Phloem unloading

Viola, R., Roberts, A.G., Haupt, S., Gazzani, S., Hancock, R.D., Marmiroli, N., Machray, G.C., and Oparka, K.J., Tuberization in potato involves a switch from apoplastic to symplastic phloem unloading, Plant Cell, 13, 385-398, 2001. 

Figure 4. Pathways of phloem unloading in sink regions. See text for details. Reproduced with permission from Ref. 13. Copyright 1983. American Society of Plant Physiologists.

Patr ick, J.W. 1997. Phloem unloading Sieve element unloading and post-sieve element transport. Annu. Rev. Plant Physiol. Plant Mol. Biol. 48 191—222. 

Recognizing that substances transported to developing seeds are unloaded in the seed coat and move to the embryo apoplastically [14], we thought it would be appropriate to determine if metabolism of ABA differed at the point of phloem unloading, the seed coat, and the point of uptake, the embryo. 

The significance of the differential metabolism of ABA by the two seed parts is not clear. While there is no question that ABA has regulatory activity, there are only a few reports that PA has biological activity [8, 12]. Soybean seed coat tissue efficiently oxidizes ABA to PA, but seems relatively inefficient in metabolizing PA. The seed coat s capacity to efficiently metabolize ABA, but not PA, could be an indication that ABA be an active agent in this tissue and while PA is not. Ross et al. [9] have shown that ABA rapidly (< 10 min) stimulates phloem unloading from pea seed coats still attached to the plant. 

Secondary metabolites can accumulate in the same cell and tissue in which they are formed, but intermediates and end-products can also be transported to other locations for further elaboration or accumulation. For example, TAs and nicotine are typically produced near the root apex, but mostly accumulate within leaf cell vacuoles. Even TA biosynthesis itself involves intercellular transport of several pathway intermediates (Fig.7.9A). P-Glucuronidase (GUS) localization in A. belladonna roots transformed with a PMT promoter-GUS fusion showed that PMT expression is restricted to the pericycle.144 Immunolocalization and in situ RNA hybridization also demonstrated the pericycle-specific expression of H6H.145,146 In contrast, TR-I was immunolocalized to the endodermis and outer root cortex, whereas TR-II was found in the pericycle, endodermis, and outer cortex.85 The localization of TR-I to a different cell type than PMT and H6H implies that an intermediate between PMT and TR-I moves from the pericycle to the endodermis (Fig.7.9A). Similarly, an intermediate between TR-I and H6H must move back to the pericycle. The occurrence of PMT in the pericycle provides the enzyme with efficient access to putrescine, ornithine, and arginine unloaded from the phloem. In the same way, scopolamine produced in the pericycle can be readily translocated to the leaves via the adjacent xylem. 

The known involvement of metabolism in translocation in the phloem could be due to active transport of solutes into the phloem of a leaf or other source, which is often referred to as loading, and/or to their removal, or unloading, in a root or other sink, such as a fruit (Fig. 9-l7a). Indeed, loading often involves proton-sucrose cotransport (Fig. 3-l4a) via a carrier located in... 

In monocots, Fe and Zn are taken up as phytosiderophore chelates by YSL transporters in the epidermis. Fe can also be taken up by OsIRTI. Metals move through the symplastic space to the vasculature. The citrate effluxer FRDLl is important for loading of citrate into the xylem and subsequent Fe transport to the shoot through the transpiration stream. YSL transporters also may play a role in unloading the xylem into the shoot and the phloem. Fe is unloaded from the phloem by OsYSL2 and OsIRTI into shoot and seed tissue. 

Braun DM, Wang L, Ruan YL. Understanding and manipulating sucrose phloem loading, unloading, metabolism, and signalling to enhance crop yield and food security. J Exp Bot. 65(7) (2014) 1713-1735. 

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