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Cytokinin N-glucosides: Occurrence, Metabolism and Biological Activities in Plants https://www.mdpi.com/939100 via @MDPIOpenAccess
This is long project on which worked many people (many from @PetrasekLab) and I joined the team only recently, but let me introduce you to some of the details.
If you follow me, you may have already noticed the word #cytokinins (do not confuse with cytokins) which are #PlantHormones which come in many types and forms. This diagram (from my upcoming review) shows the many types. The isoprenoid CKs are the ones found in nature.
Each of these types can be modified to yield various forms and each form is though to have particular function. For example, free base (no modification) is the active form, ribosides are transport form, while nucleotides are first products of biosynthesis.
N-Glucosides (CK with glucose on N7 or N9 atom of adenine) are thought to be deactivation products, because they are not active and they should not be converted back to the active free base (although there are some reports to the contrary, see my upcoming review).
First, the content of CKNGs was measured in >100 plant species. There is an increasing trend in how much CKNGs make up of total CKs. The ratio of N7 and N9 glucosides, on the other hand, shows no trend.
Next, the CK(-like) activity of CKNGs was tested in several bioassays. CKs (like other (phyto)hormones) influence several physiological processes, which we are able to quantify.
The first is based on fact, that pieces of plants in dark degrade chlorophyll. But CKs prevent that. Indeed, N9G manage to prevent chlorophyll degradation in oat leaf segments to similar degree as free base. In tomato, both N7G and N9G worked similarly.
Another is based on root growth inhibition by CKs. Maize root growth was inhibited by N9G. Expression of several CK-related genes was checked in treated plants and N9G treated plants resembled those treated with the free base, while N7G showed only little effect.
Next, both N7G and N9G were shown to induce expression of TCSv2 promoter. This is construct used to detect CK activity in planta (e.g. when you want to see specific cells in which the CK signaling is active). This is big, because that means that CKNG activate CK signaling pathway
However, in accordance with previous reports, no interaction with CK receptors was shown! 
The used compounds were extensively tested and no significant amounts of free bases - which would explain those activities - were detected.
The used compounds were extensively tested and no significant amounts of free bases - which would explain those activities - were detected.
Here is a summary of CKNG's activities in various bioassays (in blue are active, in grey are not active).
In fact, it was noted in the 1970s or so that processes regarding senescence delay are less strict about the compound structure.
In fact, it was noted in the 1970s or so that processes regarding senescence delay are less strict about the compound structure.
As you can see, the activities differ depending both on process and on species.
Moreover, some of the co-authors published their own paper where they showed only little overlap in gene expression differences upon treatment. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0232762
Moreover, some of the co-authors published their own paper where they showed only little overlap in gene expression differences upon treatment. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0232762
That's about it. In summary, the formerly thought-to-be inactive CKNGs show CK(-like) activity esp. in bioassays delaying senescence. The effect is probably not because of their hydrolysis. However, right now we have no idea what could be the mode of action.
Actually, I had a poster last year about this, where I hypothesised about the possible modes of action, as well.
That's all folks for today!
That's all folks for today!
