TRANSPARENT TESTA10 Encodes a Laccase-Like Enzyme Involved in Oxidative Polymerization of Flavonoids in Arabidopsis Seed Coat

ABSTRACTSeeds are a late land plant evolution innovation that promoted the striking spread and diversity of angiosperms. The seed coat is a specialized dead tissue protecting the plant embryo from mechanical damage. In many species, including Arabidopsis thaliana, the seed coat also achieves a remarkable balancing act: it limits oxygen uptake, avoiding premature embryo oxidative damage, but not entirely so as to enable seed dormancy release. The seed coat biophysical features implementing the striking physiological properties of the seed remain poorly understood. Tannins, a type of flavonoids, are antioxidants known to accumulate in the Arabidopsis seed coat and transparent testa (tt) mutant seeds, deficient in flavonoid synthesis, exhibit low dormancy and viability. However, their precise contribution to seed coat architecture and biophysics remains evasive. A seed coat cuticle, covering the endosperm outer surface was, intriguingly, previously shown to be more permeable in tt mutants deficient not in cuticular component synthesis, but rather in flavonoid synthesis. Investigating the role of flavonoids in cuticle permeability led us to identify cell walls, originating from the seed coat inner integument 1 cells, impregnated with tannins. We found that tannic cell walls are tightly associated with the cuticle, forming two fused layers that regulate endosperm permeability. In addition, we show that tannic cell walls are prominent building blocks of the seed coat, constituting a continuous barrier around the seed living tissues. Altogether our findings reveal the existence of tannic cell walls as a previously unrecognized biological barrier sustaining the seed’s key physiological properties.One sentence summaryThe seed coat is largely composed of plant cell walls impregnated with tannins, forming a thick and continuous protective barrier surrounding the embryo promoting seed viability and dormancy.

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MYB–bHLH–TTG1 Regulates Arabidopsis Seed Coat Biosynthesis Pathways Directly and Indirectly via Multiple Tiers of Transcription Factors

Plant and Cell Physiology ◽

10.1093/pcp/pcaa027 ◽

2020 ◽

Vol 61 (5) ◽

pp. 1005-1018 ◽

Cited By ~ 3

Author(s):

Song Feng Li ◽

Patrick J Allen ◽

Ross S Napoli ◽

Richard G Browne ◽

Hanh Pham ◽

...

Keyword(s):

Seed Coat ◽

Chromatin Immunoprecipitation ◽

Biosynthetic Pathway ◽

Plant Immunity ◽

Wax Ester ◽

Wild Type ◽

Arabidopsis Seed ◽

Developing Seeds ◽

Metabolic Genes ◽

Transparent Testa

Abstract MYB–bHLH–WDR (MBW) transcription factor (TF) complexes regulate Arabidopsis seed coat development including mucilage and tannin biosynthesis. The R2R3 MYBs MYB5, MYB23 and TRANSPARENT TESTA2 (TT2) participate in the MBW complexes with the WD-repeat protein TRANSPARENT TESTA GLABRA1 (TTG1). These complexes regulate GLABRA2 (GL2) and TTG2 expression in developing seeds. Microarray transcriptome analysis of ttg1-1- and wild-type (Ler) developing seeds identified 246 TTG1-regulated genes, which include all known metabolic genes of the tannin biosynthetic pathway. The first detailed TTG1-dependent metabolic pathways could be proposed for the biosynthesis of mucilage, jasmonic acid (JA) and cuticle including wax ester in developing seeds. We also assigned many known and previously uncharacterized genes to the activation/inactivation of hormones, plant immunity and nutrient transport. The promoters of six cuticle pathway genes were active in developing seeds. Expression of 11 genes was determined in the developing seeds of the combinatorial mutants of MYB5, MYB23 and TT2, and in the combinatorial mutants of GL2, HOMEODOMAIN GLABROUS2 (HDG2) and TTG2. These six TFs positively co-regulated the expression of four repressor genes while three of the six TFs repressed the wax biosynthesis genes examined, suggesting that the three TFs upregulate the expression of these repressor genes, which, in turn, repress the wax biosynthesis genes. Chromatin immunoprecipitation analysis identified 21 genes directly regulated by MYB5 including GL2, HDG2, TTG2, four repressor genes and various metabolic genes. We propose a multi-tiered regulatory mechanism by which MBWs regulate tannin, mucilage, JA and cuticle biosynthetic pathways.

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