Abstract
In species performing apoplasmic loading, phloem cells adjacent to sieve elements often develop into transfer cells (TCs) with wall ingrowths. The highly invaginated wall ingrowths serve to amplify plasma membrane surface area to achieve increased rates of apoplasmic transport, and may also serve as physical barriers to deter pathogen invasion. Wall ingrowth formation in TCs therefore plays an important role in phloem biology, however, the transcriptional switches regulating the deposition of this unique example of highly localized wall building remain unknown. Phloem parenchyma (PP) TCs in Arabidopsis veins provide an experimental system to identify such switches. The extent of ingrowth deposition responds to various abiotic and applied stresses, enabling bioinformatics to identify candidate regulatory genes. Furthermore, simple fluorescence staining of PP TCs in leaves enables phenotypic analysis of relevant mutants. Combining these approaches resulted in the identification of GIGANTEA as a regulatory component in the pathway controlling wall ingrowth development in PP TCs. Further utilization of this approach has identified two NAC (NAM, ATAF1/2 and CUC2)-domain and two MYB-related genes as putative transcriptional switches regulating wall ingrowth deposition in these cells.
Highlights
The plant cell wall profoundly defines cell shape and functioning
The onset of assimilate export from young leaves coincides with the differentiation of leaf minor vein transfer cells (TCs) (Gunning and Pate, 1974), and in Arabidopsis, sucrose export from leaves is affected if wall ingrowth abnormalities occur in the phloem parenchyma (PP) TCs (Maeda et al, 2006)
Low expression might be expected for genes operating as putative regulators of wall ingrowth deposition in PP TCs, since the number of PP TCs relative to most other cell types in the leaf is exceedingly low (Haritatos et al, 2000; Edwards et al, 2010), and many plant transcription factors are expressed at low levels (Czechowski et al, 2004)
Summary
The plant cell wall profoundly defines cell shape and functioning This observation is acute for transfer cells (TCs) which develop extensive wall ingrowths to aid nutrient transport. These cells trans-differentiate from various differentiated cell types at sites where nutrient distribution pathways encounter apoplasmic/symplasmic discontinuities (Pate and Gunning, 1969; Offler et al, 2003). TCs are commonly observed in cells involved in post-phloem unloading pathways (Patrick, 1997), in seed of cereal crops such as wheat and barley (Thompson et al, 2001). Wall ingrowth formation plays an important role in efficient phloem loading and post-phloem unloading strategies in many species, the genetic pathways which regulate wall ingrowth deposition in TCs remain largely unknown. We describe preliminary results using this approach to identify previously uncharacterized members of the NAC (NAM, ATAF1/2 and CUC2)-domain and MYB-related gene families as putative transcriptional regulators of wall ingrowth deposition in PP TCs
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