Biology subjects

Karami, O.

Publications and source records attributed to Karami, O..

8 recordsLinked to original sources

bioRxiv2022

The effect of short-term activation of AHL15 on long-term plant developmental change and transcriptome profile

We have previously documented that overexpression of the Arabidopsis nuclear protein AHL15 leads to reprogramming of somatic cells to embryonic cells (Karami et al., 2021) and to suppression of plant ageing (Karami et al., 2020). Here we show that transient (4 hours) activation of overexpressed AHL15-GR in Arabidopsis seedlings has long-term effects on plant development. RNA sequencing analysis detected an extensive reprogramming of the transcriptome 4 hours after AHL15-GR activation, with respectively 540 and 1107 genes showing more than 2-fold up- and down-regulation. AHL15 seemed to act in a transcription leveldependent manner, activating predominantly low expressed genes and repressing mostly highly expressed genes. Rapid decondensation of heterochromatin was observed after AHL15 activation in leaf primordia and axillary meristems, indicating that the global reprogramming of the transcriptome by transient activation of this AT-Hook domain protein might be caused by extensive modulation of the chromatin configuration. We also found that co-activated or co-repressed genes were often physically linked in small chromosomal clusters, which is in line with regulation at the chromatin level.

developmental biology↗

bioRxiv2022

Key role of auxin cellular accumulation in totipotency and pluripotency acquisition

Genome editing and in vitro based-plant propagation require efficient plant regeneration system. Somatic embryogenesis (SE) or de novo shoot regeneration are two major systems that widely used for plant in vitro regeneration. Most SE or shoot regeneration protocols rely on the exogenous application of the synthetic auxin analog 2,4-dichlorophenoxyacetic acid (2,4-D) and naphthylene acetic acid (NAA), whereas the natural auxins indole-3-acetic acid (IAA), 4-chloroindole-3-acetic acid (4-Cl-IAA) or indole-3-butyric acid (IBA) are not or less effective for plant regeneration. Although these synthetic auxins mimics the physiological activity of the main natural auxin IAA in many aspects, there are also clear differences that have been attributed to differences in stability or to different affinities for certain TIR1/AFB-Aux/IAA auxin co-receptor pairs. Here we show that the success of 2,4-D in inducing SE from Arabidopsis is related to ineffectiveness as substrate for auxin efflux, resulting in its intracellular 2,4-D accumulation. Reducing auxin efflux by addition of the auxin transport inhibitor naphthylphthalamic acid (NPA) also allowed natural auxins and other synthetic analogs to induce SE in Arabidopsis with similar efficiencies as 2,4-D. The PIN-FORMED auxin efflux carriers PIN1, PIN2 and the ATP-binding cassette-B auxin transporters ABCB1 and ABCB19 were shown to be partially responsible for the efflux of natural auxins during SE induction. Importantly, all somatic embryos induced in Arabidopsis by IAA in the presence of NPA showed a normal embryo to seedling conversion and subsequent plant development, whereas for the 2,4-D system this was limited to 50-60% of the embryos. We showed that the auxin transport inhibition promotes de novo shoot regeneration capacity from callus induced by 4-Cl-IAA in Brassica napus. In addition, we observed a obvious acceleration in shoot bud emerging from callus induced by 4-Cl-IAA than 2,4-D. Based on our data we conclude, that the efficiency of plant propagation can be significantly improved by applying the natural auxins in the presence of the auxin transport inhibitor NPA.

plant biology↗

bioRxiv2022

Structure-activity relationship of 2,4-D correlates auxin activity with the induction of somatic embryogenesis in Arabidopsis thaliana

2,4-dichlorophenoxyacetic acid (2,4-D) is a synthetic analogue of the plant hormone auxin that is commonly used in many in vitro plant regeneration systems, such as somatic embryogenesis (SE). Its effectiveness in inducing SE, compared to the natural auxin indole-3-acetic acid (IAA), has been attributed to the stress triggered by this compound rather than its auxin activity. However, this hypothesis has never been thoroughly tested. Here we used a library of 40 2,4-D analogues to test the structure-activity relationship with respect to the capacity to induce SE and auxin activity in Arabidopsis thaliana. Four analogues induced SE as effectively as 2,4-D and 13 analogues induced SE but were less effective. Based on root growth inhibition and auxin response reporter expression, the 2,4-D analogues were classified into different groups, ranging from very active auxins to not active. A halogen at the 4-position of the aromatic ring was important for auxin activity, whereas a halogen at the 3-position resulted in reduced activity. Moreover, a small substitution at the carboxylate chain was tolerated, as was extending the carboxylate chain with two but not with one carbon. In the process, we also identified two 2,4-D analogues as efficient inducers of adventitious root formation and several possible anti-auxins. The auxin activity of the 2,4-D analogues was consistent with their simulated TIR1-Aux/IAA coreceptor binding characteristics. A strong correlation was observed between SE induction efficiency and auxin activity, indicating that the stress-related effects triggered by 2,4-D that are considered important for SE induction are down-stream of auxin signaling.

plant biology↗

bioRxiv2021

Endogenous auxin directs development of embryonic stem cells into somatic proembryos in Arabidopsis

Somatic embryogenesis (SE) is the process by which embryos develop from in vitro cultured vegetative tissue explants. The synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) is widely used for SE induction, but SE can also be induced by overexpression of specific transcription factors, such as AT-HOOK MOTIF NUCLEAR LOCALIZED 15 (AHL15). 2,4-D and AHL15 both trigger the biosynthesis of the natural auxin indole-3-acetic acid (IAA). However, the role of this endogenously produced auxin in SE is yet not well understood. In this study we show that the induction of embryonic stem cells from explants does not require IAA biosynthesis, whereas an increase in IAA levels is essential to maintain embryo identity and for embryo formation from these stem cells. Further analysis showed that YUCCA (YUC) genes involved in the IPyA auxin biosynthesis pathway are up-regulated in embryo-forming tissues. Chemical inhibition of the IPyA pathway significantly reduced or completely inhibited the formation of somatic embryos in both 2,4-D-and AHL15-dependent systems. In the latter system, SE could be restored by exogenous IAA application, confirming that the biosynthesis-mediated increase in IAA levels is important. Our analyses also showed that PIN1 and AUX1 are the major auxin carriers that determine respectively auxin efflux and influx during SE. This auxin transport machinery is required for the proper transition of embryonic cells to proembryos and, later, for correct cell fate specification and differentiation. Taken together, our results indicate that auxin biosynthesis in conjunction with its polar transport are required during SE for multicellular somatic proembryo development and differentiation. One sentence summarySomatic embryogenesis in Arabidopsis requires auxin biosynthesis and polar auxin transport only after the acquisition of embryonic competence for somatic proembryo development and differentiation.

plant biology↗

bioRxiv2020

miR156-independent repression of the ageing pathway bylongevity-promoting AHL proteins in Arabidopsis

Plants age by transition through distinct developmental phases, with the juvenile- to adult vegetative and the adult vegetative to reproductive phase change as two consecutive important post-embryonic phase transitions. During the transition from the juvenile- to adult vegetative phase, also known as the vegetative phase change (VPC), many plants undergo specific morphological and physiological changes. In the model plant Arabidopsis thaliana (Arabidopsis), for example, the VPC is marked by clear heteroblastic changes in leaf shape and size and the appearance of trichomes on the abaxial side of leaves. The VPC and the vegetative to reproductive transition are promoted by the SQUAMOSA PROMOTOR BINDING PROTEIN-LIKE (SPL) family of transcription factors and repressed by miR156 and miR157 that target the SPL transcripts. Here we present data that the plant longevity promoting AT-HOOK MOTIF NUCLEAR LOCALIZED protein 15 (AHL15) and family members also repress the SPL-driven ageing pathway. Arabidopsis ahl loss-of-function mutants showed an accelerated VPC and flowering time, whereas AHL15 overexpression dramatically delayed the VPC and flowering time in both Arabidopsis and Nicotiana tabacum. Expression analysis and tissue-specific AHL15 overexpression revealed that AHL15 affects the VPC and flowering time directly through its expression in the shoot apical meristem and young leaves. In addition, we found evidence that AHL15 represses SPL gene expression in a miR156/157-independent manner. The juvenile traits of spl loss-of-function mutants appeared to be dependent on the enhanced expression of the AHL15 gene, providing evidence for a reciprocal negative feedback between AHL15 and SPL genes. Antagonistically to AHLs, SPLs promote axillary meristem (AM) maturation and thus prevent vegetative growth from these meristems by repressing AHL15 expression. Taken together, our results place AHL15 and family members at a central position in the SPL-driven ageing pathway as suppressors of the VPC, flowering time and AM maturation.

plant biology↗

bioRxiv2020

A novel pathway controlling cambium initiation and -activity via cytokinin biosynthesis in Arabidopsis

Plant secondary growth, also referred to as wood formation, includes the production of secondary xylem, which is derived from meristematic cambium cells embedded in vascular tissues. Despite the importance of secondary xylem in plant growth and wood formation, the molecular mechanism of secondary growth is not yet well understood. Here we identified an important role for the Arabidopsis thaliana (Arabidopsis) AT-HOOK MOTIF CONTAINING NUCLEAR LOCALIZED 15 (AHL15) gene, encoding for a putative transcriptional regulator, in controlling vascular cambium activity and secondary xylem formation. Secondary xylem development was significantly reduced in inflorescence stems of the Arabidopsis ahl15 loss-of-function mutant, whereas AHL15 overexpression led to extensive secondary xylem formation. AHL15 expression under a vascular meristem-specific promoter also enhanced the amount of interfascicular secondary xylem. Moreover, AHL15 appeared to be required for the enhanced secondary xylem formation in the Arabidopsis double loss-of-function mutant of the SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1) and FRUITFULL (FUL) genes. A well-known central regulator of cambial activity is the plant hormone cytokinin. We showed that the expression of two cytokinin biosynthesis genes (ISOPENTENYL TRANSERASE (IPT) 3 and 7) is decreased in ahl15 loss-of-function mutant stems, whereas the secondary xylem deficiency in these mutant stems can be resorted by cambium-specific expression of the Agrobacterium tumefaciens IPT gene, indicating that AHL15 acts through the cytokinin pathway. These findings support a model whereby AHL15 acts as a central factor inducing vascular cambium activity downstream of SOC1 and FUL and upstream of IPT3, IPT7 and LOG4, LOG5 governing the rate of secondary xylem formation in Arabidopsis inflorescence stems.

plant biology↗

bioRxiv2020

An Arabidopsis AT-hook motif nuclear protein mediates somatic embryogenesis and coinciding genome duplication

Plant somatic cells can be reprogrammed to totipotent embryonic cells that are able to form differentiated embryos in a process called somatic embryogenesis (SE), by hormone treatment or through overexpression of certain transcription factor genes, such as BABY BOOM (BBM). Here we show that overexpression of the AT-HOOK MOTIF CONTAINING NUCLEAR LOCALIZED 15 (AHL15) gene induces formation of somatic embryos on Arabidopsis thaliana seedlings in the absence of hormone treatment. During zygotic embryogenesis, AHL15 expression starts early in embryo development, and AH15 and other AHL genes are required for proper embryo patterning and development beyond the heart stage. Moreover, AHL15 and several of its homologs are upregulated and required for SE induction upon hormone treatment, and they are required for efficient BBM-induced SE as downstream targets of BBM. A significant number of plants derived from AHL15 overexpression-induced somatic embryos are polyploid. Polyploidisation occurs by endomitosis specifically during the initiation of SE, assumingly due to AHL15-mediated heterochromatin decondensation coinciding with the acquisition of embryonic competency in somatic plant cells.

plant biology↗

bioRxiv2020

A suppressor of axillary meristem maturation promotes longevity in flowering plants

Post embryonic development and growth of flowering plants are for a large part determined by the activity and maturation state of stem cell niches formed in the axils of leaves, the so-called axillary meristems (AMs)1,2. Here we identify a new role for the Arabidopsis AT-HOOK MOTIF CONTAINING NUCLEAR LOCALIZED 15 (AHL15) gene as a suppressor of AM maturation. Loss of AHL15 function accelerates AM maturation, whereas ectopic expression of AHL15 suppresses AM maturation and promotes longevity in Arabidopsis and tobacco. Together our results indicate that AHL15 expression acts as a key molecular switch, directly downstream of flowering genes (SOC1, FUL) and upstream of GA biosynthesis, in extending the plants lifespan by suppressing AM maturation.

plant biology↗