Trehalose 6-phosphate regulates photosynthesis and assimilate partitioning in reproductive tissue

Transgenic maize (Zea mays) that expresses rice (Oryza sativa) TREHALOSE PHOSPHATE PHOSPHATASE1 (TPP1) from the rice MADS6 promoter, which is active over the flowering period, produces higher yields than wild type. This yield increase occurs with or without drought conditions during flowering. To understand the mechanistic basis of the increased yield, we characterised gene expression and metabolite profiles in leaves and developing female reproductive tissue – comprising florets, node, pith and shank – over the flowering period with and without drought. The MADS6 promoter was most active in the vasculature, particularly phloem companion cells in florets and pith, consistent with the largest decreases in trehalose
6-phosphate (T6P) levels (two- to threefold) being found in pith and florets. Low T6P led to decreased gene expression for primary metabolism and increased gene expression for secondary metabolism, particularly lipid-related pathways. Despite similar changes in gene expression, the pith and floret displayed opposing assimilate profiles: sugars, sugar phosphates, amino acids and lipids increased in florets, but decreased in pith. Possibly explaining this assimilate distribution, seven SWEET genes were found to be upregulated in the transgenic plants. SnRK1 activity and the, expression of the gene for the SnRK1 beta subunit, expression of SnRK1 marker genes, and endogenous trehalose pathway genes were also altered. Furthermore, leaves of the transgenic maize maintained a higher photosynthetic rate for a longer period compared to wild type. In conclusion, we found that decreasing T6P in reproductive tissues downregulates primary metabolism and upregulates secondary metabolism, resulting in different metabolite profiles in component tissues. Our data implicate T6P/SnRK1 as a major regulator of whole-plant resource allocation for crop yield improvement.