Gravitropic plant growth regulation and ethylene: an unsought cardinal coordinate for a disused model

Plant root development: is the classical theory for auxin-regulated root growth false?

One of the longest standing theories and, therein-based, regulation-model of plant root development, posits the inhibitory action of auxin (IAA, indolylacetic acid) on elongation growth of root cells. This effect, as induced by exogenously supplied IAA, served as the foundation stone for root growth regulation. For decades, auxin ruled the day and only allowed hormonal side players to be somehow involved, or in some way affected. However, this copiously reiterated, apparent cardinal role of auxin only applies in roots immersed in solutions; it vanishes as soon as IAA-supplied roots are not surrounded by liquid. When roots grow in humid air, exogenous IAA has no inhibitory effect on elongation growth of maize roots, regardless of whether it is applied basipetally from the top of the root or to the entire residual seedling immersed in IAA solution. Nevertheless, such treatment leads to pronounced root-borne ethylene emission and lateral rooting, illustrating and confirming thereby induced auxin presence and its effect on the root - yet, not on root cell elongation. Based on these findings, a new root growth regulatory model is proposed. In this model, it is not IAA, but IAA-triggered ethylene which plays the cardinal regulatory role - taking effect, or not - depending on the external circumstances. In this model, in water- or solution-incubated roots, IAA-dependent ethylene acts due to its accumulation within the root proper by inhibited/restrained diffusion into the liquid phase. In roots exposed to moist air or gas, there is no effect on cell elongation, since IAA-triggered ethylene diffuses out of the root without an impact on growth.

Root cap-mediated evaluation of soil resistance towards graviresponding roots of maize (Zea mays L.) and the relevance of ethylene

BACKGROUND AND AIMS

Besides biological and chemical impacts, mechanical resistance represents an important obstacle that growing roots face. Graviresponding roots must assess the mechanical resistance of the substrate and take decisions on whether they change growth direction and grow around obstacles or tolerate growth conditions impaired to varying degrees. To test the significance of the root cap, we measured pressure and growth behaviour of single intact, as well as decapped, roots encountering diverse mechanical obstacles. We examined ethylene emission in intact roots as well as roots without a root cap, thereby lacking the capacity to deviate.

METHODS

Roots of fixed seedlings were grown vertically onto diverse mechanical obstacles. Developing pressure profiles of vertically growing roots encountering horizontal mechanical obstacles were measured employing electronic milligram scales, with and without root caps in given local environmental conditions. The evolution of root-borne ethylene was measured in intact roots and roots without the root cap.

KEY RESULTS

In contrast to decapped roots, intact roots develop a tentative, short-lasting pressure profile, the resolution of which is characterized by a definite change of growth direction. Similarly, pressure profiles and strengths of roots facing gradually differing surface resistances differ significantly between the two. This correlates in the short term with root cap-dependent ethylene emission which is lacking in roots without caps.

CONCLUSIONS

The way gravistimulated and graviresponding roots cope with exogenous stimuli depends on whether and how they adapt to these impacts. With respect to mechanical hindrances, roots without caps do not seem to be able to evaluate soil strengths in order to respond adequately. On encountering resistance, roots with intact caps emit ethylene, which is not observed in decapped roots. It therefore appears that it is the root cap which specifically orchestrates the resistance needed to overcome mechanical resistance by specifically inducing ethylene.

Graviperception in maize plants: is amyloplast sedimentation a red herring?

Land plants perceive gravity and respond to it in an organ-specific way; shoots typically direct growth upwards, roots typically downwards. Historically, at least with respect to maize plants, this phenomenon is attributed to three sequential processes, namely graviperception, the transduction of the perceived signal, and the graviresponse, resulting in a typical (re)positioning of the organ or entire plant body relative to the gravivector. For decades, sedimentation of starch-containing plastids within the cells of special tissues has been regarded as the primary and initiating process fundamental for gravitropic growth (starch-statolith hypothesis). Based on Popper's falsification principle, uncompromising experiments were executed. The results indicate that the model of graviperception based on amyloplast sedimentation does not apply to maize seedlings.

Localized micronutrient patches induce lateral root foraging and chemotropism in Nicotiana attenuata

Nutrients are distributed unevenly in the soil. Phenotypic plasticity in root growth and proliferation may enable plants to cope with this variation and effectively forage for essential nutrients. However, how micronutrients shape root architecture of plants in their natural environments is poorly understood. We used a combination of field and laboratory-based assays to determine the capacity of Nicotiana attenuata to direct root growth towards localized nutrient patches in its native environment. Plants growing in nature displayed a particular root phenotype consisting of a single primary root and a few long, shallow lateral roots. Analysis of bulk soil surrounding the lateral roots revealed a strong positive correlation between lateral root placement and micronutrient gradients, including copper, iron and zinc. In laboratory assays, the application of localized micronutrient salts close to lateral root tips led to roots bending in the direction of copper and iron. This form of chemotropism was absent in ethylene and jasmonic acid deficient lines, suggesting that it is controlled in part by these two hormones. This work demonstrates that directed root growth underlies foraging behavior, and suggests that chemotropism and micronutrient-guided root placement are important factors that shape root architecture in nature.

Seedling development in maize cv. B73 and blue light-mediated proteomic changes in the tip vs. stem of the coleoptile

In 2009, the draft genome of the reference inbred line of maize (Zea mays L. spp. mays cv. B73) was published so that, using this specific corn variety, molecular analyses of physiological processes became possible. However, the morphology and developmental patterns of B73 maize, compared with that of the more frequently used hybrid varieties, have not yet been analyzed. Here, we describe organ development in seedlings of B73 maize and in those of six other hybrid cultivars, and document significant morphological as well as quantitative differences between these varieties of Z. mays. In a second set of experiments, we used etiolated seedlings of B73 maize to analyze the effect of blue light (BL) on the patterns of proteins in the tip vs. growing region of this sheath-like organ. By using two-dimensional difference gel electrophoresis (2D DIGE), coupled with tandem mass spectrometry, we detected, in the microsomal fraction of maize coleoptile tips, rapid changes in the abundance of protein spots of maize phototropin 1 and several metabolic enzymes. In the sub-apical (growing) region of the coleoptile, proteomic changes were less pronounced. These results suggest that the tip of the coleoptile of B73 maize may serve as a unique model system for dissecting BL responses in a light-sensitive plant organ of known function.

Microarray analyses and comparisons of upper or lower flanks of rice shoot base preceding gravitropic bending

Gravitropism is a complex process involving a series of physiological pathways. Despite ongoing research, gravitropism sensing and response mechanisms are not well understood. To identify the key transcripts and corresponding pathways in gravitropism, a whole-genome microarray approach was used to analyze transcript abundance in the shoot base of rice (Oryza sativa sp. japonica) at 0.5 h and 6 h after gravistimulation by horizontal reorientation. Between upper and lower flanks of the shoot base, 167 transcripts at 0.5 h and 1202 transcripts at 6 h were discovered to be significantly different in abundance by 2-fold. Among these transcripts, 48 were found to be changed both at 0.5 h and 6 h, while 119 transcripts were only changed at 0.5 h and 1154 transcripts were changed at 6 h in association with gravitropism. MapMan and PageMan analyses were used to identify transcripts significantly changed in abundance. The asymmetric regulation of transcripts related to phytohormones, signaling, RNA transcription, metabolism and cell wall-related categories between upper and lower flanks were demonstrated. Potential roles of the identified transcripts in gravitropism are discussed. Our results suggest that the induction of asymmetrical transcription, likely as a consequence of gravitropic reorientation, precedes gravitropic bending in the rice shoot base.

Getting to the roots of it: Genetic and hormonal control of root architecture

Root system architecture (RSA) - the spatial configuration of a root system - is an important developmental and agronomic trait, with implications for overall plant architecture, growth rate and yield, abiotic stress resistance, nutrient uptake, and developmental plasticity in response to environmental changes. Root architecture is modulated by intrinsic, hormone-mediated pathways, intersecting with pathways that perceive and respond to external, environmental signals. The recent development of several non-invasive 2D and 3D root imaging systems has enhanced our ability to accurately observe and quantify architectural traits on complex whole-root systems. Coupled with the powerful marker-based genotyping and sequencing platforms currently available, these root phenotyping technologies lend themselves to large-scale genome-wide association studies, and can speed the identification and characterization of the genes and pathways involved in root system development. This capability provides the foundation for examining the contribution of root architectural traits to the performance of crop varieties in diverse environments. This review focuses on our current understanding of the genes and pathways involved in determining RSA in response to both intrinsic and extrinsic (environmental) response pathways, and provides a brief overview of the latest root system phenotyping technologies and their potential impact on elucidating the genetic control of root development in plants.

Getting to the roots of it: Genetic and hormonal control of root architecture

Root system architecture (RSA) - the spatial configuration of a root system - is an important developmental and agronomic trait, with implications for overall plant architecture, growth rate and yield, abiotic stress resistance, nutrient uptake, and developmental plasticity in response to environmental changes. Root architecture is modulated by intrinsic, hormone-mediated pathways, intersecting with pathways that perceive and respond to external, environmental signals. The recent development of several non-invasive 2D and 3D root imaging systems has enhanced our ability to accurately observe and quantify architectural traits on complex whole-root systems. Coupled with the powerful marker-based genotyping and sequencing platforms currently available, these root phenotyping technologies lend themselves to large-scale genome-wide association studies, and can speed the identification and characterization of the genes and pathways involved in root system development. This capability provides the foundation for examining the contribution of root architectural traits to the performance of crop varieties in diverse environments. This review focuses on our current understanding of the genes and pathways involved in determining RSA in response to both intrinsic and extrinsic (environmental) response pathways, and provides a brief overview of the latest root system phenotyping technologies and their potential impact on elucidating the genetic control of root development in plants.

Getting to the roots of it: Genetic and hormonal control of root architecture

Root system architecture (RSA) - the spatial configuration of a root system - is an important developmental and agronomic trait, with implications for overall plant architecture, growth rate and yield, abiotic stress resistance, nutrient uptake, and developmental plasticity in response to environmental changes. Root architecture is modulated by intrinsic, hormone-mediated pathways, intersecting with pathways that perceive and respond to external, environmental signals. The recent development of several non-invasive 2D and 3D root imaging systems has enhanced our ability to accurately observe and quantify architectural traits on complex whole-root systems. Coupled with the powerful marker-based genotyping and sequencing platforms currently available, these root phenotyping technologies lend themselves to large-scale genome-wide association studies, and can speed the identification and characterization of the genes and pathways involved in root system development. This capability provides the foundation for examining the contribution of root architectural traits to the performance of crop varieties in diverse environments. This review focuses on our current understanding of the genes and pathways involved in determining RSA in response to both intrinsic and extrinsic (environmental) response pathways, and provides a brief overview of the latest root system phenotyping technologies and their potential impact on elucidating the genetic control of root development in plants.

The 'root-brain' hypothesis of Charles and Francis Darwin: Revival after more than 125 years

This year celebrates the 200(th) aniversary of the birth of Charles Darwin, best known for his theory of evolution summarized in On the Origin of Species. Less well known is that, in the second half of his life, Darwin's major scientific focus turned towards plants. He wrote several books on plants, the next-to-last of which, The Power of Movement of Plants, published together with his son Francis, opened plants to a new view. Here we amplify the final sentence of this book in which the Darwins proposed that: "It is hardly an exaggeration to say that the tip of the radicle thus endowed [with sensitivity] and having the power of directing the movements of the adjoining parts, acts like the brain of one of the lower animals; the brain being seated within the anterior end of the body, receiving impressions from the sense-organs, and directing the several movements." This sentence conveys two important messages: first, that the root apex may be considered to be a 'brain-like' organ endowed with a sensitivity which controls its navigation through soil; second, that the root apex represents the anterior end of the plant body. In this article, we discuss both these statements.

From Charles Darwin's botanical country-house studies to modern plant biology

As a student of theology at Cambridge University, Charles Darwin (1809-1882) attended the lectures of the botanist John S. Henslow (1796-1861). This instruction provided the basis for his life-long interest in plants as well as the species question. This was a major reason why in his book On the Origin of Species, which was published 150 years ago, Darwin explained his metaphorical phrase 'struggle for life' with respect to animals and plants. In this article, we review Darwin's botanical work with reference to the following topics: the struggle for existence in the vegetable kingdom with respect to the phytochrome-mediated shade avoidance response; the biology of flowers and Darwin's plant-insect co-evolution hypothesis; climbing plants and the discovery of action potentials; the power of movement in plants and Darwin's conflict with the German plant physiologist Julius Sachs; and light perception by growing grass coleoptiles with reference to the phototropins. Finally, we describe the establishment of the scientific discipline of Plant Biology that took place in the USA 80 years ago, and define this area of research with respect to Darwin's work on botany and the physiology of higher plants.

The root cap determines ethylene-dependent growth and development in maize roots

Besides providing protection against mechanical damage to the root tip, the root cap is involved in the perception and processing of diverse external and internal stimuli resulting in altered growth and development. The transduction of these stimuli includes hormonal signaling pathways such as those of auxin, ethylene and cytokinin. Here, we show that the root cap is essential for the ethylene-induced regulation of elongation growth and root hair formation in maize. Exogenously applied ethylene is no longer able to inhibit elongation growth when the root cap has been surgically removed prior to hormone treatment. Reconstitution of the cap positively correlates with the developing capacity of the roots to respond to ethylene again. In contrast, the removal of the root cap does not per se affect growth inhibition controlled by auxin and cytokinin. Furthermore, our semi-quantitative RT-PCR results support earlier findings that the maize root cap is a site of high gene expression activity with respect to sensing and responding to hormones such as ethylene. From these data, we propose a novel function of the root cap which is the establishment of competence to respond to ethylene in the distal zones of the root.

HAG2/MYB76 and HAG3/MYB29 exert a specific and coordinated control on the regulation of aliphatic glucosinolate biosynthesis in Arabidopsis thaliana

In a previous transactivation screen, two Arabidopsis thaliana R2R3-MYB transcription factors, HAG2/MYB76 and HAG3/MYB29, along with the already characterized HAG1/MYB28, were identified as putative regulators of aliphatic glucosinolate biosynthesis. Molecular and biochemical characterization of HAG2/MYB76 and HAG3/MYB29 functions was performed using transformants with increased or repressed transcript levels. Real-time PCR assays, cotransformation assays and measurements of glucosinolate contents were used to assess the impact of both MYB factors on the steady-state level of glucosinolate biosynthetic genes and accumulation of aliphatic glucosinolates. Both HAG2/MYB76 and HAG3/MYB29 were shown to be positive regulators of aliphatic glucosinolate biosynthesis. Expression of promoter-beta-glucuronidase (GUS) fusions indicated GUS activities in both vegetative and generative organs, with distinct characteristics for each MYB factor. HAG1/MYB28, HAG2/MYB76 and HAG3/MYB29 reciprocally transactivated each other in the control of aliphatic glucosinolate biosynthesis and downregulated the expression of genes involved in the control of indolic glucosinolate biosynthesis, pointing to a reciprocal negative regulation of these two pathways. All three HAG transcription factors exert a coordinated control on aliphatic glucosinolate biosynthesis.

Waving and skewing: how gravity and the surface of growth media affect root development in Arabidopsis

Arabidopsis seedlings growing on inclined agar surfaces exhibit characteristic root behaviours called 'waving' and 'skewing': the former consists of a series of undulations, whereas the latter is a deviation from the direction of gravity. Even though the precise basis of these growth patterns is not well understood, both gravity and the contact between the medium and the root are considered to be the major players that result in these processes. The influence of these forces on root surface-dependent behaviours can be verified by growing seedlings at different gel pitches: plants growing on vertical plates present roots with slight waving and skewing when compared with seedlings grown on plates held at minor angles of < 90 degrees . However, other factors are thought to modulate root growth on agar; for instance, it has been demonstrated that the presence and concentration of certain compounds in the medium (such as sucrose) and of drugs able to modify the plant cell cytoskeleton also affect skewing and waving. The recent discovery of an active role of ethylene on surface-dependent root behaviour, and the finding of new mutants showing anomalous growth, pave the way for a more detailed description of these phenomena.