Wall ingrowth deposition in phloem parenchyma transfer cells in Arabidopsis: Heteroblastic variations and a potential role in pathogen defence

Abstract

Transfer cell (TCs) develop unique wall ingrowth networks which amplify plasma membrane surface area and thus maximize nutrient transporter density at key anatomic sites for nutrient exchange within plants and their external environment. These sites fall into 4 main groups corresponding to 4 categories of trans-membrane flux: absorption/secretion of solutes from or to the external environment, and absorption/secretion of solutes from or to internal, extra-cytoplasmic compartments. Research on TC biology over recent decades has demonstrated correlations between wall ingrowth deposition in TCs and enhanced transport capacity in many major agricultural species such as pea, fava bean, cotton and maize. Consequently, there is general consensus that the existence of wall ingrowth morphology implies an augmentation in membrane transport capacity. However, this may not be entirely applicable for phloem parenchyma (PP) TCs in Arabidopsis. Our recent survey of PP TC abundance and distribution in Arabidopsis veins indicated that PP TC development reflects heteroblastic status. A consequence of this observation is the suggestion that PP TCs, or at least wall ingrowth deposition in these cells, potentially act as a physical barrier to defend access of invading pathogens to sugar-rich sieve elements rather than solely in facilitating the export of photoassimilate from collection phloem in leaves.

Lateral Transport of Organic and Inorganic Solutes

“…It has been proposed, for example, that phloem parenchyma transfer cells (PP TCs), which are highly metabolically active, could fuel massive transmembrane transport, notably of sugar compounds, through the plasmodesmata as well as through active and/or passive transporter systems [66]. Finally, the role of PP TCs in controlling membrane transport capacity has been extended to their role as a physical barrier that prevents access of invading pathogens to sugar-rich sieve elements [67]. Even if progress has been made in our understanding of the role of TCs, the molecular actors involved in the transport of organic solutes (i.e., sugar and amino acids) between TCs and phloem companion cells still need to be identified.…”