incredible quantities of these toxic metals within their tissues.
Because of this extraction of metals, hyperaccumulators are commonly used in phytomining, where we use such plants to take minerals out of the soil for us.
Of course, hyperaccumulators absorb many metals, not just the ones valuable to humans.
Because of the toxicity of the metals which are absorbed by hyperaccumulating plants, scientists speculate that the primary purpose of hyperaccumulation, at least, the primary defensive purpose, is to prevent them from being eaten. The concentration of toxic metals within these plants is so high that animals which eat them will die, and so never be found eating them again.
So, here on Earth, hyperaccumulators only pad their tissues with toxic metals to decrease the likelihood of their being eaten, but you've stated specific interest in using metals to improve structure, rigidity and strength.
However, if a plant were able to pull metals from the ground like a hyperaccumulator, there's no reason those metals couldn't then be used to strengthen the plant.
First, let's have a look at which metals hyperaccumulators are known to handle:
Name Symbol UTS tensile strength
Aluminium Al 700
Silver Ag 170
Arsenic As 3
Beryllium Be 448
Chromium Cr 689
Copper Cu 220
Manganese Mn -
Mercury Hg -
Molybdenum Mo 690 Disclaimer:
Lead Pb 17 I am pretty certain that the
Palladium Pd 325 listed tensile strengths are
Platinum Pt 240 inaccurate and inconsistent.
Selenium Se 500 They should be used just as a
Zinc Zn 28 rough idea of the actual
strengths.
From this list, two metals stand out; Aluminium and Chromium.
Both incredibly strong metals; this is the same Aluminium used in skyscrapers and jet engines, and the same chromium as used in Chrome plating.
If your plants were to hyperaccumulate large enough quantities of these metals, they could capitalise on their strength in many ways. Some examples I thought of were:
* Reinforcing cell-walls by chrome plating. 561
Currently, plant cell walls are made almost entirely from cellulose, which is (compared to chromium) very weak. By reinforcing this cellulose with chromium, individual cells of your plant would become nigh-indestructable.
* Building skeletal systems. Along with the Phloem and Xylem of current plants, metal plants might have a third system of vessels, filled with Aluminium, keeping the plant structurally sound. Good luck snapping a twig laced with one of the toughest metals of which we know.
* Plating the entire plant. If the plant's epidermis were to secrete chromium instead of wax, it could build up a thick layer of chrome plating, which cannot be scratched, corroded or otherwise damaged.
Of course, now that you have the ability to absorb minerals and metals from the ground, you can use them however you want! 561