Dissecting the mechanism of abscisic acid-induced dynamic microtubule reorientation using live cell imaging

Abscisic acid induces the expression of AsKIN during the recovery period of garlic cryopreservation

Abscisic acid induced the expression of AsKIN during the recovery period of garlic cryopreservation. AsKIN was identified as a gene involved in cold and osmotic stress resistance. Cryopreservation has been proven to be effective in removing viruses from garlic. However, oxidative damage in cryopreservation has a significant impact on the survival after preservation. Abscisic acid (ABA) has been shown to reduce oxidative stress and promote the survival after cryopreservation. However, it is not clear which genes play important roles in this process. In this study, we added ABA to the dehydration step and analyzed the transcriptomic divergences between the ABA-treated group and the control group in three cryogenic steps (dehydration, unloading and recovery). By short time-series expression miner (STEM) analysis and weighted gene co-expression network analysis (WGCNA), the recovery step was identified as the period of significant changes in gene expression levels in cryopreservation. The addition of ABA promoted the upregulated expression of microtubule-related genes in the recovery step. We further identified AsKIN as a hub gene in the recovery step and verified its function. The results showed that overexpression of AsKIN enhanced the tolerance of Arabidopsis to oxidative stress in cryopreservation, influenced the expression of genes in response to cold and osmotic stress and promoted plant growth after stress. The AsKIN gene is likely to be involved in the plant response to cold stress and osmotic stress. These results reveal the molecular mechanisms of ABA in cryopreservation and elucidate the potential biological functions of the kinesin-14 subfamily.

Silver Nanoparticles Alter Microtubule Arrangement, Dynamics and Stress Phytohormone Levels

The superior properties of silver nanoparticles (AgNPs) has resulted in their broad utilization worldwide, but also the risk of irreversible environment infestation. The plant cuticle and cell wall can trap a large part of the nanoparticles and thus protect the internal cell structures, where the cytoskeleton, for example, reacts very quickly to the threat, and defense signaling is subsequently triggered. We therefore used not only wild-type Arabidopsis seedlings, but also the glabra 1 mutant, which has a different composition of the cuticle. Both lines had GFP-labeled microtubules (MTs), allowing us to observe their arrangement. To quantify MT dynamics, we developed a new microscopic method based on the FRAP technique. The number and growth rate of MTs decreased significantly after AgNPs, similarly in both lines. However, the layer above the plasma membrane thickened significantly in wild-type plants. The levels of three major stress phytohormone derivatives—jasmonic, abscisic, and salicylic acids—after AgNP (with concomitant Ag⁺) treatment increased significantly (particularly in mutant plants) and to some extent resembled the plant response after mechanical stress. The profile of phytohormones helped us to estimate the mechanism of response to AgNPs and also to understand the broader physiological context of the observed changes in MT structure and dynamics.

Associations between phytohormones and cellulose biosynthesis in land plants

BACKGROUND

Phytohormones are small molecules that regulate virtually every aspect of plant growth and development, from basic cellular processes, such as cell expansion and division, to whole plant environmental responses. While the phytohormone levels and distribution thus tell the plant how to adjust itself, the corresponding growth alterations are actuated by cell wall modification/synthesis and internal turgor. Plant cell walls are complex polysaccharide-rich extracellular matrixes that surround all plant cells. Among the cell wall components, cellulose is typically the major polysaccharide, and is the load-bearing structure of the walls. Hence, the cell wall distribution of cellulose, which is synthesized by large Cellulose Synthase protein complexes at the cell surface, directs plant growth.

SCOPE

Here, we review the relationships between key phytohormone classes and cellulose deposition in plant systems. We present the core signalling pathways associated with each phytohormone and discuss the current understanding of how these signalling pathways impact cellulose biosynthesis with a particular focus on transcriptional and post-translational regulation. Because cortical microtubules underlying the plasma membrane significantly impact the trajectories of Cellulose Synthase Complexes, we also discuss the current understanding of how phytohormone signalling impacts the cortical microtubule array.

CONCLUSION

Given the importance of cellulose deposition and phytohormone signalling in plant growth and development, one would expect that there is substantial cross-talk between these processes; however, mechanisms for many of these relationships remain unclear and should be considered as the target of future studies.

Microtubule self-organisation during seed germination in Arabidopsis

BACKGROUND

Upon water uptake and release of seed dormancy, embryonic plant cells expand, while being mechanically constrained by the seed coat. Cortical microtubules (CMTs) are key players of cell elongation in plants: their anisotropic orientation channels the axis of cell elongation through the guidance of oriented deposition of load-bearing cellulose microfibrils in the cell wall. Interestingly, CMTs align with tensile stress, and consistently, they reorient upon compressive stress in growing hypocotyls. How CMTs first organise in germinating embryos is unknown, and their relation with mechanical stress has not been investigated at such an early developing stage.

RESULTS

Here, we analysed CMT dynamics in dormant and non-dormant Arabidopsis seeds by microscopy of fluorescently tagged microtubule markers at different developmental time points and in response to abscisic acid and gibberellins. We found that CMTs first appear as very few thick bundles in dormant seeds. Consistently, analysis of available transcriptome and translatome datasets show that limiting amounts of tubulin and microtubule regulators initially hinder microtubule self-organisation. Seeds imbibed in the presence of gibberellic acid or abscisic acid displayed altered microtubule organisation and transcriptional regulation. Upon the release of dormancy, CMTs then self-organise into multiple parallel transverse arrays. Such behaviour matches the tensile stress patterns in such mechanically constrained embryos. This suggests that, as CMTs first self-organise, they also align with shape-derived tensile stress patterns.

CONCLUSIONS

Our results provide a scenario in which dormancy release in the embryo triggers microtubule self-organisation and alignment with tensile stress prior to germination and anisotropic growth.

Transcriptional analyses of two soybean cultivars under salt stress

Soybean is an economically important plant, and its production is affected in soils with high salinity levels. It is important to understand the adaptive mechanisms through which plants overcome this kind of stress and to identify potential genes for improving abiotic stress tolerance. RNA-Seq data of two Glycine max cultivars, a drought-sensitive (C08) and a tolerant (Conquista), subjected to different periods of salt stress were analyzed. The transcript expression profile was obtained using a transcriptogram approach, comparing both cultivars and different times of treatment. After 4 h of salt stress, Conquista cultivar had 1400 differentially expressed genes, 647 induced and 753 repressed. Comparative expression revealed that 719 genes share the same pattern of induction or repression between both cultivars. Among them, 393 genes were up- and 326 down-regulated. Salt stress also modified the expression of 54 isoforms of miRNAs in Conquista, by the maturation of 39 different pre-miRNAs. The predicted targets for 12 of those mature miRNAs also have matches with 15 differentially expressed genes from our analyses. We found genes involved in important pathways related to stress adaptation. Genes from both ABA and BR signaling pathways were modulated, with possible crosstalk between them, and with a likely post-transcriptional regulation by miRNAs. Genes related to ethylene biosynthesis, DNA repair, and plastid translation process were those that could be regulated by miRNA.

Abscisic acid and nitric oxide modulate cytoskeleton organization, root hair growth and ectopic hair formation in Arabidopsis

Abscisic acid (ABA) and nitric oxide (NO) are two plant growth regulators that participate in many signaling cascades in different organs all along the plant life. Here, we were interested in deciphering the effects of ABA and NO on the cytoskeleton organization in a model of polarized cell growth like root hairs. Arabidopsis roots were exposed to different concentrations of ABA, and the length of primary root, epidermal cells and root hairs were measured. The NO concentration was detected with the NO-specific fluorescent probe DAF-FM DA. To quantify the effects of ABA and NO on cytoskeleton, Arabidopsis seedlings expressing GFP-MAP4 were used to analyze microtubules (MTs) orientation. Changes in cytoplasmic streaming were quantified through fluorescence recovery after photobleaching (FRAP) experiments using confocal laser scanning microscopy (CLSM) and the probe fluorescein diacetate (FDA). Results indicate that ABA decreases root hair length and induces the differentiation of atrichoblasts into trichoblasts, increasing root hair density. ABA also triggers an increase of NO level in root hairs. Both, ABA and NO affect MT organization in root hairs. While root hairs show MT orientation close to the longitudinal axis in control roots, ABA and NO treatments induce the oblique orientation of MTs. In parallel, cytoplasmic flow, executed by actin cytoskeleton, is enhanced by NO, in an ABA-independent manner. For all experimental conditions assayed, basal levels of NO are required to keep MT organization and cytoplasmic streaming. Our findings support ABA and NO as key modulators of growth and ectopic formation of root hairs through actions on cytoskeleton functions.

ABA Affects Brassinosteroid-Induced Antioxidant Defense via ZmMAP65-1a in Maize Plants

Brassinosteroids (BRs) and ABA co-ordinately regulate water deficit tolerance in maize leaves. ZmMAP65-1a, a maize microtubule-associated protein (MAP) which plays an essential role in BR-induced antioxidant defense, has been characterized previously. However, the interactions among BR, ABA and ZmMAP65-1a in water deficit tolerance remain unexplored. In this study, we demonstrated that ABA was required for BR-induced antioxidant defense via ZmMAP65-1a by using biochemical blocking and ABA biosynthetic mutants. The expression of ZmMAP65-1a in maize leaves and mesophyll protoplasts could be increased under polyethylene glycol- (PEG) stimulated water deficit and ABA treatments. Furthermore, the importance of ABA in the early pathway of BR-induced water deficit tolerance was demonstrated by limiting ABA availability. Blocking ABA biosynthesis biochemically or by a null mutation inhibited the downstream gene expression of ZmMAP65-1a and the activity of ZmMAPK5 in the pathway. It also affected the activities of BR-induced antioxidant defense-related enzymes, namely ascorbate peroxidase (APX), catalase (CAT), glutathione reductase (GR), superoxide dismutase (SOD) and NADPH oxidase. In addition, combining results from transiently overexpressed or silenced ZmMAP65-1a in mesophyll protoplasts, we discovered that ZmMAP65-1a mediated the ABA-induced gene expression and activities of APX and SOD. Surprisingly, silencing of ZmMAP65-1a in mesophyll protoplasts did not alter the gene expression of ZmCCaMK and vice versa in response to ABA. Taken together, our data indicate that water deficit-induced ABA is a key mediator in BR-induced antioxidant defense via ZmMAP65-1a in maize.