Regulation of assimilate import into sink organs: update on molecular drivers of sink strength

Abstract

Recent developments have altered our view of molecular mechanisms that determine sink strength, defined here as the capacity of non-photosynthetic structures to compete for import of photoassimilates. We review new findings from diverse systems, including stems, seeds, flowers, and fruits. An important advance has been the identification of new transporters and facilitators with major roles in the accumulation and equilibration of sugars at a cellular level. Exactly where each exerts its effect varies among systems. Sugarcane and sweet sorghum stems, for example, both accumulate high levels of sucrose, but may do so via different paths. The distinction is central to strategies for targeted manipulation of sink strength using transporter genes, and shows the importance of system-specific analyses. Another major advance has been the identification of deep hypoxia as a feature of normal grain development. This means that molecular drivers of sink strength in endosperm operate in very low oxygen levels, and under metabolic conditions quite different than previously assumed. Successful enhancement of sink strength has nonetheless been achieved in grains by up-regulating genes for starch biosynthesis. Additionally, our understanding of sink strength is enhanced by awareness of the dual roles played by invertases (INVs), not only in sucrose metabolism, but also in production of the hexose sugar signals that regulate cell cycle and cell division programs. These contributions of INV to cell expansion and division prove to be vital for establishment of young sinks ranging from flowers to fruit. Since INV genes are themselves sugar-responsive “feast genes,” they can mediate a feed-forward enhancement of sink strength when assimilates are abundant. Greater overall productivity and yield have thus been attained in key instances, indicating that even broader enhancements may be achievable as we discover the detailed molecular mechanisms that drive sink strength in diverse systems.

Impact of foliar application of nano micronutrient fertilizers and titanium dioxide nanoparticles on the growth and yield components of barley under supplemental irrigation

“…The sink strength can be depicted as the output of sink activity and sink size (Yang et al, 2003). Sink activity is controlled by enzymes involved in starch biosynthesis and degradation (Bihmidine et al, 2013). It is assumed that phytohormones, particularly cytokinins play a major role in increasing sink size by promoting cell division during the early phase of seed filling.…”

Spatio-Temporal Dynamics of Fructan Metabolism in Developing Barley Grains

“…Invertases are also important for the production of hexose sugar signals that regulate cell cycle and cell division programs (Koch, 2004;Bihmidine et al, 2013). Certain aspects of development are regulated by the sucroseto-hexose ratio, as shown for cell division events during tuber, embryo, and root growth (Weber et al, 1997;Roessner-Tunali et al, 2003;Wind et al, 2010).…”

Tonoplast Sugar Transporters (SbTSTs) putatively control sucrose accumulation in sweet sorghum stems

“…[1][2][3][4][5] Multiple crops have been independently bred to store high concentrations of sucrose in terminal storage organs, namely, the taproots of sugar beet (Beta vulgaris L.), and the stems of sweet sorghum (Sorghum bicolor L. Moench) and sugarcane (Saccharum officinarum L.). [5][6][7][8][9] However, the sucrose contents of these crops appear to be approaching maximal levels attainable from breeding efforts 10,11 ; therefore, new approaches are needed to increase sucrose accumulation in storage organs. Hence, characterizing the genes that function in sucrose transport and storage will reveal potential new targets for future manipulations to enhance crop yields.…”