How much DNA do plants share with humans? Over 99%?
This is a number which we need to be careful with.
First, there is only one type of DNA! ALL animals and plants share the same DNA which is basically a code of only 4 'letters' which code for the same amino acids from which all proteins are made. There is a complication that some amino acids have more than one code which specifies it - there are plenty of examples where a change of codes (eg a mutation) can result in NO CHANGE in the amino acid which is specified.
It is not surprising that all animals and plants have the majority of their GENES in common. The mechanism by which sugars are oxidised to release their energy (respiration) is almost universal. There are dozens of enzymes involved with this process alone. Each enzyme is a protein and each one needs to be coded for in DNA. There are many enzymes involved in the replication of DNA itself. Other processes are almost universal too.
On the other hand, some genes are very significant. There is, I believe, only one gene responsible for setting a human embryo on the road to maleness rather than femaleness. However, this gene acts as a switch and directs other genes to produce the huge range of differences between men and women.
Some genes are present, but never used (never switched on). There seems to be a huge amount of DNA which we have inherited from our past and this may no longer be useful, but has not been "weeded out". Some genes are interrupted by long stretches of "silent" DNA for which we do not know a function. We may have mapped the whole genome of a few organisms (humans, Aarabidopsis etc), but this is little more than a "road map" and we have yet to identify the "houses" and, more significantly, the "inhabitants" of those houses.
You will be aware that DNA fingerprinting can identify one individual from another. There are clearly stretches of DNA which we don't even share with our relatives (except identical twins) - the closer the relationship, the more 'bands' on our fingerprint we share. My children only share half my DNA fingerprint.
It is said that we share about 60% of our genes with a banana. But you can see that such a statement can be very misleading.
There have been some interesting studies of proteins which ALL organisms share. One of these is Cytochrome C which is involved with part of respiration. The number of different amino acids/mutations between man and other organisms has been extensively studied. Information like this can give a numerical difference between man and other species (including plants). However, these differences are not in the part of the molecule that matters - the part of the molecule which enables Cytochrome C to carry out its function. Such data is useful for studies to tell how closely organisms are related (and indeed tell us about the relationships within that tree), but they do not tell us how "different" we are, because the Cytochrome C works the SAME in ALL organisms!