Being able to control, respond and maintain the levels of energy reserves is vital for the maintenance of a cell, but the road to understanding how plants respond and sense their energy levels is far from conclusive.
In animals, the enzyme AMPK (Adenosine Monophosphate-activated Protein Kinase) consists of three parts. One of these subunits can sense the levels of glycogen present in the cell and react accordingly. There are two key differences in plant cells: First, plant cells do not use glycogen as a sugar storage molecules; instead plants use starch for energy storage. Second, the version of AMPK in plants is structurally different and is often referred to as SnRK1.
These differences pose the question: without the specific structure to bind and detect glycogen and with a different energy storage molecule, how does SnRK1 detect the levels of energy storage in plants?
This is an ongoing topic of investigation but one big hint is in its structure. Broeckx et al. (2016) used a model with known structure of AMPK as a starting point to attempt to find the structure of this SnRK1. With this structure, it is still not clear how the sensing of the energy levels functions. The enzyme contains additional sections connected by a linker and does not have a clear binding surface with starch. Starch being the comparable plant sugar storage molecule to glycogen in animals.
"Knowing how to adjust the immunity of plants could help improve yields, and lower the dependence on pesticides."
With the process unclear, the application of different forms of sugar, other molecules and stress conditions have all been examined and identified to be a potential activation source or at least a factor which interacts with the activation source. One such example is T6P, a sugar phosphate. The phosphate part of the sugar prevents its uptake into plant cells so a small molecule was constructed to cover the phosphate group and break off from T6P when it is exposed to light. This way T6P can enter the cell and be available to alter the plants metabolism. When T6P was introduced it did activate the SnRK1 enzyme but it is not clear if this activation was directly sensed by SnRK1 or if there was something else is going on in between such as a change in the amount of glucose in the cell.
Finding out how to manipulate and regulate the function of SnRK1 could be beneficial for numerous crops because it has been demonstrated to be involved in numerous functions of the plant, not least of which is the immune system. Knowing how to adjust the immunity of plants could help improve yields, and lower the dependence on pesticides. The search for the exact mechanisms will continue.
Broeckx T., Hulsmans S., Rolland F. 2016. The plant energy sensor: evolutionary conservation and divergence of SnRK1 structure, regulation, and function. Journal of Experimental Botany. vol.67 (22) , pp. 6215-6252.
Li Y., Van den Ende W., Rolland F. 2014. Sucrose induction of anthocyanin biosynthesis is mediated by DELLA. Molecular Plant. vol.7 (3) , pp. 570-572.