Transcript Profiling Identifies NAC-Domain Genes Involved in Regulating Wall Ingrowth Deposition in Phloem Parenchyma Transfer Cells of Arabidopsis thaliana

Abstract

Transfer cells (TCs) play important roles in facilitating enhanced rates of nutrient transport at key apoplasmic/symplasmic junctions along the nutrient acquisition and transport pathways in plants. TCs achieve this capacity by developing elaborate wall ingrowth networks which serve to increase plasma membrane surface area thus increasing the cell's surface area-to-volume ratio to achieve increased flux of nutrients across the plasma membrane. Phloem parenchyma (PP) cells of Arabidopsis leaf veins trans-differentiate to become PP TCs which likely function in a two-step phloem loading mechanism by facilitating unloading of photoassimilates into the apoplasm for subsequent energy-dependent uptake into the sieve element/companion cell (SE/CC) complex. We are using PP TCs in Arabidopsis as a genetic model to identify transcription factors involved in coordinating deposition of the wall ingrowth network. Confocal imaging of pseudo-Schiff propidium iodide-stained tissue revealed different profiles of temporal development of wall ingrowth deposition across maturing cotyledons and juvenile leaves, and a basipetal gradient of deposition across mature adult leaves. RNA-Seq analysis was undertaken to identify differentially expressed genes common to these three different profiles of wall ingrowth deposition. This analysis identified 68 transcription factors up-regulated two-fold or more in at least two of the three experimental comparisons, with six of these transcription factors belonging to Clade III of the NAC-domain family. Phenotypic analysis of these NAC genes using insertional mutants revealed significant reductions in levels of wall ingrowth deposition, particularly in a double mutant of NAC056 and NAC018, as well as compromised sucrose-dependent root growth, indicating impaired capacity for phloem loading. Collectively, these results support the proposition that Clade III members of the NAC-domain family in Arabidopsis play important roles in regulating wall ingrowth deposition in PP TCs.

DNA methylation and its effects on gene expression during primary to secondary growth in poplar stems

“…7 d and e), which suggests that developmental processes are coupled with 5mC to some degree. The counterparts of some common genes in A. thaliana are involved in cell growth ( CCR4 [ 60 ] , MAP 70–1 [ 61 ] , and UGT85A [ 62 ]), secondary cell wall biosynthesis ( NAC056 [ 63 ] , PAL1 [ 64 ], FLA11 [ 65 ], C4H [ 66 ], and MYB52 [ 67 ]), and hormone and signal transduction ( WRKY27 [ 68 ] , UGT84B1 [ 69 ], anac047 [ 70 ], PIP1 [ 71 ] , and CYP716A1 [ 72 ]) (Additional files 15 and 16 ), indicating that methylation can affect some individual pathway or regulatory genes involved in wood formation. For example, expression of PtrPAL2 (Potri.008G038200) increased dramatically with the change of DNA methylation sites in PS vs TS (Additional file 15 ).…”

Developmental Biology and Induction of Phi Thickenings by Abiotic Stress in Roots of the Brassicaceae

“…Together with the regular phi thickenings, this structure appeared similar to crescent thickenings in the root cortex of red bayberry ( Myrica rubra ) and wax apple ( Syzygium samarangense ) [ 5 , 6 ]. Work by Fernandez-Garcia, Lopez-Perez, Hernandez, and Olmos [ 15 ] in B. oleracea showed high ATPase activity localised to so-called wall ingrowth regions, an area equivalent to where reticulate networks are located, while Zelko, Lux, and Czibula [ 25 ] indicated that the increased surface area in the intercellular space adhering to the cortical thickenings may play a similar role to transfer cells [ 39 ] by increasing plasma membrane surface for enhanced transport capacity. However, these “wall ingrowth” structures are highly lignified and thus structurally different from the non-lignified, primary wall-like wall ingrowths which characterise most transfer cells [ 38 , 40 ].…”

Lateral Transport of Organic and Inorganic Solutes

“…For instance, transfer cells (TCs), which can differentiate from existing vascular or non-vascular cell types (e.g., bundle sheath, xylem or phloem parenchyma cells), and which are characterized by prominent plasma membrane ingrowths, have been suggested to be a site of intense transport activity between the different vascular cell types [63]. Most studied in the phloem of minor veins, TC development occurs quite early (after an average of 10 days of growth) in cotyledons as well as in young leaves of Arabidopsis thaliana plants grown in normal conditions [64]. In adult leaves, a basipetal gradient of wall ingrowth deposition that has parallels with the sink/source transition is observed [64].…”

“…Most studied in the phloem of minor veins, TC development occurs quite early (after an average of 10 days of growth) in cotyledons as well as in young leaves of Arabidopsis thaliana plants grown in normal conditions [64]. In adult leaves, a basipetal gradient of wall ingrowth deposition that has parallels with the sink/source transition is observed [64]. Additionally, TCs can develop in response to changing environmental conditions, most probably as a result of an increased demand for solute transport [65].…”