Could the Sahara be planted with genetically altered seed?
Yup. You could even get away with planting it with natural plants and permanently covering it with vegetation if you looked after the plants and watered them to start with. Under the right conditions you can increase the rainfall over a desert by planting it. What you need is an aquifer close enough to the surface for the roots to suck up the water and pass it into the atmosphere by transpiration. You can turn a forest into desert too, by uncontrolled logging. There are hundreds of cubic kilometres of water deep below the Sahara which fell as rain during the last ice age. The Sahara has changed from being only semi-arid into a desert partly because the Earth’s weather patterns have changed over the last 12000 years but also because of man’s activities. Back about when the pyramids were being built, Southern Egypt and Ethiopia had quite productive agriculture.
A part of this question that isn’t getting answered is the part about “genetically altered” seed (or plants). I think it deserves to be said that altered plants are generally for crop use, and so they usually only work (grow) where they can be cared for by agriculture or horticulture. Wild plants grow best in the wild, and have a much more extensive adaptive honing in their background than a plant would have with a short-term and idiosyncratic human “honing” of its characteristics. BUT, wild plants CAN be planted in new places, and encouraged to grow there with a little watering. And maybe even some altered crop plants could be encouraged to grow in desert with the intention to stabilize dry sandy dunes into good ground for growing plants. Soil might even develop! That helps all plants.
