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I am considering the evolution of plants, specifically considering the chloroplast. I am getting slightly confused and cannot seem to find the information I need to build a coherent picture in my head. Specifically, I have the following questions:

  1. Is the defining feature of a plant cell that it has, or once had, chloroplasts? Is the group of plants all of the species which came from the single common ancestor eukaryotic cell which first acquired a photosynthesising bacterium as an endosymbiont?

  2. In that case, the plants Alectoria sarmentosa (witch's hair lichen) and Monotropa uniflora (Indian Pipe), which are non-photosynthesising plants and do not have chlorophyll, must have some remanant of a chloroplast inside of them (unless they are not actually plants and are just plant-like fungi). My question here is, are these really plants? If so and it is true that plants are defined as all being descendant from this first photosynthesising eukaryote ancestor, then what is their 'remnant chloroplast'? I.e. what organelle do they have that used to be a chloroplast.

  3. If it is true that there are plants which have organelles that used to be chloroplasts but not longer have chlorophyll and photosynthesise, then it is almost certainly the case that the chloroplasts carry out some function other than photosynthesis that is beneficial to the plant's survival (e.g. I know that remnant mitochondria- hydrogenosomes and mitosomes- are useful for fixing iron although they do not carry out respiration). What are the other functions of chloroplast-like organelles?

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Modern biology attempts to group organisms by clades, that is, into groups that contain all descendants of a particular organism and no others. A clade may be associated with characteristics that most or all members of the clade possess, but those characteristics do not define the clade. Previously it was common to describe biological groups by particular defining characteristics, but that has been largely abandoned because it often does not reflect true relationships.

"Plants" (or Plantae) is a term that can mean several different things, which are outlined in this Wikipedia table. In short, it might be (approximately) the clade of "land plants", the clade of "green plants", which includes land plants and green algae, or the clade containing those plus the red algae and the glaucophyte algae. That last clade, also known as the Archaeplastida, is thought to include all descendant species of the initial organism which initially established the symbiosis leading to chloroplasts (although not some organisms that acquired chloroplasts by secondary ingestion of archaeplastida). It seems like you are most concerned with that third group.

Alectoria sarmentosa (witch's hair lichen) is not a plant by any of these definitions. It is a lichen, which are symbioses of a fungi with a cyanobacterium or a green algae. Green algae of course have chloroplasts.

Monotropa uniflora (Indian Pipe) is a member of the heather family and so is a plant by any of the above three definitions. It does not have chlorophyll; my guess is that it likely has degenerate chloroplasts.

I think a separate question would be appropriate to inquire about the chloroplast remnants of non-photosynthesizing plants like Monotropa uniflora.

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  • $\begingroup$ Moreover, even among archaeplastida, the chloroplasts don't seem have the same evolutionary origin. I am not too sure about it though. $\endgroup$ – WYSIWYG Nov 15 '16 at 9:11

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