A landscape of hairy and twisted: hunting for new trichome mutants in the Saskatoon Arabidopsis T-DNA population

TRANSPARENT TESTA GLABRA1, a Key Regulator in Plants with Multiple Roles and Multiple Function Mechanisms

TRANSPARENT TESTA GLABRA1 (TTG1) is a WD40 repeat protein. The phenotypes caused by loss-of-function of TTG1 were observed about half a century ago, but the TTG1 gene was identified only about twenty years ago. Since then, TTG1 has been found to be a plant-specific regulator with multiple roles and multiple functional mechanisms. TTG1 is involved in the regulation of cell fate determination, secondary metabolisms, accumulation of seed storage reserves, plant responses to biotic and abiotic stresses, and flowering time in plants. In some processes, TTG1 may directly or indirectly regulate the expression of downstream target genes via forming transcription activator complexes with R2R3 MYB and bHLH transcription factors. Whereas in other processes, TTG1 may function alone or interact with other proteins to regulate downstream target genes. On the other hand, the studies on the regulation of TTG1 are very limited. So far, only the B3-domain family transcription factor FUSCA3 (FUS3) has been found to regulate the expression of TTG1, phosphorylation of TTG1 affects its interaction with bHLH transcription factor TT2, and TTG1 proteins can be targeted for degradation by the 26S proteasome. Here, we provide an overview of TTG1, including the identification of TTG1, the functions of TTG1, the possible function mechanisms of TTG1, and the regulation of TTG1. We also proposed potential research directions that may shed new light on the regulation and functional mechanisms of TTG1 in plants.

COP9 signalosome subunit 5A affects phenylpropanoid metabolism, trichome formation and transcription of key genes of a regulatory tri-protein complex in Arabidopsis

BACKGROUND

Trichomes and phenylpropanoid-derived phenolics are structural and chemical protection against many adverse conditions. Their production is regulated by a network that includes a TTG1/bHLH/MYB tri-protein complex in Arabidopsis. CSN5a, encoding COP9 signalosome subunit 5a, has also been implicated in trichome and anthocyanin production; however, the regulatory roles of CSN5a in the processes through interaction with the tri-protein complex has yet to be investigated.

RESULTS

In this study, a new csn5a mutant, sk372, was recovered based on its altered morphological and chemical phenotypes compared to wild-type control. Mutant characterization was conducted with an emphasis on trichome and phenylpropanoid production and possible involvement of the tri-protein complex using metabolite and gene transcription profiling and scanning electron microscopy. Seed metabolite analysis revealed that defective CSN5a led to an enhanced production of many compounds in addition to anthocyanin, most notably phenylpropanoids and carotenoids as well as a glycoside of zeatin. Consistent changes in carotenoids and anthocyanin were also found in the sk372 leaves. In addition, 370 genes were differentially expressed in 10-day old seedlings of sk372 compared to its wild type control. Real-time transcript quantitative analysis showed that in sk372, GL2 and tri-protein complex gene TT2 was significantly suppressed (p < 0.05) while complex genes EGL3 and GL3 slightly decreased (p > 0.05). Complex genes MYB75, GL1 and flavonoid biosynthetic genes TT3 and TT18 in sk372 were all significantly enhanced. Overexpression of GL3 driven by cauliflower mosaic virus 35S promotor increased the number of single pointed trichomes only, no other phenotypic recovery in sk372.

CONCLUSIONS

Our results indicated clearly that COP9 signalosome subunit CSN5a affects trichome production and the metabolism of a wide range of phenylpropanoid and carotenoid compounds. Enhanced anthocyanin accumulation and reduced trichome production were related to the enhanced MYB75 and suppressed GL2 and some other differentially expressed genes associated with the TTG1/bHLH/MYB complexes.

TRANSPARENT TESTA GLABRA 1-Dependent Regulation of Flavonoid Biosynthesis

The flavonoid composition of various tissues throughout plant development is of biological relevance and particular interest for breeding. Arabidopsis thaliana TRANSPARENT TESTA GLABRA 1 (AtTTG1) is an essential regulator of late structural genes in flavonoid biosynthesis. Here, we provide a review of the regulation of the pathway’s core enzymes through AtTTG1-containing R2R3-MYELOBLASTOSIS-basic HELIX-LOOP-HELIX-WD40 repeat (MBW(AtTTG1)) complexes embedded in an evolutionary context. We present a comprehensive collection of A. thalianattg1 mutants and AtTTG1 orthologs. A plethora of MBW(AtTTG1) mechanisms in regulating the five major TTG1-dependent traits is highlighted.

Adaptor proteins GIR1 and GIR2. I. Interaction with the repressor GLABRA2 and regulation of root hair development

Plants use specialized root outgrowths, termed root hairs, to enhance acquisition of nutrients and water, help secure anchorage, and facilitate interactions with soil microbiome. One of the major regulators of this process is GLABRA2 (GL2), a transcriptional repressor of root hair differentiation. However, regulation of the GL2-function is relatively well characterized, it remains completely unknown whether GL2 itself functions in complex with other transcriptional regulators. We identified GIR1 and GIR2, a plant-specific two-member family of closely related proteins that interact with GL2. Loss-of-function mutants of GIR1 and GIR2 enhanced development of root hair whereas gain-of-function mutants repressed it. Thus, GIR1 and GIR2 might function as adaptor proteins that associate with GL2 and participate in control of root hair formation.

Adaptional evolution of trichome in Caragana korshinskii to natural drought stress on the Loess Plateau, China

Caragana korshinskii is commonly employed to improve drought ecosystems on the Loess Plateau, although the molecular mechanism at work is poorly understood, particularly in terms of the plant's ability to tolerate drought stress. Water is the most severe limiting factor for plant growth on the Loess Plateau. The trichome is known to play an efficient role in reducing water loss through decreasing the rate of transpiration, so in this study, we focused on the trichome‐related gene expression of ecological adaptation in C. korshinskii under low precipitation conditions. In order to explore the responses of trichomes to drought, we selected two experimental sites from wet to dry along the Loess Plateau latitude gradient for observation. Micro‐phenomena through which trichomes grew denser and larger under reduced precipitation were observed using a scanning electron microscope; de novo transcriptomes and quantitative PCR were then used to explore and verify gene expression patterns of C. korshinskii trichomes. Results showed that GIS2,TTG1, and GL2 were upregulated (as key positive‐regulated genes on trichome development), while CPC was downregulated (negative‐regulated gene). Taken together, our data indicate that downstream genes of gibberellin and cytokinin signaling pathways, alongside several cytoskeleton‐related genes, contribute to modulating trichome development to enhance transpiration resistance ability and increase the resistance to drought stress in C. korshinskii.

Hairy Canola (Brasssica napus) re-visited: Down-regulating TTG1 in an AtGL3-enhanced hairy leaf background improves growth, leaf trichome coverage, and metabolite gene expression diversity

BACKGROUND

Through evolution, some plants have developed natural resistance to insects by having hairs (trichomes) on leaves and other tissues. The hairy trait has been neglected in Brassica breeding programs, which mainly focus on disease resistance, yield, and overall crop productivity. In Arabidopsis, a network of three classes of proteins consisting of TTG1 (a WD40 repeat protein), GL3 (a bHLH factor) and GL1 (a MYB transcription factor), activates trichome initiation and patterning. Introduction of a trichome regulatory gene AtGL3 from Arabidopsis into semi-glabrous Brassica napus resulted in hairy canola plants which showed tolerance to flea beetles and diamondback moths; however plant growth was negatively affected. In addition, the role of BnTTG1 transcription in the new germplasm was not understood.

RESULTS

Here, we show that two ultra-hairy lines (K-5-8 and K-6-3) with BnTTG1 knock-down in the hairy AtGL3+ B. napus background showed stable enhancement of trichome coverage, density, and length and restored wild type growth similar to growth of the semi-glabrous Westar plant. In contrast, over-expression of BnTTG1 in the hairy AtGL3+ B. napus background gave consistently glabrous plants of very low fertility and poor stability, with only one glabrous plant (O-3-7) surviving to the T3 generation. Q-PCR trichome gene expression data in leaf samples combining several leaf stages for these lines suggested that BnGL2 controlled B. napus trichome length and out-growth and that strong BnTTG1 transcription together with strong GL3 expression inhibited this process. Weak expression of BnTRY in both glabrous and trichome-bearing leaves of B. napus in the latter Q-PCR experiment suggested that TRY may have functions other than as an inhibitor of trichome initiation in the Brassicas. A role for BnTTG1 in the lateral inhibition of trichome formation in neighbouring cells was also proposed for B. napus. RNA sequencing of first leaves identified a much larger array of genes with altered expression patterns in the K-5-8 line compared to the hairy AtGL3(+) B. napus background (relative to the Westar control plant). These genes particularly included transcription factors, protein degradation and modification genes, but also included pathways that coded for anthocyanins, flavonols, terpenes, glucosinolates, alkaloids, shikimates, cell wall biosynthesis, and hormones. A 2nd Q-PCR experiment was conducted on redox, cell wall carbohydrate, lignin, and trichome genes using young first leaves, including T4 O-3-7-5 plants that had partially reverted to yield two linked growth and trichome phenotypes. Most of the trichome genes tested showed to be consistant with leaf trichome phenotypes and with RNA sequencing data in three of the lines. Two redox genes showed highest overall expression in K-5-8 leaves and lowest in O-3-7-5 leaves, while one redox gene and three cell wall genes were consistently higher in the two less robust lines compared with the two robust lines.

CONCLUSION

The data support the strong impact of BnTTG1 knockdown (in the presence of strong AtGL3 expression) at restoring growth, enhancing trichome coverage and length, and enhancing expression and diversity of growth, metabolic, and anti-oxidant genes important for stress tolerance and plant health in B. napus. Our data also suggests that the combination of strong (up-regulated) BnTTG1 expression in concert with strong AtGL3 expression is unstable and lethal to the plant.