Nuclear phytochrome B regulates leaf flattening through phytochrome interacting factors

Light and temperature regulation of leaf morphogenesis in Arabidopsis

Leaves are the main photosynthetic organs in plants, and their anatomy is optimized for light interception and gas exchange. Although each species has a characteristic leaf anatomy, which depends on the genotype, leaves also show a large degree of developmental plasticity. Light and temperature regulate leaf development from primordia differentiation to late stages of blade expansion. While the molecular mechanisms of light and temperature signaling have been mostly studied in seedlings, in the latest years, research has focused on leaf development. Here, I will describe the latest work carried out in the environmental regulation of Arabidopsis leaf development, comparing signaling mechanisms between leaves and seedlings, highlighting the new discoveries, and pointing out the most exciting open questions.

KANADI promotes thallus differentiation and FR-induced gametangiophore formation in the liverwort Marchantia

In angiosperms, KANADI transcription factors have roles in the sporophyte generation regulating tissue polarity, organogenesis and shade avoidance responses, but are not required during the gametophyte generation. Whether these roles are conserved in the gametophyte-dominant bryophyte lineages is unknown, which we examined by characterising the sole KANADI ortholog, MpKAN, in the liverwort Marchantia polymorpha. In contrast to angiosperm orthologs, MpKAN functions in the gametophyte generation in Marchantia, where it regulates apical branching and tissue differentiation, but does not influence tissue polarity in either generation. MpKAN can partially rescue the sporophyte polarity defects of kanadi mutants in Arabidopsis, indicating that MpKAN has conserved biochemical activity to its angiosperm counterparts. Mpkan loss-of-function plants display defects in far-red (FR) light responses. Mpkan plants have reduced FR-induced growth tropisms, have a delayed transition to sexual reproduction and fail to correctly form gametangiophores. Our results indicate that MpKAN is a modulator of FR responses, which may reflect a conserved role for KANADI across land plants. Under FR, MpKAN negatively regulates MpDELLA expression, suggesting that MpKAN and MpDELLA act in a pathway regulating FR responses, placing MpKAN in a gene regulatory network exhibiting similarities with those of angiosperms.

Phototropin-mediated perception of light direction in leaves regulates blade flattening

One conserved feature among angiosperms is the development of flat thin leaves. This developmental pattern optimizes light capture and gas exchange. The blue light (BL) receptors phototropins are required for leaf flattening, with the null phot1phot2 mutant showing curled leaves in Arabidopsis (Arabidopsis thaliana). However, key aspects of their function in leaf development remain unknown. Here, we performed a detailed spatiotemporal characterization of phototropin function in Arabidopsis leaves. We found that phototropins perceive light direction in the blade, and, similar to their role in hypocotyls, they control the spatial pattern of auxin signaling, possibly modulating auxin transport, to ultimately regulate cell expansion. Phototropin signaling components in the leaf partially differ from hypocotyls. Moreover, the light response on the upper and lower sides of the leaf blade suggests a partially distinct requirement of phototropin signaling components on each side. In particular, NON PHOTOTROPIC HYPOCOTYL 3 showed an adaxial-specific function. In addition, we show a prominent role of PHYTOCHROME KINASE SUBSTRATE 3 in leaf flattening. Among auxin transporters, PIN-FORMED 3,4,7 and AUXIN RESISTANT 1 (AUX1)/LIKE AUXIN RESISTANT 1 (LAX1) are required for the response while ABCB19 has a regulatory role. Overall, our results show that directional BL perception by phototropins is a key aspect of leaf development, integrating endogenous and exogenous signals.

A chloroplast heat shock protein modulates growth and abiotic stress response in creeping bentgrass

Small heat shock proteins (sHSPs), a family of the ubiquitous stress proteins in plants acting as molecular chaperones to protect other proteins from stress-induced damage, have been implicated in plant growth and development as well as plant response to environmental stress, especially heat stress. In this study, a chloroplast-localized sHSP, AsHSP26.8, was overexpressed in creeping bentgrass (Agrostis stolonifera L.) to study its role in regulating plant growth and stress response. Transgenic (TG) creeping bentgrass plants displayed arrested root development, slow growth rate, twisted leaf blades and are more susceptible to heat and salt but less sensitive to drought stress compared to wild-type (WT) controls. RNA-seq analysis revealed that AsHSP26.8 modulated the expression of genes in auxin signalling and stress-related genes such as those encoding HSPs, heat shock factors and other transcription factors. Our results provide new evidence demonstrating that AsHSP26.8 negatively regulates plant growth and development and plays differential roles in plant response to a plethora of diverse abiotic stresses.

Photomorphogenic responses to ultraviolet-B light

Exposure to ultraviolet B (UV-B) light regulates numerous aspects of plant metabolism, morphology and physiology through the differential expression of hundreds of genes. Photomorphogenic responses to UV-B are mediated by the photoreceptor UV RESISTANCE LOCUS8 (UVR8). Considerable progress has been made in understanding UVR8 action: the structural basis of photoreceptor function, how interaction with CONSTITUTIVELY PHOTOMORPHOGENIC 1 initiates signaling and how REPRESSOR OF UV-B PHOTOMORPHOGENESIS proteins negatively regulate UVR8 action. In addition, recent research shows that UVR8 mediates several responses through interaction with other signaling pathways, in particular auxin signaling. Nevertheless, many aspects of UVR8 action remain poorly understood. Most research to date has been undertaken with Arabidopsis, and it is important to explore the functions and regulation of UVR8 in diverse plant species. Furthermore, it is essential to understand how UVR8, and UV-B signaling in general, regulates processes under natural growth conditions. Ultraviolet B regulates the expression of many genes through UVR8-independent pathways, but the activity and importance of these pathways in plants growing in sunlight are poorly understood.

Leaf epinasty and auxin: A biochemical and molecular overview

Leaf epinasty involves the downward bending of leaves as a result of disturbances in their growth, with a greater expansion in adaxial cells as compared to abaxial surface cells. The co-ordinated anisotropy of growth in epidermal, palisade mesophyll and vascular tissues contributes to epinasty. This phenotype, which is regulated by auxin (indole-3-acetic acid, IAA), controls plant cell division and elongation by regulating the expression of a vast number of genes. Other plant hormones, such as ethylene, abscisic acid and brassinosteroids, also regulate epinasty and hyponasty. Reactive oxygen species (ROS) accumulation induced by auxins and 2,4-dichlorophenoxyacetic acid (2,4-D) triggers epinasty. The role of ROS and nitric oxide (NO) in the regulation of epinasty has recently been established. Thus, treatment with synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) induces disturbances in the actin cytoskeleton through ROS and NO-dependent post-translational modifications in actin by carbonylation and S-nitrosylation, which cause a reduction in the actin filament. Reorientation of microtubules has become a major feature of the response to auxin. The cytoskeleton is therefore a key player in epinastic development.

Ultraviolet-B radiation stimulates downward leaf curling in Arabidopsis thaliana

Plants are very well adapted to growth in ultraviolet-B (UV-B) containing light. In Arabidopsis thaliana, many of these adaptations are mediated by the UV-B receptor UV resistance locus 8 (UVR8). Using small amounts of supplementary UV-B light, we observed changes in the shape of rosette leaf blades. Wild type plants show more pronounced epinasty of the blade edges, while this is not the case in uvr8 mutant plants. The UVR8 effect thus mimics the effect of phytochrome (phy) B in red light. In addition, a meta-analysis of transcriptome data indicates that the UVR8 and phyB signaling pathways have over 70% of gene regulation in common. Moreover, in low levels of supplementary UV-B light, mutant analysis revealed that phyB signaling is necessary for epinasty of the blade edges. Analysis of auxin levels and the auxin signal reporter DR5::GUS suggest that the epinasty relies on altered auxin distribution, keeping auxin at the leaf blade edges in the presence of UV-B. Together, our results suggest a co-action of phyB and UVR8 signaling, with auxin as a downstream factor.