The role of phytochrome C in gravitropism and phototropism in Arabidopsis thaliana

Genetic analysis of grapevine root system architecture and loci associated gene networks

Own-rooted grapevines and grapevine rootstocks are vegetatively propagated from cuttings and have an adventitious root system. Unraveling the genetic underpinnings of the adventitious root system architecture (RSA) is important for improving own-rooted and grafted grapevine sustainability for a changing climate. Grapevine RSA genetic analysis was conducted in an Vitis sp. 'VRS-F2' population. Nine root morphology, three total root system morphology, and two biomass traits that contribute to root anchorage and water and nutrient uptake were phenotyped. Quantitative trait loci (QTL) analysis was performed using a high density integrated GBS and rhAmpSeq genetic map. Thirty-one QTL were detected for eleven of the RSA traits (surface area, root volume, total root length, fresh weight, number of tips, forks or links, longest root and average root diameter, link length, and link surface area) revealing many small effects. Several QTL were colocated on chromosomes 1, 9, 13, 18, and 19. QTL with identical peak positions on chromosomes 1 or 13 were enriched for AP2-EREBP, AS2, C2C2-CO, HMG, and MYB transcription factors, and QTL on chromosomes 9 or 13 were enriched for the ALFIN-LIKE transcription factor and regulation of autophagy pathways. QTL modeling for individual root traits identified eight models explaining 13.2 to 31.8% of the phenotypic variation. 'Seyval blanc' was the grandparent contributing to the allele models that included a greater surface area, total root length, and branching (number of forks and links) traits promoting a greater root density. In contrast, V. riparia 'Manitoba 37' contributed the allele for greater average branch length (link length) and diameter, promoting a less dense elongated root system with thicker roots. LATERAL ORGAN BOUNDARY DOMAIN (LBD or AS2/LOB) and the PROTODERMAL FACTOR (PFD2 and ANL2) were identified as important candidate genes in the enriched pathways underlying the hotspots for grapevine adventitious RSA. The combined QTL hotspot and trait modeling identified transcription factors, cell cycle and circadian rhythm genes with a known role in root cell and epidermal layer differentiation, lateral root development and cortex thickness. These genes are candidates for tailoring grapevine root system texture, density and length in breeding programs.

Signaling events for photomorphogenic root development

A germinating seedling incorporates environmental signals such as light into developmental outputs. Light is not only a source of energy, but also a central coordinative signal in plants. Traditionally, most research focuses on aboveground organs' response to light; therefore, our understanding of photomorphogenesis in roots is relatively scarce. However, root development underground is highly responsive to light signals from the shoot and understanding these signaling mechanisms will give a better insight into early seedling development. Here, we review the central light signaling hubs and their role in root growth promotion of Arabidopsis thaliana seedlings.

Phototropism of Arabidopsis thaliana in microgravity and fractional gravity on the International Space Station

While there is a great deal of knowledge regarding plant growth and development in microgravity aboard orbiting spacecraft, there is little information available about these parameters in reduced or fractional gravity conditions (less than the nominal 1g on Earth). Thus, in these experiments using the European Modular Cultivation System on the International Space Station, we studied the interaction between phototropism and gravitropism in the WT and mutants of phytochrome A and B of Arabidopis thaliana. Fractional gravity and the 1 g control were provided by centrifuges in the spaceflight hardware, and unidirectional red and blue illumination followed a white light growth period in the time line of the space experiments. The existence of red-light-based positive phototropism in hypocotyls of seedlings that is mediated by phytochrome was confirmed in these microgravity experiments. Fractional gravity studies showed an attenuation of red-light-based phototropism in both roots and hypocotyls of seedlings occurring due to gravitational accelerations ranging from 0.l to 0.3 g. In contrast, blue-light negative phototropism in roots, which was enhanced in microgravity compared with the 1g control, showed a significant attenuation at 0.3 g. In addition, our studies suggest that the well-known red-light enhancement of blue-light-induced phototropism in hypocotyls is likely due to an indirect effect by the attenuation of gravitropism. However, red-light enhancement of root blue-light-based phototropism may occur via a more direct effect on the phototropism system itself, most likely through the phytochrome photoreceptors. To our knowledge, these experiments represent the first to examine the behavior of flowering plants in fractional or reduced gravity conditions.

Phototropism and gravitropism in transgenic lines of Arabidopsis altered in the phytochrome pathway

Phytochromes are a family of photoreceptor molecules, absorbing primarily in red and far-red, that are important in many aspects of plant development. These studies investigated the role of phytochromes in phototropism and gravitropism of seedlings of Arabidopsis thaliana. We used two transgenic lines, one which lacked phytochromes specifically in the roots (M0062/UASBVR) and the other lacked phytochromes in the shoots (CAB3::pBVR). These transgenic plants are deficient in the phytochrome chromophore in specific tissues due the expression of biliverdin IXa reductase (BVR), which binds to precursors of the chromophore. Experiments were performed in both light and dark conditions to determine whether roots directly perceive light signals or if the signal is perceived in the shoot and then transmitted to the root during tropistic curvature. Kinetics of tropisms and growth were assayed by standard methods or with a computer-based feedback system. We found that the perception of red light occurs directly in the root during phototropism in this organ and that signaling also may occur from root to shoot in gravitropism.

A novel phototropic response to red light is revealed in microgravity

The aim of this study was to investigate phototropism in plants grown in microgravity conditions without the complications of a 1-g environment. Experiments performed on the International Space Station (ISS) were used to explore the mechanisms of both blue-light- and red-light-induced phototropism in plants. This project utilized the European Modular Cultivation System (EMCS), which has environmental controls for plant growth as well as centrifuges for gravity treatments used as a 1-g control. Images captured from video tapes were used to analyze the growth, development, and curvature of Arabidopsis thaliana plants that developed from seed in space. A novel positive phototropic response to red light was observed in hypocotyls of seedlings that developed in microgravity. This response was not apparent in seedlings grown on Earth or in the 1-g control during the space flight. In addition, blue-light-based phototropism had a greater response in microgravity compared with the 1-g control. Although flowering plants are generally thought to lack red light phototropism, our data suggest that at least some flowering plants may have retained a red light sensory system for phototropism. Thus, this discovery may have important implications for understanding the evolution of light sensory systems in plants.

Modelling leaf phototropism in a cucumber canopy

Leaf phototropism might have significant effects on the light interception, dry matter production and yield of cucumber (Cucumis sativus L.). The objective of the present study was to model the phototropism of leaves in a greenhouse cucumber canopy. The dynamic structural model of cucumber using a parametric L-system was extended to mimic the leaf movement induced by gradients in the local light environment of each leaf. The red to far-red (R : FR) ratio is known to be a driving force in shade avoidance reactions of plants. In the model, R : FR ratios on the left and right leaf half of each individual leaf lamina were calculated and the change in tropism angle per phyllochron was based on the R : FR gradient across the leaf halves. The tropism angle of a leaf describes the deviation of the present leaf azimuth from the initial leaf orientation, which is predefined by the phyllotaxis of the plant. Even in its simple form, the model simulated photo-morphogenic canopy responses.