Space-time analysis of gravitropism in etiolated Arabidopsis hypocotyls using bioluminescence imaging of the IAA19 promoter fusion with a destabilized luciferase reporter

Multiple light signaling pathways control solar tracking in sunflowers

Sunflowers are famous for their ability to track the sun throughout the day and then reorient at night to face east the following morning. This occurs by differential growth patterns, with the east sides of stems growing more during the day and the west sides of stems growing more at night. This process, termed heliotropism, is generally believed to be a specialized form of phototropism; however, the underlying mechanism is unknown. To better understand heliotropism, we compared gene expression patterns in plants undergoing phototropism in a controlled environment and in plants initiating and maintaining heliotropic growth in the field. We found the expected transcriptome signatures of phototropin-mediated phototropism in sunflower stems bending towards monochromatic blue light. Surprisingly, the expression patterns of these phototropism-regulated genes are quite different in heliotropic plants. Most genes rapidly induced during phototropism display only minor differences in expression across solar tracking stems. However, some genes that are both rapidly induced during phototropism and are implicated in growth responses to foliar shade are rapidly induced on the west sides of stems at the onset of heliotropism, suggesting a possible role for red light photoreceptors in solar tracking. To test the involvement of different photoreceptor signaling pathways in heliotropism, we modulated the light environment of plants initiating solar tracking. We found that depletion of either red and far-red light or blue light did not hinder the initiation or maintenance of heliotropism in the field. Together, our results suggest that the transcriptional regulation of heliotropism is distinct from phototropin-mediated phototropism and likely involves inputs from multiple light signaling pathways.

ZEITLUPE enhances expression of PIF4 and YUC8 in the upper aerial parts of Arabidopsis seedlings to positively regulate hypocotyl elongation

Microarray and genetic analyses reveal that ZTL induces the expression of genes related to auxin synthesis, thereby promoting hypocotyl elongation. ZTL is a blue-light receptor that possesses a light-oxygen-voltage-sensing (LOV) domain, an F-box motif, and a kelch repeat domain. ZTL promotes hypocotyl elongation under high temperature (28 °C) in Arabidopsis thaliana; however, the mechanism of this regulation is unknown. Here, we divided seedlings into hypocotyls and upper aerial parts, and performed microarray analyses. In hypocotyl, 1062 genes were down-regulated in ztl mutants (ztl-3 and ztl-105) compared with wild type; some of these genes encoded enzymes involved in cell wall modification, consistent with reduced hypocotyl elongation. In upper aerial parts, 1038 genes were down-regulated in the ztl mutants compared with wild type; these included genes involved in auxin synthesis and auxin response. Furthermore, the expression of the PHYTOCHROME INTERACTING FACTOR 4 (PIF4) gene, which encodes a transcription factor known to positively regulate YUCCA genes (YUCs), was also decreased in the ztl mutants. Genetic analysis revealed that overexpression of PIF4 and YUC8 could restore the suppressed hypocotyl length in the ztl mutants. Our results suggest that ZTL induces expression of YUC8 via PIF4 in upper aerial parts and promotes hypocotyl elongation.

Beyond the darkness: recent lessons from etiolation and de-etiolation studies

The state of etiolation is generally defined by the presence of non-green plastids (etioplasts) in plant tissues that would normally contain chloroplasts. In the commonly used dark-grown seedling system, etiolation is coupled with a type of growth called skotomorphogenesis. Upon illumination, de-etiolation occurs, marked by the transition from etioplast to chloroplast, and, at the seedling level, a switch to photomorphogenic growth. Etiolation and de-etiolation systems are therefore important for understanding both the acquisition of photosynthetic capacity during chloroplast biogenesis and plant responses to light-the most relevant signal in the life and growth of the organism. In this review, we discuss recent discoveries (within the past 2-3 years) in the field of etiolation and de-etiolation, with a particular focus on post-transcriptional processes and ultrastructural changes. We further discuss ambiguities in definitions of the term 'etiolation', and benefits and biases of common etiolation/de-etiolation systems. Finally, we raise several open questions and future research possibilities.

Quantitative Measurements of Curvature Along the Growth Axis in Tropic Responses Using Free Software Environments

Tropic responses in plants have usually been studied by measuring changes in the deflection angle of the organ tip. However, the measurement of other geometric parameters, such as curvature along the entire length, may give us better understanding of tropic responses, particularly in shoots. Here, we describe methods for obtaining quantitative measurements of local curvature and other parameters based on digital images of bending Arabidopsis hypocotyls using the free software packages, ImageJ and R.

Growth-mediated plant movements: hidden in plain sight

While fast plant movements are spectacular but rare, almost all plants exhibit relatively slow, growth-mediated tropic movements that are key to their survival in the natural world. In this brief review, we discuss recent insights into the molecular mechanisms underlying phototropism, gravitropism, hydrotropism, and autostraightening. Careful molecular genetic and physiological studies have helped confirm the importance of lateral auxin gradients in gravitropic and phototropic responses. However, auxin signaling does not explain all tropisms: recent work has shown that abscisic acid signaling mediates root hydrotropism and has implicated mechanosensing in autostraightening, the organ straightening process recently modeled as a proprioceptive response. The interactions between distinct tropic signaling pathways and other internal and external sensory processes are also now being untangled.