Obviously, only green plant tissues will need chloroplasts. A plant starts life as a single cell which presumably does not contain chloroplasts. Where do they come from and how do they get their unique DNA inside them?
The single cell *does *contain chloroplasts. They get their unique DNA inside of them by having evolved from an independent organism that became an endosymbiont in plant cells. Some genes that were no longer needed were lost, others that weren’t needed to be used quickly moment by moment relocated to the cell’s nucleus. A handful that are needed and are needed to be accessed quickly were kept within the chloroplast itself. (All the above also applies to mitochondria.)
OK. So how do some cells lose their chloroplasts? And why aren’t seeds green?
Dunno, off the top of my head. Maybe during differentiation into different cell types there is a mechanism to isolate chloroplasts outside of cell types that don’t need them. Maybe the differentiated cells have chloroplasts but tell them to self destruct.
Either way, once a cell looses all its chloroplasts there is no going back. Since they contain some of their own DNA, there aren’t enough instructions in the cell’s nuclear DNA to recreate them if they are gone. So the germ-line cells must remain in an unbroken chain of chloroplast containers, even if they don’t see sunlight.
Most of a seed isn’t the baby plant itself, just food for the baby plant. You know how when you eat a peanut, there are two large lobes, with a tiny little nub at one end between them? That tiny little nub is what the new peanut plant would grow from, so that’s the only part that needs any chloroplasts at all. And it doesn’t need very many, either, so while there’s some green pigment in there, there’s just as much or more of the other pigments that make it brownish.
That cell does contain chloroplasts. The first cell comes from the egg cell of the female parent plant, which contains chloroplasts within the cytoplasm. (Pollen, like sperm, does not contain much cytoplasm or contribute chloroplasts.)
As noted, chloroplasts, like mitochondria, contain their own DNA and their inheritance is separate from that of nuclear DNA.
Some some cell types must have a mechanism for destroying the chloroplasts. This is what I am interested in.
A mechanism for destroying the chloroplasts is easy. The hard part is a mechanism for some cells to not destroy the chloroplasts.
Okay, it seems that my knowledge of chloroplasts was more simplistic than I realized. I was correct in thinking that chloroplasts reproduce themselves by binary fusion like the bacteria that they evolved from. But I didn’t know that there was a more simplified “proplastid” that can develop into a chloroplast or into several other types of “plast” organelle–and that a mature chloroplast can also bud off new small undifferentiated proplastid organelles. So, while I still can’t give you a direct explanation (which would probably be something dry about the expression of protein ABC123 anyway) but it looks like the proplastid–which is needed by all types of plant cells to produce some of the various other “plast” types–is just stopped from developing into chloroplasts by cells that don’t need them, but can be switched on when needed. But it remains true that the proplast contains genes that aren’t found in the cell nucleus, and if the proplastid (and all their specialized versions) were to be removed, the cell couldn’t grow them again.
There probably aren’t many plant tissues with zero chlorop[lasts (although I’m not an expert, this is just reasonable surmise on my part).
A plant cell with more than zero chloroplasts may still have very few chloroplasts. The plant tissue may look white to us. We all have experience with vegetables that may be quite pale or white in areas sheltered from the sun, but green elsewhere on the leaf. Leeks are a great example, but common iceberg lettuce works well to illustrate this , too.
It’s similar with people. My skin is lilly-white (NOT green, but white) in areas without sun exposure and darker where the sun shines. But I do have melanocytes all through my skin.
Thanks everyone.