Phloem Transport of Resources

Abstract

This chapter reviews the principles and concepts of resource transport in the phloem in general and then moves to describe the physiology of phloem translocation in sugarcane. The movement of resources through the phloem is interpreted through the Münch pressure flow hypothesis. The phloem transport rate is determined by magnitudes of pressure differences between the source and sink ends of the pathway and the phloem sap concentration of each transported nutrient. Phloem rate is regulated by loading and unloading activities, sink hydrostatic pressures, and hydraulic conductivities of plasmodesmata interconnecting sieve element/companion cell complexes with phloem parenchyma cells. A description of the leaf structural framework in sugarcane is provided and the phloem pathways in sugarcane are dissected from loading of collection phloem in source leaves, transit through transport phloem, to release phloem where resources are unloaded into growth and storage sinks. Phloem conductivities in sugarcane, mechanisms of loading and unloading of solutes, and the environmental effects on phloem translocation are considered.

Sucrose Transporter Localization and Function in Phloem Unloading in Developing Stems

“…Similar roles might apply to SUT3 clade members localized to SE-CC complexes of mature stems of wheat and rice (Aoki et al, 2004;Scofield et al, 2007). Suc released to the stem apoplasm could be hydrolyzed by cell wall invertases active in the intercalary meristem and elongation zone of sugarcane stems (Grof et al, 2014) and taken up by hexose/proton symporters (Casu et al, 2003) or SWEETs expressed by SP cells (Sosso et al, 2015). Transitioning to a mature state, cell wall invertase activity decreases (Grof et al, 2014), and uptake from the stem apoplasm by SP cells is likely to be mediated by Suc/proton symporters.…”

“…The above analysis points to a central role played by plasma membrane sugar transporters in facilitating radial apoplasmic transfer of Suc or its hexose derivatives derived from hydrolysis of apoplasmic Suc by cell wall invertase (Grof et al, 2014), from SE lumens to SP cells in the intercalary meristems, elongating and transition zones of monocot stems. In some sweet Sorghum cultivars, this scenario could extend to mature stem zones (Bihmidine et al, 2015).…”

“…Suc released to the stem apoplasm could be hydrolyzed by cell wall invertases active in the intercalary meristem and elongation zone of sugarcane stems (Grof et al, 2014) and taken up by hexose/proton symporters (Casu et al, 2003) or SWEETs expressed by SP cells (Sosso et al, 2015). Transitioning to a mature state, cell wall invertase activity decreases (Grof et al, 2014), and uptake from the stem apoplasm by SP cells is likely to be mediated by Suc/proton symporters. In this context, ShSUT1 was highly expressed in maturing internodes of sugarcane and was localized to cells surrounding vascular bundles (Rae et al, 2005).…”

“…While there is a growing mechanistic understanding of phloem unloading into terminal sinks, unloading from the transport phloem has attracted less attention. This status particularly applies to stems of monocot species that accumulate Suc to high concentrations such as sugarcane (Saccharum officinarum) and sweet Sorghum (Slewinski, 2012;Grof et al, 2014), in which stem storage is the predominant sink, limited by its capacity to accumulate Suc (Watt et al, 2013).…”

“…While there is a growing mechanistic understanding of phloem unloading into terminal sinks, unloading from the transport phloem has attracted less attention. This status particularly applies to stems of monocot species that accumulate Suc to high concentrations such as sugarcane (Saccharum officinarum) and sweet Sorghum (Slewinski, 2012;Grof et al, 2014), in which stem storage is the predominant sink, limited by its capacity to accumulate Suc (Watt et al, 2013).Transport phloem of monocot stems traverses their intercalary meristems, located immediately above the basal node of each elongating internode, cell elongation and mature zones. Hence, Suc unloaded from the transport phloem supports stem growth (cell division and expansion) and storage (elongating and mature zones; Milne et al, 2015).…”