Early axis growth during seed germination is gravitropic and mediated by ROS and calcium

Sucrose and Mannans Affect Arabidopsis Shoot Gravitropism at the Cell Wall Level

Gravitropism is the plant organ bending in response to gravity. Gravitropism, phototropism and sufficient mechanical strength define the optimal position of young shoots for photosynthesis. Etiolated wild-type Arabidopsis seedlings grown horizontally in the presence of sucrose had a lot more upright hypocotyls than seedlings grown without sucrose. We studied the mechanism of this effect at the level of cell wall biomechanics and biochemistry. Sucrose strengthened the bases of hypocotyls and decreased the content of mannans in their cell walls. As sucrose is known to increase the gravitropic bending of hypocotyls, and mannans have recently been shown to interfere with this process, we examined if the effect of sucrose on shoot gravitropism could be partially mediated by mannans. We compared cell wall biomechanics and metabolomics of hypocotyls at the early steps of gravitropic bending in Col-0 plants grown with sucrose and mannan-deficient mutant seedlings. Sucrose and mannans affected gravitropic bending via different mechanisms. Sucrose exerted its effect through cell wall-loosening proteins, while mannans changed the walls' viscoelasticity. Our data highlight the complexity of shoot gravitropism control at the cell wall level.

Melatonin delays ABA-induced leaf senescence via H2 O2 -dependent calcium signalling

Precocious leaf senescence can reduce crop yield and quality by limiting the growth stage. Melatonin has been shown to delay leaf senescence; however, the underlying mechanism remains obscure. Here, we show that melatonin offsets abscisic acid (ABA) to protect photosystem II and delay the senescence of attached old leaves under the light. Melatonin induced H₂ O₂ accumulation accompanied by an upregulation of melon respiratory burst oxidase homolog D (CmRBOHD) under ABA-induced stress. Both melatonin and H₂ O₂ induced the accumulation of cytoplasmic-free Ca²⁺ ([Ca²⁺ ]_cyt ) in response to ABA, while blocking of Ca²⁺ influx channels attenuated melatonin- and H₂ O₂ -induced ABA tolerance. CmRBOHD overexpression induced [Ca²⁺ ]_cyt accumulation and delayed leaf senescence, whereas deletion of Arabidopsis AtRBOHD, a homologous gene of CmRBOHD, compromised the melatonin-induced [Ca²⁺ ]_cyt accumulation and delay of leaf senescence in Arabidopsis under ABA stress. Furthermore, melatonin, H₂ O₂  and Ca²⁺ attenuated ABA-induced K⁺ efflux and subsequent cell death. CmRBOHD overexpression and AtRBOHD deletion alleviated and aggravated the ABA-induced K⁺ efflux, respectively. Taken together, our study unveils a new mechanism by which melatonin offsets ABA action to delay leaf senescence via RBOHD-dependent H₂ O₂ production that triggers [Ca²⁺ ]_cyt accumulation and subsequently inhibits K⁺ efflux and delays cell death/leaf senescence in response to ABA.

H2O2 and Ca2+ Signaling Crosstalk Counteracts ABA to Induce Seed Germination

Seed germination is a critical stage and the first step in the plant's life cycle. H2O2 and Ca2+ act as important signal molecules in regulating plant growth and development and in providing defense against numerous stresses; however, their crosstalk in modulating seed germination remains largely unaddressed. In the current study, we report that H2O2 and Ca2+ counteracted abscisic acid (ABA) to induce seed germination in melon and Arabidopsis by modulating ABA and gibberellic acid (GA3) balance. H2O2 treatment induced a Ca2+ influx in melon seeds accompanied by the upregulation of cyclic nucleotide-gated ion channel(CNGC) 20, which encodes a plasma membrane Ca2+-permeable channel. However, the inhibition of cytoplasmic free Ca2+ elevation in the melon seeds and Arabidopsis mutant atcngc20 compromised H2O2-induced germination under ABA stress. CaCl2 induced H2O2 accumulation accompanied by the upregulation of respiratory burst oxidase homologue(RBOH) D and RBOHF in melon seeds with ABA pretreatment. However, inhibition of H2O2 accumulation in the melon seeds and Arabidopsis mutant atrbohd and atrbohf abolished CaCl2-induced germination under ABA stress. The current study reveals a novel mechanism in which H2O2 and Ca2+ signaling crosstalk offsets ABA to induce seed germination. H2O2 induces Ca2+ influx, which in turn increases H2O2 accumulation, thus forming a reciprocal positive-regulatory loop to maintain a balance between ABA and GA3 and promote seed germination under ABA stress.

Brassinosteroids Influence Arabidopsis Hypocotyl Graviresponses through Changes in Mannans and Cellulose

The force of gravity is a constant environmental factor. Plant shoots respond to gravity through negative gravitropism and gravity resistance. These responses are essential for plants to direct the growth of aerial organs away from the soil surface after germination and to keep an upright posture above ground. We took advantage of the effect of brassinosteroids (BRs) on the two types of graviresponses in Arabidopsis thaliana hypocotyls to disentangle functions of cell wall polymers during etiolated shoot growth. The ability of etiolated Arabidopsis seedlings to grow upward was suppressed in the presence of 24-epibrassinolide (EBL) but enhanced in the presence of brassinazole (BRZ), an inhibitor of BR biosynthesis. These effects were accompanied by changes in cell wall mechanics and composition. Cell wall biochemical analyses, confocal microscopy of the cellulose-specific pontamine S4B dye and cellular growth analyses revealed that the EBL and BRZ treatments correlated with changes in cellulose fibre organization, cell expansion at the hypocotyl base and mannan content. Indeed, a longitudinal reorientation of cellulose fibres and growth inhibition at the base of hypocotyls supported their upright posture whereas the presence of mannans reduced gravitropic bending. The negative effect of mannans on gravitropism is a new function for this class of hemicelluloses. We also found that EBL interferes with upright growth of hypocotyls through their uneven thickening at the base.

Metabolomics and Physiological Approach to Understand Allelopathic Effect of Horseradish Extract on Onion Root and Lettuce Seed as Model Organism

In the present study, we assessed the allelopathic effects of various concentrations (0%, 0.1%, 0.2%, and 0.3%) of horseradish root extract (HRE) on onion root. The average growth of onion root tips during the 0% HRE treatment (deionized water treatment) was 0.9 cm/day, which was the highest among the growth rates obtained with all HRE treatments. Moreover, the average growth during 0.3% HRE treatment was 0.1 cm/day. During cell cycle analysis, the mitotic phase fraction of the control (deionized water treatment) cells was 6.5% of all dividing cells, with this percentage being the highest among the values obtained for all treatment groups. In the control group, all cell cycle phases were identified; however, in the 0.1%, 0.2%, and 0.3% treatment groups, telophase was not identified. The ROS accumulation area of the onion root decreased, as the HRE treatment concentration increased. In the control root, the area of dead tissue was 0%; however, in the 0.1% and 0.2% HRE treatment roots, the ratio was 5% and 50%, respectively. These findings indicate that the allelopathic effect of HRE depends on the concentration of HRE applied to the onion root.

Melatonin antagonizes ABA action to promote seed germination by regulating Ca2+ efflux and H2O2 accumulation

Seed germination is a vital stage in the plant life-cycle that greatly contributes to plant establishment. Melatonin has been shown to promote seed germination under various environmental stresses; however, the mechanism remains largely underexplored. Here, we reported that melatonin antagonized abscisic acid (ABA) to promote seed germination by regulating ABA and gibberellic acid (GA₃) balance. Transcriptomic analysis revealed that such a role of melatonin was associated with Ca²⁺ and redox signaling. Melatonin pretreatment induced Ca²⁺ efflux accompanied by an up-regulation of vacuolar H+/Ca²⁺ antiporter 3 (CAX3). AtCAX3 deletion in Arabidopsis exhibited reduced Ca²⁺ efflux. Inhibition of Ca²⁺ efflux in the seeds of melon and Arabidopsis mutant AtCAX3 compromised melatonin-induced germination under ABA stress. Melatonin increased H₂O₂ accumulation, and H₂O₂ pretreatment decreased ABA/GA₃ ratio and promoted seed germination under ABA stress. However, complete inhibition of H₂O₂ accumulation abolished melatonin-induced ABA and GA₃ balance and seed germination. Our study reveals a novel regulatory mechanism in which melatonin counteracts ABA to induce seed germination that essentially involves CAX3-mediated Ca²⁺ efflux and H₂O₂ accumulation, which, in turn, regulate ABA and GA₃ balance by promoting ABA catabolism and/or GA₃ biosynthesis.

Quantitative proteomics reveals dual effects of calcium on radicle protrusion in soybean

To reveal calcium-mediated germination in soybean, a gel-free/label-free proteomics was performed in radicle of seed imbibed with CaCl2. Morphological analysis presented promoting and suppressing performance of seed growth under 5 and 50 mM CaCl2, respectively. A total of 106 and 581 proteins were identified in response to 5 and 50 mM CaCl2, respectively. Among 33 proteins, which were simultaneously affected by 5 and 50 mM CaCl2 imbibition, proteins related to protein metabolism, cell, development, and stress showed reversed abundance in response to CaCl2 on dose-dependent manner. Notably, protein abundance of late embryogenesis abundant (LEA) 4-5, LEA4, and dehydrin decreased and increased by 5 and 50 mM CaCl2, respectively, consistent with the transcript level. Moreover, inhibited biosynthesis of gibberellic acid repressed growth of 5 mM CaCl2-imbibed soybean, while inhibition of abscisic acid biosynthesis released the suppressing effects of 50 mM CaCl2. Taken together, these results suggest that decreased or increased protein abundance of LEA4-5, LEA4, and dehydrin might determine promoting or suppressing effects of low or high level of calcium on soybean through enhancing seed sensitivity to gibberellic acid or abscisic acid during radicle protrusion. SIGNIFICANCE: Calcium serves as a versatile signal in plant growth; however, calcium-mediated germination on dose-dependent manner remains elusive. In this study, dual effects of calcium on radicle protrusion in soybean were investigated using proteomic approach. Radicle growth of germinating seed was improved by 5 mM CaCl2; however, it was retarded by 50 mM CaCl2. Late embryogenesis abundant (LEA) 4-5, LEA4, and dehydrin displayed converse profiles in response to low and high concentrations of CaCl2 at both protein abundance and gene expression level. Inhibited biosynthesis of gibberellic acid (GA) significantly impeded radicle protrusion in presence of low concentration of CaCl2, while inhibiting of abscisic acid (ABA) biosynthesis released suppression induced by high concentration of CaCl2. These findings suggest that LEA proteins are associated with calcium-mediated radicle protrusion on dose-dependent manner, and seed sensitivity to GA and ABA might determine promoting and suppressing effects of calcium on radicle protrusion in soybean.

The signalling role of ROS in the regulation of seed germination and dormancy

Reactive oxygen species (ROS) are versatile compounds which can have toxic or signalling effects in a wide range living organisms, including seeds. They have been reported to play a pivotal role in the regulation of seed germination and dormancy but their mechanisms of action are still far from being fully understood. In this review, we sum-up the major findings that have been carried out this last decade in this field of research and which altogether shed a new light on the signalling roles of ROS in seed physiology. ROS participate in dormancy release during seed dry storage through the direct oxidation of a subset of biomolecules. During seed imbibition, the controlled generation of ROS is involved in the perception and transduction of environmental conditions that control germination. When these conditions are permissive for germination, ROS levels are maintained at a level which triggers cellular events associated with germination, such as hormone signalling. Here we propose that the spatiotemporal regulation of ROS production acts in concert with hormone signalling to regulate the cellular events involved in cell expansion associated with germination.

A calcium-dependent lipid binding protein, OsANN10, is a negative regulator of osmotic stress tolerance in rice

Annexin, a multi-gene family in plants, is essential for plant growth and stress responses. Recent studies demonstrated a positive effect of annexin in abiotic stress responses. Interestingly, we found OsANN10, a putative annexin gene in rice, negatively regulated plant responses to osmotic stress. Knocking down OsANN10 significantly decreased the content of H₂O₂ by increasing Peroxidase (POD) and Catalase (CAT) activities, further reducing oxidative damage in rice leaves, suggesting a negative regulation of OsANN10 in protecting cell membrane against oxidative damage via scavenging ROS under osmotic stress.

Orchestration of Cu-Zn SOD and class III peroxidase with upstream interplay between NADPH oxidase and PM H+-ATPase mediates root growth in Vigna radiata (L.) Wilczek

Post-germination plant growth depends on the regulation of reactive oxygen species (ROS) metabolism, spatiotemporal pH changes and Ca⁺² homeostasis, whose potential integration has been studied during Vigna radiata (L.) Wilczek root growth. The dissipation of proton (H⁺) gradients across plasma membrane (PM) by CCCP (protonophore) and the inhibition of PM H⁺-ATPase by sodium orthovanadate repressed SOD (superoxide dismutase; EC 1.15.1.1) activity as revealed by spectrophotometric and native PAGE assay results. Similar results derived from treatment with DPI (NADPH oxidase inhibitor) and Tiron (O₂- scavenger) denote a functional synchronization of SOD, PM H⁺-ATPase and NOX, as the latter two enzymes are substrate sources for SOD (H⁺ and O₂-, respectively) and are involved in a feed-forward loop. After SOD inactivation, a decline in apoplastic H₂O₂ content was observed in each treatment group, emerging as a possible cause of the diminution of class III peroxidase (Prx; EC 1.11.1.7), which utilizes H₂O₂ as a substrate. In agreement with the pivotal role of Ca⁺² in PM H⁺-ATPase and NOX activation, Ca⁺² homeostasis antagonists, i.e., LaCl₃ (Ca⁺² channel inhibitor), EGTA (Ca⁺² chelator) and LiCl (endosomal Ca⁺² release blocker), inhibited both SOD and Prx. Finally, a drastic reduction in apoplastic OH (hydroxyl radical) concentrations (induced by each treatment, leading to Prx inhibition) was observed via fluorometric analysis. A consequential inhibition of root growth observed under each treatment denotes the importance of the orchestrated functioning of PM H⁺-ATPase, NOX, Cu-Zn SOD and Prx during root growth. A working model demonstrating postulated enzymatic synchronization with an intervening role of Ca⁺² is proposed.