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.
Highlights
Arabidopsis (Arabidopsis thaliana) leaf trichomes are single-cell structures with a well-studied development, but little is understood about their function
Two Arabidopsis SALK insertional mutants in the EXO70H4 single-exon gene were used for this study: exo70H4-1 and exo70H4-3 (SALK_023593 and SALK_003200, respectively)
Because jasmonic acid (JA) induction has been linked to increased herbivore resistance in Arabidopsis, we suggest that the influence of trichome stiffness is hazed by other effects, likely methyl jasmonate (MeJA)-induced glucosinolate synthesis and overall resistance
Summary
Arabidopsis (Arabidopsis thaliana) leaf trichomes are single-cell structures with a well-studied development, but little is understood about their function. Exocyst in Arabidopsis Trichome Maturation group of glassy mutants lacks surface papillae, rendering them a lustrous and transparent appearance These include chablis, chardonnay, retsina (Hülskamp et al, 1994), glassy hair (GLH1), GLH2, GLH3, GLH4, and GLH6 (Suo et al, 2013). Transcription factors involved in the trichome cell wall development include HOMEODOMAIN GLABROUS2 (Marks et al, 2009) and MYB DOMAIN PROTEIN106 (NOECK; Jakoby et al, 2008). Both of these mutants have underdeveloped trichome papillae. Most of the published work on cell walls formation was done on stem/xylem or whole-plant analysis and is related to cell wall component biosynthesis (Liepman et al, 2010), with recent emphasis on transcriptional networks reconstructed from transcriptional analyses (Wang et al, 2012)
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