The Inducible Accumulation of Cell Wall-Bound p-Hydroxybenzoates Is Involved in the Regulation of Gravitropic Response of Poplar

Lignin in Populus species is acylated with p-hydroxybenzoate. Monolignol p-hydroxybenzoyltransferase 1 (PHBMT1) mediates p-hydroxybenzoylation of sinapyl alcohol, eventually leading to the modification of syringyl lignin subunits. Angiosperm trees upon gravistimulation undergo the re-orientation of their growth along with the production of specialized secondary xylem, i.e., tension wood (TW), that generates tensile force to pull the inclined stem or leaning branch upward. Sporadic evidence suggests that angiosperm TW contains relatively a high percentage of syringyl lignin and lignin-bound p-hydroxybenzoate. However, whether such lignin modification plays a role in gravitropic response remains unclear. By imposing mechanical bending and/or gravitropic stimuli to the hybrid aspens in the wild type (WT), lignin p-hydroxybenzoate deficient, and p-hydroxybenzoate overproduction plants, we examined the responses of plants to gravitropic/mechanical stress and their cell wall composition changes. We revealed that mechanical bending or gravitropic stimulation not only induced the overproduction of crystalline cellulose fibers and increased the relative abundance of syringyl lignin, but also significantly induced the expression of PHBMT1 and the increased accumulation of p-hydroxybenzoates in TW. Furthermore, we found that although disturbing lignin-bound p-hydroxybenzoate accumulation in the PHBMT1 knockout and overexpression (OE) poplars did not affect the major chemical composition shifts of the cell walls in their TW as occurred in the WT plants, depletion of p-hydroxybenzoates intensified the gravitropic curving of the plantlets in response to gravistimulation, evident with the enhanced stem secant bending angle. By contrast, hyperaccumulation of p-hydroxybenzoates mitigated gravitropic response. These data suggest that PHBMT1-mediated lignin modification is involved in the regulation of poplar gravitropic response and, likely by compromising gravitropism and/or enhancing autotropism, negatively coordinates the action of TW cellulose fibers to control the poplar wood deformation and plant growth.

A Talk between Flavonoids and Hormones to Reorient the Growth of Gymnosperms

Plants reorient the growth of affected organs in response to the loss of gravity vector. In trees, this phenomenon has received special attention due to its importance for the forestry industry of conifer species. Sustainable management is a key factor in improving wood quality. It is of paramount importance to understand the molecular and genetic mechanisms underlying wood formation, together with the hormonal and environmental factors that affect wood formation and quality. Hormones are related to the modulation of vertical growth rectification. Many studies have resulted in a model that proposes differential growth in the stem due to unequal auxin and jasmonate allocation. Furthermore, many studies have suggested that in auxin distribution, flavonoids act as molecular controllers. It is well known that flavonoids affect auxin flux, and this is a new area of study to understand the intracellular concentrations and how these compounds can control the gravitropic response. In this review, we focused on different molecular aspects related to the hormonal role in flavonoid homeostasis and what has been done in conifer trees to identify molecular players that could take part during the gravitropic response and reduce low-quality wood formation.

Evaluation and GWAS of radicle gravitropic response in a core rice germplasm population

Aims Since gravitropism is one of the primary determinants of root development, facilitating root penetration into soil and subsequent absorption of water and nutrients, we studied this response in rice. Methods The gravitropism of 226 Chinese rice micro-core accessions and drought-resistant core accessions were assessed through the modified gravity-bending experiment and genome-wide association analysis (GWAS) was used to map the associated QTLs. Results The average value of gravitropic response speed of seminal roots was 41.05°/h, ranging from 16.77°/h to 62.83°/h. The gravity response speed of Indica (42.49°/h) was significantly (P < 0.002) higher than Japonica (39.71°/h) subspecies. The gravitational response speed of seminal roots was significantly positively correlated with the number of deep roots (r = 0.16), the growth speed of seminal roots (r = 0.21) and the drought resistance coefficient (r = 0.14). Conclusions In total, 3 QTLs (quantitative traits) associated with gravitropic response speed were identified on chromosome 4, 11 and 12. There are some known QTLs relating to roots traits and drought resistance located nearby the QTLs identified here, which confirms the close relationship between radicle gravitropism and the drought resistance. From within these intervals, 5 candidate genes were screened and verified by qPCR in a few rice varieties with extreme phenotypic values, demonstrating that gene LOC_Os12g29350 may regulate gravitropism negatively. This may be a promising candidate to be confirmed in further studies.

The AtCRK5 Protein Kinase Is Required to Maintain the ROS NO Balance Affecting the PIN2-Mediated Root Gravitropic Response in Arabidopsis

The Arabidopsis AtCRK5 protein kinase is involved in the establishment of the proper auxin gradient in many developmental processes. Among others, the Atcrk5-1 mutant was reported to exhibit a delayed gravitropic response via compromised PIN2-mediated auxin transport at the root tip. Here, we report that this phenotype correlates with lower superoxide anion (O2•−) and hydrogen peroxide (H2O2) levels but a higher nitric oxide (NO) content in the mutant root tips in comparison to the wild type (AtCol-0). The oxidative stress inducer paraquat (PQ) triggering formation of O2•− (and consequently, H2O2) was able to rescue the gravitropic response of Atcrk5-1 roots. The direct application of H2O2 had the same effect. Under gravistimulation, correct auxin distribution was restored (at least partially) by PQ or H2O2 treatment in the mutant root tips. In agreement, the redistribution of the PIN2 auxin efflux carrier was similar in the gravistimulated PQ-treated mutant and untreated wild type roots. It was also found that PQ-treatment decreased the endogenous NO level at the root tip to normal levels. Furthermore, the mutant phenotype could be reverted by direct manipulation of the endogenous NO level using an NO scavenger (cPTIO). The potential involvement of AtCRK5 protein kinase in the control of auxin-ROS-NO-PIN2-auxin regulatory loop is discussed.

Evaluation and GWAS of radicle gravitropic response in a core rice germplasms population

Aims: Gravitropism is one of the primary determinants of root development, facilitating root penetration into soil and subsequent absorption of water and nutrients. To study the gravitropism of the radicle roots, we conducted this research.Methods: The gravitropism of 226 Chinese rice micro-core accessions and drought-resistant core accessions were assessed through the modified gravity-bending experiment and genome-wide association analysis (GWAS) was used to map the associated QTLs.Results: The average value of gravitropic response speed of radicle roots was 41.05°/h from 16.77°/h to 62.83°/h. Significant difference (p < 0.002) in gravity response speed between Indica (42.49°/h) and Japonica (39.71°/h) subspecies was found. The gravitational response speed of radicle roots was significantly positively correlated with the number of deep roots (r =0.16), the growth speed of radicle roots (r =0.21) and the drought resistance coefficient (r=0.14).Conclusions: In total, 3 QTLs (quantitative traits) associated with gravitropic response speed were identified on chromosome 4,11 and 12. There are some known QTLs relating to roots traits and drought resistance located nearby the QTLs identified here, which confirms the close relationship between radicle gravitropism and the drought resistance. From within these intervals, 5 candidate genes were screened for qPCR in 6 extreme rice varieties, demonstrating that gene LOC_Os12g29350 may regulate gravitropism negatively and confirming its candidacy for further study.

The Circadian-clock Regulates the Arabidopsis Gravitropic Response

For long-term space missions, it is necessary to understand how organisms respond to changes in gravity. Plant roots are positively gravitropic; the primary root grows parallel to gravity's pull even after being turned away from the direction of gravity. We examined if this gravitropic response varies depending on the time of day reorientation occurs. When plants were reoriented in relation to the gravity vector or placed in simulated microgravity, the magnitude of the root gravitropic response varied depending on the time of day the initial change in gravity occurred. The response was greatest when plants were reoriented at dusk, just before a period of rapid growth, and were minimal just before dawn as the plants entered a period of reduced root growth. We found that this variation in the magnitude of the gravitropic response persisted in constant light (CL) suggesting the variation is circadian-regulated. Gravitropic responses were disrupted in plants with disrupted circadian clocks, including plants overexpressing Circadian-clock Associated 1 (CCA1) and elf3-2, in the reorientation assay and on a 2D clinostat. These findings indicate that circadian-regulated pathways modulate the gravitropic responses, thus, highlighting the importance of considering and recording the time of day gravitropic experiments are performed.

Evaluation and GWAS of radicle gravitropic response in a core rice germplasms population

Gravitropism is one of the primary determinants of root development, facilitating root penetration into soil and subsequent absorption of water and nutrients. In this study, the gravitropism of 226 Chinese rice micro-core accessions and drought-resistant core accessions were assessed. The average value of gravitropic response speed of radicle roots was 41.05°/h in the population ranging from 16.77°/h to 62.83°/h. We observed a highly significant difference in gravity response speed between Indica (42.49°/h) and Japonica (39.71°/h) subspecies with p-value < 0.002. The correlation analysis showed that the gravitational response speed of radicle roots was significantly positively correlated with the number of deep roots (correlation coefficient = 0.16), the growth speed of radicle roots (correlation coefficient = 0.21) and the drought resistance coefficient (correlation coefficient = 0.14). Using genome-wide association analysis (GWAS), 4 QTLs(quantitative traits) associating with gravitropic response speed were identified on chromosome 4,11 and 12. From within these intervals 5 candidate genes were screened for qPCR verification in 6 extreme rice varieties, demonstrating that gene LOC_Os12g29350 may regulate gravitropism negatively and confirming it’s candidacy for further study.