Figure 7.

A, Model of interactions affecting growth and starch accumulation on trehalose when T6P accumulates. Glc and Suc feeding cause AGPase redox activation and thus starch synthesis by different pathways that are likely also relevant for the growth responses to these sugars (Tiessen et al., 2003; Michalska et al., 2009; Geigenberger, 2011). When feeding Glc, T6P does not accumulate (T.L. Delatte, P. Sedijani, and H. Schluepmann, unpublished data), and Glc-6-P (G6P) in plastids is shunted through the oxidative part of the pentose phosphate pathway (OPP), generating NADPH for NADPH-thioredoxin reductase C (NTRC)-dependent reduction of AGPase and thus activation. In contrast, feeding Suc or trehalose leads to T6P increase, which acts upon AGPase redox by an unknown mechanism (Schluepmann et al., 2004; Kolbe et al., 2005; Lunn et al., 2006; Michalska et al., 2009). Suc inhibits the translation of bZIP11 by way of uORF2 (Wiese et al., 2004), but trehalose does not. When feeding trehalose, T6P accumulates. T6P accumulation inhibits SnRK1; this inhibition of SnRK1 depends on an intermediary factor I, present in young tissues (Zhang et al. 2009). Possibly, SnRK1 phosphorylation activates bZIP11 transfer to the nucleus or complexing of the transcription factor in such a way that bZIP11 controls part of the SnRK1 output that is required for growth. Thus, when T6P accumulates and inhibits SnRK1 in young tissues, overexpression of bZIP11 may act as a surrogate for SnRK1. B, Antisense SnRK1 restricted to individual pollen of barley in particular (Zhang et al., 2001) but also work in developing potato tubers (Purcell et al., 1998) show that SnRK1 is required in growing heterotrophic cells for growth and starch accumulation. Therefore, it is possible that SnRK1 is needed to respond to nutrient stress so as to make carbon available in growing sinks. SnRK1 inhibition (by artificially increasing T6P when feeding trehalose or by antisense SnRK1) would thus uncouple growth from carbon starvation responses, leading to the swollen cells observed in the growing zones of roots of Arabidopsis seedlings on trehalose (tre) compared with sorbitol (sorb). Root tips were stained with propidium iodide in water immediately prior to visualization with the confocal microscope. [See online article for color version of this figure.]