What are the implications of the recent discovery that plants regulate their temperature around 21°C?
I am shooting a bit for the hip here to be honest, this answer is opinion, not a researched position, but...

To imply that a object, be it plant, animal, or inanimate object with the ability to self regulate their temperature has the ability to have implications on climate change on a global scale, at least a positive one, ignores the physical laws of entropy.  That statement assumes that by positive effects means slowing of warming.

Entropy as a basic rule of physics always increases.  For this purpose, you can consider entropy to be the total energy in the system, with the energy of concern here being "heat".  If a plant (or anything) raises its temperature, it also extracts that heat from somewhere else.  If the surrounding environment is warmer, it can do that by simply absorbing heat, otherwise it does that by using other energy by chemical or physical processes.  These processes always are net zero in entropy or increase entropy thus increase total system energy usually seen as heat.  The same is true when the plant needs to lower its temperature, it can exchange heat with surrounding environment or through physical or chemical means lower its temperature, but doing so will again either be net zero energy or an increase in energy/heat.

Plants do have an almost exemption of this rule though, they grow, and in doing so the store some energy, temporarily taking some of that energy out of the equation.  This stored energy, and thus heat, is then released when the plant dies and decays, is used as food, or when millions of years of the stored energy is used in the form of fossil fuels, again resulting in a net increase in total entropy in the system.

Plants can and are used for localized climate moderation, but the contribution that would be applied to their ability to regulate their own temperature I would think is somewhere between insignificant and a slight negative in terms of slowing global warming.  The more significant aspect is in areas like slowing ground heating of denuded areas, cleaning pollutants from the air, storing of energy for later use, and many other aspects of plant chemistry and relationship with its environment.  For instance, the shade provided by a single tree I would guess outweighs any effect cause by temperature regulation of many trees.  I would be happy to see any study to the contrary, but I do not see that as possible within the known laws of physics.