Plant roots: recycled auxin energizes patterning and growth

Mathematical Modelling of Auxin Transport in Plant Tissues: Flux Meets Signalling and Growth

Plant hormone auxin has critical roles in plant growth, dependent on its heterogeneous distribution in plant tissues. Exactly how auxin transport and developmental processes such as growth coordinate to achieve the precise patterns of auxin observed experimentally is not well understood. Here we use mathematical modelling to examine the interplay between auxin dynamics and growth and their contribution to formation of patterns in auxin distribution in plant tissues. Mathematical models describing the auxin-related signalling pathway, PIN and AUX1 dynamics, auxin transport, and cell growth in plant tissues are derived. A key assumption of our models is the regulation of PIN proteins by the auxin-responsive ARF-Aux/IAA signalling pathway, with upregulation of PIN biosynthesis by ARFs. Models are analysed and solved numerically to examine the long-time behaviour and auxin distribution. Changes in auxin-related signalling processes are shown to be able to trigger transition between passage- and spot-type patterns in auxin distribution. The model was also shown to be able to generate isolated cells with oscillatory dynamics in levels of components of the auxin signalling pathway which could explain oscillations in levels of ARF targets that have been observed experimentally. Cell growth was shown to have influence on PIN polarisation and determination of auxin distribution patterns. Numerical simulation results indicate that auxin-related signalling processes can explain the different patterns in auxin distributions observed in plant tissues, whereas the interplay between auxin transport and growth can explain the ‘reverse-fountain’ pattern in auxin distribution observed at plant root tips.

The interaction between strigolactones and other plant hormones in the regulation of plant development

Plant hormones are small molecules derived from various metabolic pathways and are important regulators of plant development. The most recently discovered phytohormone class comprises the carotenoid-derived strigolactones (SLs). For a long time these compounds were only known to be secreted into the rhizosphere where they act as signaling compounds, but now we know they are also active as endogenous plant hormones and they have been in the spotlight ever since. The initial discovery that SLs are involved in the inhibition of axillary bud outgrowth, initiated a multitude of other studies showing that SLs also play a role in defining root architecture, secondary growth, hypocotyl elongation, and seed germination, mostly in interaction with other hormones. Their coordinated action enables the plant to respond in an appropriate manner to environmental factors such as temperature, shading, day length, and nutrient availability. Here, we will review the current knowledge on the crosstalk between SLs and other plant hormones-such as auxin, cytokinin, abscisic acid (ABA), ethylene (ET), and gibberellins (GA)-during different physiological processes. We will furthermore take a bird's eye view of how this hormonal crosstalk enables plants to respond to their ever changing environments.

A plausible mechanism for auxin patterning along the developing root

Background

In plant roots, auxin is critical for patterning and morphogenesis. It regulates cell elongation and division, the development and maintenance of root apical meristems, and other processes. In Arabidopsis, auxin distribution along the central root axis has several maxima: in the root tip, in the basal meristem and at the shoot/root junction. The distal maximum in the root tip maintains the stem cell niche. Proximal maxima may trigger lateral or adventitious root initiation.

Results

We propose a reflected flow mechanism for the formation of the auxin maximum in the root apical meristem. The mechanism is based on auxin's known activation and inhibition of expressed PIN family auxin carriers at low and high auxin levels, respectively. Simulations showed that these regulatory interactions are sufficient for self-organization of the auxin distribution pattern along the central root axis under varying conditions. The mathematical model was extended with rules for discontinuous cell dynamics so that cell divisions were also governed by auxin, and by another morphogen Division Factor which combines the actions of cytokinin and ethylene on cell division in the root. The positional information specified by the gradients of these two morphogens is able to explain root patterning along the central root axis.

Conclusion

We present here a plausible mechanism for auxin patterning along the developing root, that may provide for self-organization of the distal auxin maximum when the reverse fountain has not yet been formed or has been disrupted. In addition, the proximal maxima are formed under the reflected flow mechanism in response to periods of increasing auxin flow from the growing shoot. These events may predetermine lateral root initiation in a rhyzotactic pattern. Another outcome of the reflected flow mechanism - the predominance of lateral or adventitious roots in different plant species - may be based on the different efficiencies with which auxin inhibits its own transport in different species, thereby distinguishing two main types of plant root architecture: taproot vs. fibrous.

Developmental consequences of the tumorous shoot development1 mutation, a novel allele of the cellulose-synthesizing KORRIGAN1 gene

This work describes the further characterization of the tumorous shoot development1 (tsd1) mutant of Arabidopsis thaliana, which develops disorganized tumorous-like shoot tissue instead of organized leaves and stems. Map-based cloning revealed that tsd1 is a novel strong allele of the KOR1 gene, encoding a membrane-bound endo-1,4-beta-D-glucanase involved in cellulose synthesis. To study developmental changes accompanying the aberrant growth of the tsd1 mutant, patterning in the meristems and the hormonal status were analysed by marker genes. Expression of key regulators of meristem maintenance, the CLV3 and STM genes, indicated the presence of numerous meristems in the tsd1 shoot callus. Expression of the LFY::GUS marker supported the ability of the tsd1 callus to form organ primordia, which however failed to develop further. An epidermal marker showed that the L1 layer was maintained only in distinct areas of the tsd1 callus, which could be a reason of disorganized shoot growth. In the tsd1 root meristem, quiescent center activity was lost early after germination, which caused differentiation of the root meristem. The spatial expression of genes reporting the auxin and cytokinin status was altered in the tsd1 mutant. Modifying the endogenous levels of these hormones partially rescued shoot and root development of the tsd1 mutant. Together, the work shows that TSD1/KOR1 is required for maintaining a correct meristematic pattern and organ growth as well as for a normal hormonal response.

Hydrogen sulfide promotes root organogenesis in Ipomoea batatas, Salix matsudana and Glycine max

In this report, we demonstrate that sodium hydrosulfide (NaHS), a hydrogen sulfide (H(2)S) donor, promoted adventitious root formation mediated by auxin and nitric oxide (NO). Application of the H(2)S donor to seedling cuttings of sweet potato (Ipomoea batatas L.) promoted the number and length of adventitious roots in a dose-dependent manner. It was also verified that H(2)S or HS(-) rather than other sulfur-containing components derived from NaHS could be attributed to the stimulation of adventitious root formation. A rapid increase in endogenous H(2)S, indole acetic acid (IAA) and NO were sequentially observed in shoot tips of sweet potato seedlings treated with HaHS. Further investigation showed that H(2)S-mediated root formation was alleviated by N-1-naphthylphthalamic acid (NPA), an IAA transport inhibitor, and 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), an NO scavenger. Similar phenomena in H(2)S donor-dependent root organogenesis were observed in both excised willow (Salix matsudana var. tortuosa Vilm) shoots and soybean (Glycine max L.) seedlings. These results indicated that the process of H(2)S-induced adventitious root formation was likely mediated by IAA and NO, and that H(2)S acts upstream of IAA and NO signal transduction pathways.

ABCB19/PGP19 stabilises PIN1 in membrane microdomains in Arabidopsis

Auxin transport is mediated at the cellular level by three independent mechanisms that are characterised by the PIN-formed (PIN), P-glycoprotein (ABCB/PGP) and AUX/LAX transport proteins. The PIN and ABCB transport proteins, best represented by PIN1 and ABCB19 (PGP19), have been shown to coordinately regulate auxin efflux. When PIN1 and ABCB19 coincide on the plasma membrane, their interaction enhances the rate and specificity of auxin efflux and the dynamic cycling of PIN1 is reduced. However, ABCB19 function is not regulated by the dynamic cellular trafficking mechanisms that regulate PIN1 in apical tissues, as localisation of ABCB19 on the plasma membrane was not inhibited by short-term treatments with latrunculin B, oryzalin, brefeldin A (BFA) or wortmannin--all of which have been shown to alter PIN1 and/or PIN2 plasma membrane localisation. When taken up by endocytosis, the styryl dye FM4-64 labels diffuse rather than punctuate intracellular bodies in abcb19 (pgp19), and some aggregations of PIN1 induced by short-term BFA treatment did not disperse after BFA washout in abcb19. Although the subcellular localisations of ABCB19 and PIN1 in the reciprocal mutant backgrounds were like those in wild type, PIN1 plasma membrane localisation in abcb19 roots was more easily perturbed by the detergent Triton X-100, but not other non-ionic detergents. ABCB19 is stably associated with sterol/sphingolipid-enriched membrane fractions containing BIG/TIR3 and partitions into Triton X-100 detergent-resistant membrane (DRM) fractions. In the wild type, PIN1 was also present in DRMs, but was less abundant in abcb19 DRMs. These observations suggested a rationale for the observed lack of auxin transport activity when PIN1 is expressed in a non-plant heterologous system. PIN1 was therefore expressed in Schizosaccharomyces pombe, which has plant-like sterol-enriched microdomains, and catalysed auxin transport in these cells. These data suggest that ABCB19 stabilises PIN1 localisation at the plasma membrane in discrete cellular subdomains where PIN1 and ABCB19 expression overlaps.