Figure 6.

Scheme illustrating the links between sugar metabolism/signaling and glycosylation and the trade-off between growth and stress responses. The figure illustrates the effect of sugar metabolism and signaling during unstressed conditions and stressed conditions in terms of sugar availability. Under non-stressed conditions, photosynthesis provides sugars to be used in metabolism and directly in glycosylation reactions. Sugar availability stimulates Target of Rapamycin (TOR), promoting growth and development and inhibiting stress responses. Sugar availability can also directly influence glycosylation by serving as the substrates, for instance through O-GlcNAc transferases (OGTs). Invertases (INVs) are key components, specifically in sink tissue, to convert sucrose to metabolizable hexoses. Under these conditions, Snf1-related protein kinase 1 (SnRK1) is inhibited by trehalose-6-phosphate (T6P). When exposed to severe stresses, photosynthesis is hampered, resulting in sugar deprivation, reflected through T6P levels (mirror sucrose levels). Low T6P results in an active SnRK1, inhibiting growth and possibly also glycosylation by regulating OGTs as is the case in animals. Active SnRK1 also promotes stress responses. It is important to note that mild abiotic stresses result in temporal leaf sweetening which can also potentially stimulate SnRK1 and/or stress responses. During sugar deprivation, TOR is typically inactive. Apart from sugar signaling pathways, limited sugar availability can also directly hamper glycosylation due to limited substrate availability and low energy levels. It is unclear whether a sugar sensor exists that translates sugar availability to glycosylation reactions in plants; however, candidates include OGTs themselves or SnRK1, since this appears to be the case in animals. In animals, AMP-activate protein kinase (AMPK), the homolog of SnRK1, and OGTs co-regulate one another to alter their activities and localization. It is important to note that glycosylation will never be completely inhibited even under severe stresses, as a low level of glycosylation is also important during these conditions. Arrows with arrow heads indicate stimulatory effects whereas arrows with blunt ends indicate inhibitory effects. Solid lines are known effects and dashed lines predicted effects. Grey arrows indicate an inactive pathway under a specific condition. This figure was created with BioRender.com.