Biphasic control of cell expansion by auxin coordinates etiolated seedling development

Auxin concentration–dependent cell expansion coordinates hypocotyl elongation and apical hook development for soil emergence.

Chemical inhibition of auxin inactivation pathway uncovers the metabolic turnover of auxin homeostasis

The phytohormone auxin, specifically indole-3-acetic acid (IAA) plays a prominent role in plant development. Cellular auxin concentration is coordinately regulated by auxin synthesis, transport, and inactivation to maintain auxin homeostasis; however, the physiological contribution of auxin inactivation to auxin homeostasis has remained elusive. The GH3 genes encode auxin amino acid conjugating enzymes that perform a central role in auxin inactivation. The chemical inhibition of GH3s in planta is challenging because the inhibition of GH3 enzymes leads to IAA overaccumulation that rapidly induces GH3 expression. Here, we developed a potent GH3 inhibitor, designated as kakeimide (KKI), that selectively targets auxin-conjugating GH3s. Chemical knockdown of the auxin inactivation pathway demonstrates that auxin turnover is very rapid (about 10 min), indicating auxin biosynthesis and inactivation dynamically regulate auxin homeostasis.

Tissue-wide integration of mechanical cues promotes effective auxin patterning

New plant organs form by local accumulation of auxin, which is transported by PIN proteins that localize following mechanical stresses. As auxin itself modifies tissue mechanics, a feedback loop between tissue mechanics and auxin patterning unfolds—yet the impact of tissue-wide mechanical coupling on auxin pattern emergence remains unclear. Here, we use a model composed of a vertex model for plant tissue mechanics and a compartment model for auxin transport to explore the collective mechanical response of the tissue to auxin patterns and how it feeds back onto auxin transport. We compare a model accounting for a tissue-wide mechanical integration to a model that regards cells as mechanically isolated. We show that tissue-wide mechanical coupling not only leads to more focused auxin spots via stress redistribution, but that it also mitigates the disruption to patterning when considering noise in the mechanical properties of each cell of the tissue. We find that this mechanism predicts that a local turgor increase correlates with auxin concentration, and yet auxin spots can exist regardless of the exact local turgor distribution.

Effects of Phosphate Shortage on Root Growth and Hormone Content of Barley Depend on Capacity of the Roots to Accumulate ABA

Although changes in root architecture in response to the environment can optimize mineral and water nutrient uptake, mechanisms regulating these changes are not well-understood. We investigated whether P deprivation effects on root development are mediated by abscisic acid (ABA) and its interactions with other hormones. The ABA-deficient barley mutant Az34 and its wild-type (WT) were grown in P-deprived and P-replete conditions, and hormones were measured in whole roots and root tips. Although P deprivation decreased growth in shoot mass similarly in both genotypes, only the WT increased primary root length and number of lateral roots. The effect was accompanied by ABA accumulation in root tips, a response not seen in Az34. Increased ABA in P-deprived WT was accompanied by decreased concentrations of cytokinin, an inhibitor of root extension. Furthermore, P-deficiency in the WT increased auxin concentration in whole root systems in association with increased root branching. In the ABA-deficient mutant, P-starvation failed to stimulate root elongation or promote branching, and there was no decline in cytokinin and no increase in auxin. The results demonstrate ABA’s ability to mediate in root growth responses to P starvation in barley, an effect linked to its effects on cytokinin and auxin concentrations.

Glutathione Enhances Auxin Sensitivity in Arabidopsis Roots

Root development is regulated by the tripeptide glutathione (GSH), a strong non-enzymatic antioxidant found in plants but with a poorly understood function in roots. Here, Arabidopsis mutants deficient in GSH biosynthesis (cad2, rax1, and rml1) and plants treated with the GSH biosynthesis inhibitor buthionine sulfoximine (BSO) showed root growth inhibition, significant alterations in the root apical meristem (RAM) structure (length and cell division), and defects in lateral root formation. Investigation of the molecular mechanisms of GSH action showed that GSH deficiency modulated total ubiquitination of proteins and inhibited the auxin-related, ubiquitination-dependent degradation of Aux/IAA proteins and the transcriptional activation of early auxin-responsive genes. However, the DR5 auxin transcriptional response differed in root apical meristem (RAM) and pericycle cells. The RAM DR5 signal was increased due to the up-regulation of the auxin biosynthesis TAA1 protein and down-regulation of PIN4 and PIN2, which can act as auxin sinks in the root tip. The transcription auxin response (the DR5 signal and expression of auxin responsive genes) in isolated roots, induced by a low (0.1 µM) auxin concentration, was blocked following GSH depletion of the roots by BSO treatment. A higher auxin concentration (0.5 µM) offset this GSH deficiency effect on DR5 expression, indicating that GSH deficiency does not completely block the transcriptional auxin response, but decreases its sensitivity. The ROS regulation of GSH, the active GSH role in cell proliferation, and GSH cross-talk with auxin assume a potential role for GSH in the modulation of root architecture under stress conditions.

Oxidative stress inducing agents’ copper and alloxan accelerate cell cycle re-entering of somatic plant cells in the presence of suboptimal exogenous auxin

The physiological status of differentiated somatic plant cells and kinetics of re-entering in cell cycle were investigated in the case of Medicago sativa leaf protoplasts after the application of oxidative stress-inducing agents. Excess copper (30 μM) and alloxan (0.5 mM) accelerated cell cycle re-entry at an exogenous auxin concentration that alone was insufficient to induce cell activation. Application of stress-inducing agents accelerated changes in the nuclei landscape with further faster re-entry in DNA replication and cytokinesis. This acceleration was accompanied by a lower level of reactive oxygen species (ROS) accumulations. At later stages, stress-agents treated cells resemble stem cells in planta with a smaller size, higher cell viability, lower ROS level, and lower activities of major ROS scavenging enzymes. A similar cellular response could be achieved by increasing the exogenous auxin concentration. Based on these experimental results, it is suggested that sub-lethal stress treatments evoke a transient cell state that accelerates cellular reprogramming. We also speculate that this transient cell state serves as an effective mechanism for protection against oxidative stress.

Quantificação de açúcares totais e auxina no desenvolvimento inicial de mini-toletes de cana-de-açúcar

Reserve carbohydrate and auxinspresent in the stalk are involved in the development of sugarcane plant. Thus, this study aimed to quantify total sugarsand auxin of type indoleacetic acid (IAA) content in the initial development of stalks and sugarcane buds, as a function of diameter and position in the stalk. The experiment was conducted in a 3x3 factorial scheme (diameter x stalk section), with four replicates. Eleven-month-old plants stalks were classified into three diameters (fine < 2 cm; medium 2-3 cm; thick > 3 cm) and divided into three sections (apex, middle and base). Data were submitted to analysis of variance, and when significant the means were compared by the Tukey (p≤0.05). The diameter factor resulted in greater interference in the content of the biochemical variables evaluated (sugars and auxin), in comparation to stalk section. Biometric variables (length and dry mass of shoot and roots, budding and budding speed index), in general, were mostly affected by stalk section factor. The diameter factor interfered in the content of endogenous auxin in the stalk and buds. Root growth was related to endogenous auxin concentration and the highest sprouting percentage was obtained from the apical section of the talk due to the availability of reducing sugars content.The use of mini-stalks for the propagation of sugarcane proved feasible.Keyword:apical dominance;plant hormone; propagation;Saccharum spp.

Temporal integration of auxin information for the regulation of patterning

Positional information is essential for coordinating the development of multicellular organisms. In plants, positional information provided by the hormone auxin regulates rhythmic organ production at the shoot apex, but the spatio-temporal dynamics of auxin gradients is unknown. We used quantitative imaging to demonstrate that auxin carries high-definition graded information not only in space but also in time. We show that, during organogenesis, temporal patterns of auxin arise from rhythmic centrifugal waves of high auxin travelling through the tissue faster than growth. We further demonstrate that temporal integration of auxin concentration is required to trigger the auxin-dependent transcription associated with organogenesis. This provides a mechanism to temporally differentiate sites of organ initiation and exemplifies how spatio-temporal positional information can be used to create rhythmicity.

Rooting stimulation of “Victoria” and “Korinka russkaya” grape hardwood cuttings as influenced by potassium salt of Indolyl-3-Acetic Acid (KIAA)

Relevance. Applying exogenous auxins to the cuttings of grapevines is a common practice in viticulture to improve the rooting process. The potassium salt formulations of auxins have been documented to be more, less, or equally efficient as acid formulations in rooting stimulation of cuttings depending on the genetic features of the plant, type and concentration of auxin, type of cuttings, and many other factors.Methods. The present study aimed at evaluating the effect of potassium salt of indolyl-3-acetic acid (KIAA) on rooting of the hardwood cuttings of two grape cultivars namely, Korinka russkaya and Victoria, which are commonly planted by grape growers of the Tatarstan Republic. The study was conducted in 2018 and 2019 under hydroponic conditions in a controlled-environment growth chamber in Kazan State Agrarian University. The cuttings were treated with 1) KIAA at two concentrations (5,000 and 10,000) ppm; 2) β-indolyl-butyric acid (IBA) 3,000 ppm (as a check treatment) and 3) distilled water as a control.Results. Based upon the overall results, KIAA significantly improved the rooting process of both investigated cultivars. Rooting percentage, average number of roots on cuttings as well as the quality of the root system was enhanced with increasing auxin concentration up to 10,000 ppm. IBA also improved the rooting quality parameters over the controls. The two grape cultivars differed significantly in their rooting capacity: cuttings of Korinka russkaya in both years had greater values of all the studied parameters compared with those of Victoria cultivar except for the callusing percentage, which was higher in Victoria.

Effects of auxin and urea derivatives on adventitious rooting in chestnut and oak microshoots

The present study investigated how auxin concentration and the method of application affected the formation of adventitious roots in microshoots of chestnut (Castanea sativa) and oak (Quercus robur). The activity of two urea derivatives (2, 3-MDPU and 3, 4-MDPU) was also evaluated. Microshoots were derived from basal sprouts of two mature chestnut trees (P1 and P2) and one adult oak genotype (Sainza). In chestnut, rooting percentage was positively affected by auxin in a dose-dependent manner, particularly in shoots treated with the hormone for 24 h. The effect of auxin on rooting also differed depending on the application method. In shoots treated for 24 h, the highest concentration of auxin produced the healthiest rooted plantlets, in terms of the root system and shoot quality. By contrast, in shoots treated by the basal quick-dip method, the shoot quality was best at the lowest auxin concentration. The effect of urea derivatives on the root system depended on the species as well as on the auxin concentration and application period. Use of the MDPUs improved the root system architecture of auxin-treated shoots by promoting lateral root development and triggering the synchronous initiation of root primordia at the base of the shoot. Shoot quality was also improved by MDPUs, which promoted resumption of growth and reduced shoot-tip necrosis.