Plant egg cell fate determination depends on its exact position in female gametophyte

Plant fertilization involves both an egg cell, which fuses with a sperm cell, and synergid cells, which guide pollen tubes for sperm cell delivery. Therefore, egg and synergid cell functional specifications are prerequisites for successful fertilization. However, how the egg and synergid cells, referred to as the “egg apparatus,” derived from one mother cell develop into distinct cell types remains an unanswered question. In this report, we show that the final position of the nuclei in female gametophyte determines the cell fate of the egg apparatus. We established a live imaging system to visualize the dynamics of nuclear positioning and cell identity establishment in the female gametophyte. We observed that free nuclei should migrate to a specific position before egg apparatus specialization. Artificial changing in the nuclear position on disturbance of the actin cytoskeleton, either in vitro or in vivo, could reset the cell fate of the egg apparatus. We also found that nuclei of the same origin moved to different positions and then showed different cell identities, whereas nuclei of different origins moved to the same position showed the same cell identity, indicating that the final positions of the nuclei, rather than specific nucleus lineage, play critical roles in the egg apparatus specification. Furthermore, the active auxin level was higher in the egg cell than in synergid cells. Auxin transport inhibitor could decrease the auxin level in egg cells and impair egg cell identity, suggesting that directional and accurate auxin distribution likely acts as a positional cue for egg apparatus specialization.

Response of primary root to nitrogen-doped carbon dots in Arabidopsis thaliana: Alterations in auxin level and cell division activity

Fluorescent carbon dots have attracted wide attentions in bioscience and bioimaging. However, the molecular mechanisms of the effects of carbon dots on plants growth and development have not been elucidated....

Ex vitro Rooting, Acclimatization and Genetic Stability of Lonicera caerulea var. kamtschatica

Ex vitro rooting and acclimatization of two cultivars ‘Wojtek’ and ‘Zojka’ of blue honeysuckle (Lonicera caerulea var. kamtschatica Sevast.) were studied. To the ex vitro conditions were transferred rooted and unrooted shoots. The post-effect of auxin type and concentration as well as microcutting and soil substrate types were tested. The genetic stability of the plantlets in relation to the mother plants by using amplified fragment length polymorphism (AFLP) and inter simple sequence repeat (ISSR) markers has been also determined. It has been found that in vitro rooted cuttings of both cultivars showed a higher survival rate (max. 88%) and better growth and development when they were rooted on a medium containing a low auxin level (1.0 mg·dm−3). The results of the second experiment showed successful ex vitro rooting of blue honeysuckle shoots without auxin treatment. Higher ex vitro rooting and survival rate in the greenhouse have been observed for ‘Wojtek’ (max. 96%) than ‘Zojka’ (max. 88%). Better growth and development of shoots and roots were observed on peat alone or a mixture of peat and perlite as compared to a mixture of peat and sand. The micropropagated plantlets appeared similar to mother plants. Molecular analysis confirmed a high level of genetic stability of blue honeysuckle after 2 years of in vitro propagation. However, among the cultivars studied, ‘Wojtek’ showed slightly higher genetic stability than ‘Zojka’ (99.5% and 97.7%, respectively). For ‘Zojka’ plants, the degree of variation was comparable for AFLP and ISSR markers. For ‘Wojtek’, no polymorphism was detected using the ISSR analysis in contrast to the AFLP analysis.

A role of DAO1 in oxidation of IAA amino acid conjugates revealed through metabolite, high throughput transcript and protein level analysis

SummaryAuxin metabolism is, together with auxin transport, a key determinant of auxin signalling output in plant cells, yet details on the underlying mechanisms and factors involved are still largely unknown. Processes involved in the auxin metabolism are subject to regulation based on numerous signals, including auxin concentration itself. Altered auxin availability and the subsequent changes of auxin metabolite profiles can therefore elucidate the function and regulatory role of individual elements in the auxin metabolic machinery.After analysing auxin metabolism in auxin dependent tobacco BY-2 cell line grown in presence or absence of synthetic auxin 2,4-D we found that both conditions were similarly characterized by very low levels of endogenous indole-3-acetic acid (IAA) and its metabolites. However, metabolic profiling after exogenous application of IAA uncovered that the concentration of N-(2-oxindole-3-acetyl)-L-aspartic acid (oxIAA-Asp), the most abundantly formed auxin metabolite in the control culture, dramatically decreased in auxin-starved conditions. To describe the molecular mechanism behind this regulation, we analysed transcriptome and proteome changes caused by auxin starvation. While no changes in the expression of auxin biosynthetic machinery were observed, many genes related to auxin conjugation and degradation showed differential expression. Selected putative auxin glycosylating enzymes as well as members of the Gretchen Hagen 3 gene family involved in auxin amino acid conjugation showed both up- and down-regulation. Contrarily to that, all tobacco homologs of Arabidopsis thaliana DIOXYGENASE FOR AUXIN OXIDATION 1 (DAO1), known to be responsible for the formation of oxIAA from IAA, showed significant downregulation at both transcript and protein levels. To validate the role of DAO1 in auxin metabolism, we performed auxin metabolite profiling in BY-2 mutants carrying either siRNA-silenced or CRISPR-Cas9-mutated NtDAO1, as well as in dao1-1 Arabidopsis thaliana plants. Both mutants showed not only expectedly lower levels of oxIAA, but also significantly lower abundance of oxidated amino acid conjugates of IAA (oxIAA-Asp). Our results thus represent the first direct evidence on DAO1 activity on IAA amino acid conjugates.Statement of significanceHere we present an analysis of auxin metabolism on metabolite, transcript and protein levels in tobacco BY-2 cell line, collectively identifying oxidation of IAA amino acid conjugates as a new role of DIOXYGENASE FOR AUXIN OXIDATION 1 within an auxin-level-responsive metabolic system.

Expression Analysis of Key Auxin Biosynthesis, Transport, and Metabolism Genes of Betula pendula with Special Emphasis on Figured Wood Formation in Karelian Birch

Auxin status in woody plants is believed to be a critical factor for the quantity and quality of the wood formed. It has been previously demonstrated that figured wood formation in Karelian birch (Betula pendula Roth var. carelica (Merckl.) Hämet-Ahti) is associated with a reduced auxin level and elevated sugar content in the differentiating xylem, but the molecular mechanisms of the abnormal xylogenesis remained largely unclear. We have identified genes involved in auxin biosynthesis (Yucca), polar auxin transport (PIN) and the conjugation of auxin with amino acids (GH3) and UDP-glucose (UGT84B1) in the B. pendula genome, and analysed their expression in trunk tissues of trees differing in wood structure. Almost all the investigated genes were overexpressed in Karelian birch trunks. Although Yucca genes were overexpressed, trunk tissues in areas developing figured grain had traits of an auxin-deficient phenotype. Overexpression of GH3s and UGT84B1 appears to have a greater effect on figured wood formation. Analysis of promoters of the differentially expressed genes revealed a large number of binding sites with various transcription factors associated with auxin and sugar signalling. These data agree with the hypothesis that anomalous figured wood formation in Karelian birch may be associated with the sugar induction of auxin conjugation.

N-glucosyltransferase GbNGT1 from Ginkgo complement auxin metabolic pathway

As a group of the most important phytohormone, auxin homeostasis is regulated in a complex manner. Generally, auxin conjugations especially IAA glucosides are dominant on high auxin level conditions. Former terminal glucosylation researches mainly focus on O-position, while IAA-N-glucoside or IAA-Asp-N-glucoside has been neglected since their found in 2001. In our study, IAA-Asp-N-glucoside was firstly found specifically abundant (as high as 4.13 mg/g) in ginkgo seeds of 58 cultivars from Ginkgo Resource Nursery built in 1990. Furthermore, a novel N-glucosyltransferase GbNGT1, which could catalyze IAA-Asp and IAA to form their corresponding N-glucoside, was identified through differential transcriptome analysis and in vitro enzymatic test. The enzyme was demonstrated to possess specific catalyze capacity toward the N-position of IAA-amino acid or IAA among 52 substrates, and was typical of acid tolerance, metal ion independence and high temperature sensitivity. Docking and site-directed mutagenesis of this enzyme confirmed that E15G mutant could almost abolish enzyme catalytic activity towards IAA-Asp and IAA in vitro and in vivo. The IAA modification of GbNGT1 and GbGH3.5 was verified by transient expression assay in Nicotiana benthamiana. In conclusion, our results complement the terminal metabolic pathway of auxin, and the specific catalytic function of GbNGT1 towards IAA-amino acid provide a new way to biosynthesis indole-amide compounds.HighlightThe N-glucosylation of IAA or IAA-amino acids in auxin metabolism had been neglected over decades, our work for GbNGT1 redeems the missing chain of auxin metabolic pathway.

IPyA glucosylation mediates light and temperature signaling to regulate auxin-dependent hypocotyl elongation inArabidopsis

Auxin is a class of plant hormone that plays a crucial role in the life cycle of plants, particularly in the growth response of plants to ever-changing environments. Since the auxin responses are concentration-dependent and higher auxin concentrations might often be inhibitory, the optimal endogenous auxin level must be closely controlled. However, the underlying mechanism governing auxin homeostasis remains largely unknown. In this study, a UDP-glycosyltransferase (UGT76F1) was identified fromArabidopsis thaliana, which participates in the regulation of auxin homeostasis by glucosylation of indole-3-pyruvic acid (IPyA), a major precursor of the auxin indole-3-acetic acid (IAA) biosynthesis, in the formation of IPyA glucose conjugates (IPyA-Glc). In addition, UGT76F1 was found to mediate hypocotyl growth by modulating active auxin levels in a light- and temperature-dependent manner. Moreover, the transcription ofUGT76F1was demonstrated to be directly and negatively regulated by PIF4, which is a key integrator of both light and temperature signaling pathways. This study sheds a light on the trade-off between IAA biosynthesis and IPyA-Glc formation in controlling auxin levels and reveals a regulatory mechanism for plant growth adaptation to environmental changes through glucosylation of IPyA.