You can use enzymes to digest plant tissues (cells) to remove cell walls and release single cells (protoplasts). The hard part is that at present there is no CHEAP and simple tools (such as single-cell pipettes (see attached 2014 paper)) available commercially to move these single cells around.

In this attached 2015 review paper, it includes a section describing useful protocols, where they listed several references (see below). You can find these papers and look into their protocols.

Paper title: "The use of comet assay in plant toxicology: recent advances (2015)"

"In plants, the presence of a cell wall causes technical issues for performing the comet assay on plant tissues. To overcome these problems, a simple and efficient mechanical extraction to isolate cell nuclei was developed by Cerda et al. (1993), and then improved by Koppen and Angelis (1998), Navarrete et al. (1997), and Gichner and Plewa (1998). Since then, most of the researchers used directly those protocols or derived versions, such as described in Gichner and Plewa (1998). Recently, Pourrut et al. (2015) identified the key steps of comet assay in plants and proposed an optimized protocol to increase its reliability and its throughput." (Yellow highlights in the paper)

Here also a link with an article "Comet assay in plants": [http://www.scorecomets.com/comet-assay-news/199-comet-assay-in-plants ].

It describes a case study based on the 2015 paper listed above. (also attached below). The paper title is: "Recommendations for increasing alkaline comet assay reliability in plants (2015)"

Thank you very much Sir, Yuan-Yeu Yau for your kind help.