Adaptive divergence in shoot gravitropism creates hybrid sterility in an Australian wildflower

Natural selection is responsible for much of the diversity we see in nature. Just as it drives the evolution of new traits, it can also lead to new species. However, it is unclear whether natural selection conferring adaptation to local environments can drive speciation through the evolution of hybrid sterility between populations. Here, we show that adaptive divergence in shoot gravitropism, the ability of a plant’s shoot to bend upwards in response to the downward pull of gravity, contributes to the evolution of hybrid sterility in an Australian wildflower, Senecio lautus. We find that shoot gravitropism has evolved multiple times in association with plant height between adjacent populations inhabiting contrasting environments, suggesting that these traits have evolved by natural selection. We directly tested this prediction using a hybrid population subjected to eight rounds of recombination and three rounds of selection in the field. Our experiments revealed that shoot gravitropism responds to natural selection in the expected direction of the locally adapted population. Using the advanced hybrid population, we discovered that individuals with extreme differences in gravitropism had more sterile crosses than individuals with similar gravitropic responses, which were largely fertile, indicating that this adaptive trait is genetically correlated with hybrid sterility. Our results suggest that natural selection can drive the evolution of locally adaptive traits that also create hybrid sterility, thus revealing an evolutionary connection between local adaptation and the origin of new species.

CsLAZY1 mediates shoot gravitropism and branch angle in tea plants (Camellia sinensis)

Background Branch angle is a pivotal component of tea plant architecture. Tea plant architecture not only affects tea quality and yield but also influences the efficiency of automatic tea plant pruning. However, the molecular mechanism controlling the branch angle, which is an important aspect of plant architecture, is poorly understood in tea plants. Results In the present study, three CsLAZY genes were identified from tea plant genome data through sequence homology analysis. Phylogenetic tree displayed that the CsLAZY genes had high sequence similarity with LAZY genes from other plant species, especially those in woody plants. The expression patterns of the three CsLAZYs were surveyed in eight tissues. We further verified the expression levels of the key CsLAZY1 transcript in different tissues among eight tea cultivars and found that CsLAZY1 was highly expressed in stem. Subcellular localization analysis showed that the CsLAZY1 protein was localized in the plasma membrane. CsLAZY1 was transferred into Arabidopsis thaliana to investigate its potential role in regulating shoot development. Remarkably, the CsLAZY1 overexpressed plants responded more effectively than the wild-type plants to a gravity inversion treatment under light and dark conditions. The results indicate that CsLAZY1 plays an important role in regulating shoot gravitropism in tea plants. Conclusions The results provide important evidence for understanding the functions of CsLAZY1 in regulating shoot gravitropism and influencing the stem branch angle in tea plants. This report identifies CsLAZY1 as a promising gene resource for the improvement of tea plant architecture.

The CsLAZY1 Mediates Shoot Gravitropism and Branch Angle in Tea Plant (Camellia Sinensis)

Background: The tea plant (Camellia sinensis) architecture not only affects tea quality and yield, but also influences the efficiency of automatic pruning of tea plants. However, the molecular mechanism of branch angle that important aspect of plant architecture is poorly understood in tea plant. Results: In the present study, three CsLAZY genes were identified from the tea plant genome data through sequence homology. Phylogenetic tree displayed that the CsLAZY genes have high sequence similarity with LAZY genes from other plant species, especially those in woody plants. The expression patterns of the three CsLAZYs in eight tissues were surveyed, and we further verified the expression levels of the key CsLAZY1 transcript in different tissues among eight tea cultivars, demonstrating that CsLAZY1 was highly expressed in stem. Subcellular localization analysis showed that CsLAZY1 protein was localized in the plasma membrane. Remarkably, CsLAZY1 was transferred into Arabidopsis thaliana to investigate its potential role in regulating shoot development, demonstrating that the over-expressed plants responded more effectively than the wild types under gravity processing in light and dark. The results indicate that CsLAZY1 plays an important role in regulating shoot gravitropism in tea plant.Conclusions: The results provide evidence that CsLAZY1 may plays a critical role in regulating shoot gravitropism, and further affecting stem branch angle in tea plant.

LAZY1 Controls Tiller Angle and Shoot Gravitropism by Regulating the Expression of Auxin Transporters and Signaling Factors in Rice

Tiller angle is a key factor determining rice plant architecture, planting density, light interception, photosynthetic efficiency, disease resistance, and grain yield. However, the mechanisms underlying tiller angle control are far from clear. In this study, we identified a mutant, termed bta1–1, with an enlarged tiller angle throughout its life cycle. A detailed analysis reveals that BTA1 has multiple functions because tiller angle, shoot gravitropism, and tolerance to drought stress are changed in bta1–1 plants. Moreover, BTA1 is a positive regulator of shoot gravitropism in rice. Shoot responses to gravistimulation are disrupted in bta1-1 under both light and dark conditions. Gene cloning reveals that bta1-1 is a novel mutant allele of LA1 renamed la1-SN. LA1 is able to rescue the tiller angle and shoot gravitropism defects observed in la1-SN. The nuclear localization signal of LA1 is disrupted by la1-SN, causing changes of its subcellular localization. LA1 is required to regulate the expression of auxin transporters and signaling factors that control shoot gravitropism and tiller angle. High-throughput mRNA sequencing is performed to elucidate the molecular and cellular functions of LA1. The results show that LA1 may be involved in the nucleosome and chromatin assembly, and protein-DNA interactions to control gene expression, shoot gravitropism, and tiller angle. Our results provide new insight into the mechanisms whereby LA1 controls shoot gravitropism and tiller angle in rice.

A mathematical model explores the contributions of bending and stretching forces to shoot gravitropism in Arabidopsis

Plant shoot gravitropism is a complex phenomenon resulting from gravity sensing, curvature sensing (proprioception), the ability to uphold self-weight and growth. Although recent data analysis and modelling have revealed the detailed morphology of shoot bending, the relative contribution of bending force (derived from the gravi-proprioceptive response) and stretching force (derived from shoot axial growth) behind gravitropism remains poorly understood. To address this gap, we combined morphological data with a theoretical model to analyze shoot bending in wild-type and lazy1-like 1 mutant Arabidopsis thaliana. Using data from actual bending events, we searched for model parameters that minimized discrepancies between the data and mathematical model. The resulting model suggests that both the bending force and the stretching force differ significantly between the wild type and mutant. We discuss the implications of the mechanical forces associated with differential cell growth and present a plausible mechanical explanation of shoot gravitropism.

Adaptive divergence in shoot gravitropism creates intrinsic reproductive isolation in an Australian wildflower

Adaptation to the local environment is a major driver of speciation. Yet, it remains largely unknown whether natural selection directly causes intrinsic reproductive isolation (hybrid sterility or inviability) between locally adapted populations. Here, we show that adaptive divergence in shoot gravitropism, the ability of a plant’s shoot to bend upwards in response to the downward pull of gravity, contributes to the evolution of intrinsic reproductive isolation in an Australian wildflower, Senecio lautus. We find that shoot gravitropism has evolved multiple times in association with plant height between adjacent populations inhabiting contrasting environments, suggesting that these traits have evolved by natural selection. We directly tested this prediction in multi-year reciprocal transplant experiments using hybrid populations. We show that shoot gravitropism and plant height respond to natural selection in the expected direction of the locally adapted population. Remarkably, we find that crossing F11 hybrid individuals with extreme differences in gravitropism significantly reduces their ability to produce seeds, providing strong evidence that this adaptive trait is genetically correlated with intrinsic reproductive isolation. Our results suggest that natural selection can drive the evolution of locally adaptive traits that incidentally create intrinsic reproductive isolation, thus increasing our understanding of the origin and maintenance of new species.