Auxin plays important roles during the entire life span of a plant. This small organic acid influences cell division, cell elongation and cell differentiation, and has great impact on the final shape and function of cells and tissues in all higher plants. Auxin metabolism is not well understood but recent discoveries, reviewed here, have started to shed light on the processes that regulate the synthesis and degradation of this important plant hormone.

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Auxin metabolism and homeostasis during plant development.

The plant hormone auxin drives plant growth and morphogenesis. The levels and distribution of the active auxin indole-3-acetic acid (IAA) are tightly controlled through synthesis, inactivation, and transport. Many auxin precursors and modified auxin forms, used to regulate auxin homeostasis, have been identified; however, very little is known about the integration of multiple auxin biosynthesis and inactivation pathways. This review discusses the many ways auxin levels are regulated through biosynthesis, storage forms, and inactivation, and the potential roles modified auxins play in regulating the bioactive pool of auxin to affect plant growth and development.

Auxin biosynthesis and storage forms

Peroxisomes are eukaryotic organelles that are highly dynamic both in morphology and metabolism. Plant peroxisomes are involved in numerous processes, including primary and secondary metabolism, development, and responses to abiotic and biotic stresses. Considerable progress has been made in the identification of factors involved in peroxisomal biogenesis, revealing mechanisms that are both shared with and diverged from non-plant systems. Furthermore, recent advances have begun to reveal an unexpectedly large plant peroxisomal proteome and have increased our understanding of metabolic pathways in peroxisomes. Coordination of the biosynthesis, import, biochemical activity, and degradation of peroxisomal proteins allows for highly dynamic responses of peroxisomal metabolism to meet the needs of a plant. Knowledge gained from plant peroxisomal research will be instrumental to fully understanding the organelle’s dynamic behavior and defining peroxisomal metabolic networks, thus allowing the development of molecular strategies for rational engineering of plant metabolism, biomass production, stress tolerance, and pathogen defense.

Plant Peroxisomes: Biogenesis and Function

The plant hormone auxin is believed to influence almost every aspect of plant growth and development. Auxin transport, biosynthesis and degradation combine to form gradients of the hormone that influence a range of key developmental and environmental response processes. There is abundant genetic evidence for the existence of multiple pathways for auxin biosynthesis and degradation. The complexity of these pathways makes it difficult to obtain a clear picture of the relative importance of specific metabolic pathways during development. We have developed a sensitive mass spectrometry-based method to simultaneously profile the majority of known auxin precursors and conjugates/catabolites in small amounts of Arabidopsis tissue. The method includes a new derivatization technique for quantification of the most labile of the auxin precursors. We validated the method by profiling the auxin metabolome in root and shoot tissues from various Arabidopsis thaliana ecotypes and auxin over-producing mutant lines. Substantial differences were shown in metabolite patterns between the lines and tissues. We also found differences of several orders of magnitude in the abundance of auxin metabolites, potentially indicating the relative importance of these compounds in the maintenance of auxin levels and activity. The method that we have established will enable researchers to obtain a better understanding of the dynamics of auxin metabolism and activity during plant growth and development.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-313X.2012.05085.x

Tissue‐specific profiling of the Arabidopsis thaliana auxin metabolome

Long before its chemical identity was known, the phytohormone auxin was postulated to regulate plant growth. In the late 1800s, Sachs hypothesized that plant growth regulators, present in small amounts, move differentially throughout the plant to regulate growth. Concurrently, Charles Darwin and Francis Darwin were discovering that light and gravity were perceived by the tips of shoots and roots and that the stimulus was transmitted to other tissues, which underwent a growth response. These ideas were improved upon by Boysen-Jensen and Paal and were later developed into the Cholodny-Went hypothesis that tropisms were caused by the asymmetric distribution of a growth-promoting substance. These observations led to many efforts to identify this elusive growth-promoting substance, which we now know as auxin. In this review of auxin field advances over the past century, we start with a seminal paper by Kenneth Thimann and Charles Schneider titled "The relative activities of different auxins" from the American Journal of Botany, in which they compare the growth altering properties of several auxinic compounds. From this point, we explore the modern molecular understanding of auxin-including its biosynthesis, transport, and perception. Finally, we end this review with a discussion of outstanding questions and future directions in the auxin field. Over the past 100 yr, much of our progress in understanding auxin biology has relied on the steady and collective advance of the field of auxin researchers; we expect that the next 100 yr of auxin research will likewise make many exciting advances.

Auxin activity: Past, present, and future

Levels of auxin, which regulates both cell division and cell elongation in plant development, are controlled by synthesis, inactivation, transport, and the use of storage forms. However, the specific contributions of various inputs to the active auxin pool are not well understood. One auxin precursor is indole-3-butyric acid (IBA), which undergoes peroxisomal β-oxidation to release free indole-3-acetic acid (IAA). We identified ENOYL-COA HYDRATASE2 (ECH2) as an enzyme required for IBA response. Combining the ech2 mutant with previously identified iba response mutants resulted in enhanced IBA resistance, diverse auxin-related developmental defects, decreased auxin-responsive reporter activity in both untreated and auxin-treated seedlings, and decreased free IAA levels. The decreased auxin levels and responsiveness, along with the associated developmental defects, uncover previously unappreciated roles for IBA-derived IAA during seedling development, establish IBA as an important auxin precursor, and suggest that IBA-to-IAA conversion contributes to the positive feedback that maintains root auxin levels.

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Multiple Facets of Arabidopsis Seedling Development Require Indole-3-Butyric Acid–Derived Auxin

Mutations in peroxisome biogenesis proteins (peroxins) can lead to developmental deficiencies in various eukaryotes. PEX14 and PEX13 are peroxins involved in docking cargo-receptor complexes at the peroxisomal membrane, thus aiding in the transport of the cargo into the peroxisomal matrix. Genetic screens have revealed numerous Arabidopsis thaliana peroxins acting in peroxisomal matrix protein import; the viable alleles isolated through these screens are generally partial loss-of-function alleles, whereas null mutations that disrupt delivery of matrix proteins to peroxisomes can confer embryonic lethality. In this study, we used forward and reverse genetics in Arabidopsis to isolate four pex14 alleles. We found that all four alleles conferred reduced PEX14 mRNA levels and displayed physiological and molecular defects suggesting reduced but not abolished peroxisomal matrix protein import. The least severe pex14 allele, pex14-3, accumulated low levels of a C-terminally truncated PEX14 product that retained partial function. Surprisingly, even the severe pex14-2 allele, which lacked detectable PEX14 mRNA and PEX14 protein, was viable, fertile, and displayed residual peroxisome matrix protein import. As pex14 plants matured, import improved. Together, our data indicate that PEX14 facilitates, but is not essential for peroxisomal matrix protein import in plants.

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Matrix proteins are inefficiently imported into Arabidopsis peroxisomes lacking the receptor-docking peroxin PEX14.

In recent years, LASER has been introduced as a minimally invasive treatment for a broad range of vaginal and vulvar symptoms and diseases. However, the efficacy and safety of vaginal and vulvar LASER has continuously been questioned. The aim of this study is to create an overview of the current literature and discuss the controversies within the use of LASER for genitourinary syndrome of menopause, vulvovaginal atrophy, urinary incontinence and lichen sclerosus. A search string was built in PubMed. The search was commenced on August 25, 2021 and closed on October 27, 2021. Two authors screened the studies in Covidence for inclusion according to the eligibility criteria in the protocol. The data were extracted from the studies and are reported in both text and tables. This review included 114 papers, of which 15 were randomized controlled trials (RCTs). The effect of LASER as a vaginal treatment was investigated for genitourinary syndrome of menopause in 36 studies (six RCTs), vulvovaginal atrophy in 34 studies (four RCTs) and urinary incontinence in 30 studies (two RCTs). Ten studies (three RCTs) investigated the effect of vulvar treatment for lichen sclerosus. Half of the included RCTs, irrespective of indication, did not find a significant difference in improvement in women treated with vaginal CO2 or Er:YAG LASER compared with their respective controls. However, most non‐comparative studies reported significant improvement after exposure to vaginal or vulvar LASER across all indications. Included studies generally had a short follow‐up period and only a single RCT followed their participants for more than 6 months post treatment. Adverse events were reported as mild and transient and 99 studies including 51 094 patients provided information of no serious adverse events. In conclusion, this review found that the effect of vaginal and vulvar LASER decreases with higher study quality where potential biases have been eliminated. We therefore stress that all patients who are treated with vaginal or vulvar LASER should be carefully monitored and that LASER for those indications as a treatment should be kept on a research level until further high‐quality evidence is available.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1111/aogs.14353

The evidence behind the use of LASER for genitourinary syndrome of menopause, vulvovaginal atrophy, urinary incontinence and lichen sclerosus: A state‐of‐the‐art review

Sarah E. Christensen

Papers

Multiple Facets of Arabidopsis Seedling Development Require Indole-3-Butyric Acid–Derived Auxin

Matrix proteins are inefficiently imported into Arabidopsis peroxisomes lacking the receptor-docking peroxin PEX14.

The evidence behind the use of LASER for genitourinary syndrome of menopause, vulvovaginal atrophy, urinary incontinence and lichen sclerosus: A state‐of‐the‐art review