Wow this one has gone for so long without an answer. I am so sorry to have missed this one.
I do hope you are still around and will find my answer.
Officially no attempt has been made to engineer any plant (or animal) for extra-terrestrial use - meaning all DNA engineering and gene splicing and even breeding has been for applications here on earth.
There is work in progress to engineer crops like Corn, wheat and several forms of vegetable for conditions above the arctic circle and for arid wasteland conditions.
Granted not exactly Mars conditions, more like aspects of Mars-like conditions. But when we think of Mars as a place of future habitation, then we must consider it as it will be after preparatory work is done to get it halfway from its current conditions to something like earth.
Speculation and terraformations plans do not call for creating trees or advanced plants to exist under current Mars conditions.
Focus would be on algae and lichens, perhaps as complex as moss. Those would be the first 'plants' seeded on Mars to switch its atmosphere from mostly Carbon dioxide to one with higher levels of Oxygen.
Mars Atmosphere is about 0.7% the density of Earth's, currently composed of 95% Carbon dioxide and 3% Nitrogen, 1.6% Argon and 0.13% Oxygen, the rest are trace amounts of gasses.
While some of the nitrogen on Mars may have escaped into space, it is assumed that most of it is bound up in the soils in the forms of nitrates. If so, then denitrating bacteria would be seeded into the soils most likely before the algae seeding stage since Anaerobic bacteria are the denitrating type. Anaerobic is a fancy name that means that they do not use oxygen.
Even then the atmosphere would have a density of maybe (if we are real lucky and unbind nitrogen and oxygen at the same time) 3-5% that of Earth. That is not even close to the atmospheric pressure on top of Everest (about 33% of sea level) and men need oxygen tanks in order to be up there.
At this point in things most speculative fiction call upon 'Magic' methods to fill out the atmosphere, such as a find of frozen atmosphere on Mars, or the miraculous "total Recall" atmosphere machine that can suddenly melt enough water ice to create livable atmosphereic pressure within seconds.
The reality is that we would most likely need to find a source of atmosphere. Nitrogen or another inert gas would be needed for the most of the atmosphere. Fortunately 'recent' discoveries have revealed that Titan has high levels of nitrogen. We can only speculate on how Titan nitrogen would be harvested and shipped to Mars, most likely it would be a process of using the cold of space to liquefy and keep the nitrogen liquid (77.36K or -195.79 °C (-320.42 °F)) one could freeze the gas solid by putting it in space and giving it a reflective shield from solar radiations - when you need to melt it just remove the shield. However getting it from Titan's surface to space is going to require a good deal of energy.
Water will most likely be a problem. It is now thought that a good deal of Mars' free water broke down into hydrogen and oxygen and rose into the atmosphere and then was 'blown' away by the solar wind. Water Ice is found in comets, however those only come by at random times, the Rings of Saturn are composed of dirt, debris and water ice most no larger than a snow flake. Due to its reflectivity we assume that the main composition is water ice. However it is possible that low freezing point gasses could make up a substantial larger percentage of that.
Once we get the atmospheric pressure up to approximately 1/2 to 1/3 of that of earth's atmosphere then we would have to only slightly modify plants like trees to exist in conditions that are found in high mountain - above treeline - elevations.
On Earth Alpine (Mountain) tree line is a problem of too cold for too long and too much persistent snow. While 'forest trees' Tall and mature and full of health do not fair well at those elevations, there are trees there, usually stunted, warped by wind and weather into bushes and shrubs. So trees can technically exist in those areas, the are just barely strong enough to survive.
On Mars near the equator it is highly possible that temperatures will be consistently above freezing year round. In that case high alpine trees (evergreens like Fir and Pine) may be able to exist starting somewhere near the middle of importing atmosphere to Mars.
Assuming that only part of the existing CO2 is turned into oxygen by bacteria, algae and the like, it is highly possible that higher levels of CO2 will help fir and pine to grow to full maturity, over coming the cold and lower atmospheric pressure and the lack of free flowing water.
Most theorists agree that many earth adapted plants and animals will not need to be re-engineered to exist on terraformed (Greened) Mars. The processes that we will use to geoengineer Mars to make it into a sustainable ecology to suit our needs will naturally make it suitable for many other species as well.
It is possible that we may not even need to genetically change some algae, bacteria and lichens - species are found that exist in the extremes of Antarctica
http://www.coolantarctica.com/gallery/scenic/icebergs2/Antarctica_iceberg2.htm shows an image of red tint on snow, that red tint is caused by algae.
quote:
There are over 300 species of such algae that live in such harsh and cold conditions. The red colour is a protective chemical (carotenoids such as astaxanthin) that the alga produces against exceptionally high concentrations of visible and ultra violet light that bounces off the snow and ice surfaces and so saturates them to a point where it become harmful and destructive. Such algae are also found in other parts of the world, often in high mountains where extra u-v light due to the thinner atmosphere and again increased light scattering by ice and snow requires protection by similar pigments.
Due to these 'recent' finds of live in the harsh arctic conditions, some speculators on the Greening of Mars have concluded that what we will need to do is cause a 'snow' to fall on Mars. once again we look toward the Rings of Saturn, collecting the ice and sending it to Mars on low energy trajectories. the most popular method is to make a snow ball - compressing ice into a denser 'ball' of ice/snow then using a rail gun (most often powered by solar panels) and 'shooting' the ball out of the Saturn system on an orbit that goes counter to the orbit of the planets. While such a shot would be of escape velocity from Saturn, it would be in the opposite direction of the orbits of the planets, thus losing its velocity relative to the sun, thus it would 'descend' in orbit around the sun. With the correct amount of energy and semi-careful calculations a snow ball from the Rings of Saturn could 'fall' into the orbital path of Mars. as Mars swing by it would pick up the snow balls in its way. The short journey through the atmosphere would melt the snow ball (well before reaching the surface. Some of the melt would fract the hydrogen from the oxygen while most of the melt would remain as water vapor. given enough water vapor and the naturally occurring dust and the temperatures on Mars condensation of water vapor into ice crystals is a 90+% certainty.
Given the present temperatures on most of Mars precipitation would most likely be in the form of snow, while in some areas (low valleys, equatorial zones) day time temperatures could result in snow melt and free flowing water would thus exist (during the day, maybe at night if fog also takes form).
Our current level of technology is the guiding forces in most of the plans (serious plans) dealing with the geoengineering of Mars. Thus a good deal of consideration is placed in using naturally occurring earth species to do the hard work of converting Frozen, arid Red Mars into a spring warm, green Mars.
