Cell Wall Maturation of Arabidopsis Trichomes Is Dependent on Exocyst Subunit EXO70H4 and Involves Callose Deposition  

Abstract

Arabidopsis (Arabidopsis thaliana) leaf trichomes are single-cell structures with a well-studied development, but little is understood about their function. Developmental studies focused mainly on the early shaping stages, and little attention has been paid to the maturation stage. We focused on the EXO70H4 exocyst subunit, one of the most up-regulated genes in the mature trichome. We uncovered EXO70H4-dependent development of the secondary cell wall layer, highly autofluorescent and callose rich, deposited only in the upper part of the trichome. The boundary is formed between the apical and the basal parts of mature trichome by a callose ring that is also deposited in an EXO70H4-dependent manner. We call this structure the Ortmannian ring (OR). Both the secondary cell wall layer and the OR are absent in the exo70H4 mutants. Ecophysiological aspects of the trichome cell wall thickening include interference with antiherbivore defense and heavy metal accumulation. Ultraviolet B light induces EXO70H4 transcription in a CONSTITUTIVE PHOTOMORPHOGENIC1-dependent way, resulting in stimulation of trichome cell wall thickening and the OR biogenesis. EXO70H4-dependent trichome cell wall hardening is a unique phenomenon, which may be conserved among a variety of the land plants. Our analyses support a concept that Arabidopsis trichome is an excellent model to study molecular mechanisms of secondary cell wall deposition.

Callose-associated silica deposition in Arabidopsis

“…Herein we have seen similarities with the development of trichomes with silica deposition appearing to mirror the role of callose in the development of these epidermal hair cells [9] (Figure 4). Silicification in the 35S::PMR4-GFP mutants was particularly evident in the apical part of trichomes above the Ortmannian ring (Figure 3c,d) which is known to be rich in callose [9]. Biogenic silica is clearly intimately associated with the machinery of the plant extracellular matrix [2] and almost certainly adventitiously so.…”

“…In horsetail we were able to demonstrate almost mirror-like depositions of callose and silica during the development of stomata [3]. Herein we have seen similarities with the development of trichomes with silica deposition appearing to mirror the role of callose in the development of these epidermal hair cells [9] (Figure 4). Silicification in the 35S::PMR4-GFP mutants was particularly evident in the apical part of trichomes above the Ortmannian ring (Figure 3c,d) which is known to be rich in callose [9].…”

Transient cell-specific EXO70A1 activity in the CASP domain and Casparian strip localization

“…Working with single knockouts of different EXO70 subunits, however, either gave no phenotypes or led to only weak, partial effects on a given trafficking substrate or process 18 . A notable exception might be exo70h4, which was recently found to specifically lack Ortmannian rings, a localized callose deposition at the trichome base 35 , a potentially cell-type-specific role for this subunit. exo70a1 is a pleiotropic mutant with developmental defects ranging from reproductive organ function, to tracheary element formation, root hair elongation, auxin transport or mucilage secretion 17,18,33,36,37 .…”

Analysis of Exocyst Subunit EXO70 Family Reveals Distinct Membrane Polar Domains in Tobacco Pollen Tubes

“…For most EXO70 paralogs, the available studies are based on genetic approaches combined with cytological analyses. This way, AtEXO70H4 was shown to be indispensable for correct trichome cell wall thickening and callose deposition (Kulich et al, 2015). The results of gene-silencing experiments suggest that the EXO70F-like subunits are essential in resistance to fungal penetration in barley (Hordeum vulgare; Ostertag et al, 2013).…”