Auxins are plant hormones that play a central role in controlling plant growth and development across different environmental conditions. Even at low concentrations, auxins can regulate gene expression through specific transcription factors and proteins that are modulated to environmental responses in the signalling cascade. Auxins are synthesized in tissues with high cell division activity and distributed by specific transmembrane proteins that regulate efflux and influx. This review presents recent advances in understanding the biosynthetic pathways, both dependent and independent of tryptophan, highlighting the intermediate indole compounds (indole-3-acetamide, indole-3-acetaldoxime, indole-3-pyruvic acid and tryptamine) and the key enzymes for auxin biosynthesis, such as YUCs and TAAs. In relation to the signalling cascade, it has been shown that auxins influence gene expression regulation by the connection between synthesis and distribution. Moreover, the molecular action of the auxin response factors and auxin/indole-3-acetic acid transcription factors with the F-box TIR1/AFB auxin receptors regulates gene expression. In addition, the importance of microRNAs in the auxin signalling pathway and their influence on plant plasticity to environmental fluctuations is also demonstrated. Finally, this review describes the chemical and biological processes involving auxins in plants.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/plb.13303

Auxin and its role in plant development: structure, signalling, regulation and response mechanisms

Stem cell populations in meristematic tissues at distinct locations in the plant body provide the potency of continuous plant growth. Primary meristems, at the apices of the plant body, contribute ...

The Dynamics of Cambial Stem Cell Activity

Brassinosteroids have been implicated in the differentiation of vascular cell types in herbaceous plants, but their roles during secondary growth and wood formation are not well defined. Here we pharmacologically and genetically manipulated brassinosteroid levels in poplar trees and assayed the effects on secondary growth and wood formation, and on gene expression within stems. Elevated brassinosteroid levels resulted in increases in secondary growth and tension wood formation, while inhibition of brassinosteroid synthesis resulted in decreased growth and secondary vascular differentiation. Analysis of gene expression showed that brassinosteroid action is positively associated with genes involved in cell differentiation and cell-wall biosynthesis. The results presented here show that brassinosteroids play a foundational role in the regulation of secondary growth and wood formation, in part through the regulation of cell differentiation and secondary cell wall biosynthesis.

Brassinosteroid regulation of wood formation in poplar

Populus spp. are among the most economically important species worldwide. These trees are used not only for wood and fiber production, but also in the rehabilitation of degraded lands. Since they are clonally propagated, the ability of stem cuttings to form adventitious roots is a critical point for plant establishment and survival in the field, and consequently for the forest industry. Adventitious rooting in different Populus clones has been an agronomic trait targeted in breeding programs for many years, and many factors have been identified that affect this quantitative trait. A huge variation in the rooting capacity has been observed among the species in the Populus genus, and the responses to some of the factors affecting this trait have been shown to be genotype-dependent. This review analyses similarities and differences between results obtained from studies examining the role of internal and external factors affecting rooting of Populus species cuttings. Since rooting is the most important requirement for stand establishment in clonally propagated species, understanding the physiological and genetic mechanisms that promote this trait is essential for successful commercial deployment.

/pdf/adventitious-rooting-in-populus-species-update-and-d0b2q5wgtp.pdf

Adventitious Rooting in Populus Species: Update and Perspectives.

Wood formation is the terminal differentiation of xylem mother cells derived from cambial initials, and negative regulators play important roles in xylem differentiation. The molecular mechanism of the negative regulator of xylem differentiation PagKNAT2/6b was investigated. PagKNAT2/6b is an ortholog of Arabidopsis KNAT2 and KNAT6 that is highly expressed in phloem and xylem. Compared to nontransgenic control plants, transgenic poplar plants overexpressing PagKNAT2/6b present with altered vascular patterns, characterized by decreased secondary xylem with thin cell walls containing less cellulose, xylose and lignin. RNA sequencing analyses revealed that differentially expressed genes are enriched in xylem differentiation and secondary wall synthesis functions. Expression of NAM/ATAF/CUC (NAC) domain genes including PagSND1-A1, PagSND1-A2, PagSND1-B2 and PagVND6-C1 is downregulated by PagKNAT2/6b, while PagXND1a is directly upregulated. Accordingly, the dominant repression form of PagKNAT2/6b leads to increased xylem width per stem diameter through downregulation of PagXND1a. PagKNAT2/6b can inhibit cell differentiation and secondary wall deposition during wood formation in poplar by modulating the expression of NAC domain transcription factors. Direct activation of PagXND1a by PagKNAT2/6b is a key node in the negative regulatory network of xylem differentiation by KNOXs.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/nph.16036

KNAT2/6b, a class I KNOX gene, impedes xylem differentiation by regulating NAC domain transcription factors in poplar

Adventitious roots occur naturally in many species and can also be induced from explants of some tree species including Populus, providing an important means of clonal propagation. Auxin has been identified as playing a crucial role in adventitious root formation, but the associated molecular regulatory mechanisms need to be elucidated. In this study, we examined the role of PagFBL1, the hybrid poplar (Populus alba × P. glandulosa clone 84K) homolog of Arabidopsis auxin receptor TIR1, in adventitious root formation in poplar. Similar to the distribution pattern of auxin during initiation of adventitious roots, PagFBL1 expression was concentrated in the cambium and secondary phloem in stems during adventitious root induction and initiation phases, but decreased in emerging adventitious root primordia. Overexpressing PagFBL1 stimulated adventitious root formation and increased root biomass, while knock-down of PagFBL1 transcript levels delayed adventitious root formation and decreased root biomass. Transcriptome analyses of PagFBL1 overexpressing lines indicated that an extensive remodelling of gene expression was stimulated by auxin signalling pathway during early adventitious root formation. In addition, PagIAA28 was identified as downstream targets of PagFBL1. We propose that the PagFBL1-PagIAA28 module promotes adventitious rooting and could be targeted to improve Populus propagation by cuttings.

https://www.fs.fed.us/psw/publications/groover/psw_2019_groover002_shu.pdf

The auxin receptor TIR1 homolog (PagFBL 1) regulates adventitious rooting through interactions with Aux/IAA28 in Populus.

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The auxin receptor TIR1 homolog (PagFBL 1) regulates adventitious rooting through interactions with Aux/IAA28 in Populus.