An ATP-Binding Cassette Transporter, ABCB19, Regulates Leaf Position and Morphology during Phototropin1-Mediated Blue Light Responses

ABCB transporters: functionality extends to more than auxin transportation

MAIN CONCLUSION

ABCs transport diverse compounds; with plant's most abundant ABCG and ABCB subfamilies. ABCBs are multi-functional transporter proteins having role in plant adaptation. ATP-binding cassette (ABC) proteins have been known for the transportation of various structurally diverse compounds in all kingdoms of life. Plants possess a particularly high number of ABC transporters compared to other eukaryotes: the most abundant being ABCG followed by the ABCB subfamilies. While members of the ABCB subfamily are primarily known for auxin transportation, however, studies have shown their involvement in variety of other functions viz. growth and development, biotic and abiotic stresses, metal toxicity and homeostasis, cellular redox state stability, stomatal regulation, cell shape maintenance, and transport of secondary metabolites and phytohormones. These proteins are able to perform various biological processes due to their widespread localization in the plasma membrane, mitochondrial membrane, chloroplast, and tonoplast facilitating membrane transport influenced by various environmental and biological cues. The current review compiles published insights into the role of ABCB transporters, and also provides brief insights into the role of ABCB transporters in a medicinal plant, where the synthesis of its bioactive secondary metabolite is linked to the primary function of ABCBs, i.e., auxin transport. The review discusses ABCB subfamily members as multi-functional protein and comprehensively examines their role in various biological processes that help plants to survive under unfavorable environmental conditions.

PIN2-mediated self-organizing transient auxin flow contributes to auxin maxima at the tip of Arabidopsis cotyledons

Directional auxin transport and formation of auxin maxima are critical for embryogenesis, organogenesis, pattern formation, and growth coordination in plants, but the mechanisms underpinning the initiation and establishment of these auxin dynamics are not fully understood. Here we show that a self-initiating and -terminating transient auxin flow along the marginal cells (MCs) contributes to the formation of an auxin maximum at the tip of Arabidopsis cotyledon that globally coordinates the interdigitation of puzzle-shaped pavement cells in the cotyledon epidermis. Prior to the interdigitation, indole butyric acid (IBA) is converted to indole acetic acid (IAA) to induce PIN2 accumulation and polarization in the marginal cells, leading to auxin flow toward and accumulation at the cotyledon tip. Once IAA levels at the cotyledon tip reaches a maximum, it activates pavement cell interdigitation as well as the accumulation of the IBA transporter TOB1 in MCs, which sequesters IBA to the vacuole and reduces IBA availability and IAA levels. The reduction of IAA levels results in PIN2 down-regulation and cessation of the auxin flow. Hence, our results elucidate a self-activating and self-terminating transient polar auxin transport system in cotyledons, contributing to the formation of localized auxin maxima that spatiotemporally coordinate pavement cell interdigitation.

Defining context-dependent m6A RNA methylomes in Arabidopsis

N6-Methyladenosine (m6A) prevalently occurs on cellular RNA across almost all kingdoms of life. It governs RNA fate and is essential for development and stress responses. However, the dynamic, context-dependent m6A methylomes across tissues and in response to various stimuli remain largely unknown in multicellular organisms. Here, we generate a comprehensive census that identifies m6A methylomes in 100 samples during development or following exposure to various external conditions in Arabidopsis thaliana. We demonstrate that m6A is a suitable biomarker to reflect the developmental lineage, and that various stimuli rapidly affect m6A methylomes that constitute the regulatory network required for an effective response to the stimuli. Integrative analyses of the census and its correlation with m6A regulators identify multiple layers of regulation on highly context-dependent m6A modification in response to diverse developmental and environmental stimuli, providing insights into m6A modification dynamics in the myriad contexts of multicellular organisms.

Genome-wide analysis of Citrus medica ABC transporters reveals the regulation of fruit development by CmABCB19 and CmABCC10

ATP-binding cassette (ABC) transporters are vital for plant growth and development as they facilitate the transport of essential molecules. Despite the family's significance, limited information exists about its functional distinctions in Citrus medica. Our study identified 119 genes encoding ABC transporter proteins in the C. medica genome. Through an evolutionary tree and qPCR analysis, two ABC genes, CmABCB19 and CmABCC10, were implicated in C. medica fruit development, showing upregulation in normal fruits compared to malformed fruits. CmABCB19 was found to localize to the plasma membrane of Nicotiana tabacum, exhibiting indole-3-acetic acid (IAA) efflux activity in the yeast mutant strain yap1. CmABCC10, a tonoplast-localized transporter, exhibited efflux of diosmin, nobiletin, and naringin, with rutin influx in strain ycf1. Transgenic expression of CmABCB19 and CmABCC10 in Arabidopsis thaliana induced alterations in auxin and flavonoid content, impacting silique and seed size. This effect was attributed to the modulation of structural genes in the auxin biosynthesis (YUC5/9, CYP79B2, CYP83B1, SUR1) and flavonoid biosynthesis (4CL2/3, CHS, CHI, FLS1/3) pathways. In summary, the functional characterization of CmABCB19 and CmABCC10 illuminates auxin and flavonoid transport, offering insights into their interplay with biosynthetic pathways and providing a foundation for understanding the transporter's role in fruit development.

Mining Candidate Genes for Leaf Angle in Brassica napus L. by Combining QTL Mapping and RNA Sequencing Analysis

Leaf angle (LA) is an important trait of plant architecture, and individuals with narrow LA can better capture canopy light under high-density planting, which is beneficial for increasing the overall yield per unit area. To study the genetic basis and molecular regulation mechanism of leaf angle in rapeseed, we carried out a series of experiments. Quantitative trait loci (QTL) mapping was performed using the RIL population, and seven QTLs were identified. Transcriptome analysis showed that the cell wall formation/biogenesis processes and biosynthesis/metabolism of cell wall components were the most enrichment classes. Most differentially expressed genes (DEGs) involved in the synthesis of lignin, xylan, and cellulose showed down-regulated expression in narrow leaf material. Microscopic analysis suggested that the cell size affected by the cell wall in the junction area of the stem and petiole was the main factor in leaf petiole angle (LPA) differences. Combining QTL mapping and RNA sequencing, five promising candidate genes BnaA01G0125600ZS, BnaA01G0135700ZS, BnaA01G0154600ZS, BnaA10G0154200ZS, and BnaC03G0294200ZS were identified in rapeseed, and most of them were involved in cell wall biogenesis and the synthesis/metabolism of cell wall components. The results of QTL, transcriptome analysis, and cytological analysis were highly consistent, collectively revealing that genes related to cell wall function played a crucial role in regulating the LA trait in rapeseed. The study provides further insights into LA traits, and the discovery of new QTLs and candidate genes is highly beneficial for genetic improvement.

PIN2-mediated self-organizing transient auxin flow contributes to auxin maxima at the tip of Arabidopsis cotyledons

Directional auxin transport and formation of auxin maxima are critical for embryogenesis, organogenesis, pattern formation, and growth coordination in plants, but the mechanisms underpinning the initiation and establishment of these auxin dynamics are not fully understood. Here we show that a self-initiating and -terminating transient auxin flow along the marginal cells (MCs) contributes to the formation of an auxin maximum at the tip of Arabidopsis cotyledon that globally coordinates the interdigitation of puzzle-shaped pavement cells in the cotyledon epidermis. Prior to the interdigitation, indole butyric acid (IBA) is converted to indole acetic acid (IAA) to induce PIN2 accumulation and polarization in the marginal cells, leading to auxin flow toward and accumulation at the cotyledon tip. When IAA levels at the cotyledon tip reaches a maximum, it activates pavement cell interdigitation as well as the accumulation of the IBA transporter TOB1 in MCs, which sequesters IBA to the vacuole and reduces IBA availability and IAA levels. The reduction of IAA levels results in PIN2 down-regulation and cessation of the auxin flow. Hence, our results elucidate a self-activating and self-terminating transient polar auxin transport system in cotyledons, contributing to the formation of localized auxin maxima that spatiotemporally coordinate pavement cell interdigitation.

Major quantitative trait locus qLA3.1 is related to tomato leaf angle by regulating cell length at the petiole base

KEY MESSAGE

qLA3.1, controlling leaf angle in tomato, was fine-mapped to an interval of 4.45 kb on chromosome A03, and one gene encoding auxin response factor was identified as a candidate gene. Leaf angle is a crucial trait in plant architecture that plays an important role in achieving optimal plant structure. However, there are limited reports on gene localization, cloning, and the function of plant architecture in horticultural crops, particularly regarding leaf angle. In this study, we selected 'Z3' with erect leaves and 'Heinz1706' with horizontal leaves as the phenotype and cytological observation. We combined bulked segregant analysis and fine genetic mapping to identify a candidate gene, known as, i.e., qLA3.1, which was related to tomato leaf angle. Through multiple analyses, we found that Solyc03g113410 was the most probably candidate for qLA3.1, which encoded the auxin response factor SlARF11 in tomato and was homologous to OsARF11 related to leaf angle in rice. We discovered that silencing SlARF11 resulted in upright leaves, while plants with over-expressed SlARF11 exhibited horizontal leaves. We also found that cultivars with erect leaves had a mutation from base G to base A. Moreover, quantitative analysis of plants treated with hormones indicated that SlARF11 might participate in cell elongation and the activation of genes related to auxin and brassinosteroid pathways. Transcriptome analysis further validated that SlARF11 may regulate leaf angle through hormone signaling pathways. These data support the idea that the auxin response factor SlARF11 may have an important function in tomato leaf petiole angles.

Mechanism of the Pulvinus-Driven Leaf Movement: An Overview

Leaf movement is a manifestation of plant response to the changing internal and external environment, aiming to optimize plant growth and development. Leaf movement is usually driven by a specialized motor organ, the pulvinus, and this movement is associated with different changes in volume and expansion on the two sides of the pulvinus. Blue light, auxin, GA, H+-ATPase, K+, Cl-, Ca2+, actin, and aquaporin collectively influence the changes in water flux in the tissue of the extensor and flexor of the pulvinus to establish a turgor pressure difference, thereby controlling leaf movement. However, how these factors regulate the multicellular motility of the pulvinus tissues in a species remains obscure. In addition, model plants such as Medicago truncatula, Mimosa pudica, and Samanea saman have been used to study pulvinus-driven leaf movement, showing a similarity in their pulvinus movement mechanisms. In this review, we summarize past research findings from the three model plants, and using Medicago truncatula as an example, suggest that genes regulating pulvinus movement are also involved in regulating plant growth and development. We also propose a model in which the variation of ion flux and water flux are critical steps to pulvinus movement and highlight questions for future research.

Identification and expression analysis of ATP-binding cassette (ABC) transporters revealed its role in regulating stress response in pear (Pyrus bretchneideri)

BACKGROUND

ATP-binding cassette (ABC) transporter proteins constitute a plant gene superfamily crucial for growth, development, and responses to environmental stresses. Despite their identification in various plants like maize, rice, and Arabidopsis, little is known about the information on ABC transporters in pear. To investigate the functions of ABC transporters in pear development and abiotic stress response, we conducted an extensive analysis of ABC gene family in the pear genome.

RESULTS

In this study, 177 ABC transporter genes were successfully identified in the pear genome, classified into seven subfamilies: 8 ABCAs, 40 ABCBs, 24 ABCCs, 8 ABCDs, 9 ABCEs, 8 ABCFs, and 80 ABCGs. Ten motifs were common among all ABC transporter proteins, while distinct motif structures were observed for each subfamily. Distribution analysis revealed 85 PbrABC transporter genes across 17 chromosomes, driven primarily by WGD and dispersed duplication. Cis-regulatory element analysis of PbrABC promoters indicated associations with phytohormones and stress responses. Tissue-specific expression profiles demonstrated varied expression levels across tissues, suggesting diverse functions in development. Furthermore, several PbrABC genes responded to abiotic stresses, with 82 genes sensitive to salt stress, including 40 upregulated and 23 downregulated genes. Additionally, 91 genes were responsive to drought stress, with 22 upregulated and 36 downregulated genes. These findings highlight the pivotal role of PbrABC genes in abiotic stress responses.

CONCLUSION

This study provides evolutionary insights into PbrABC transporter genes, establishing a foundation for future research on their functions in pear. The identified motifs, distribution patterns, and stress-responsive expressions contribute to understanding the regulatory mechanisms of ABC transporters in pear. The observed tissue-specific expression profiles suggest diverse roles in developmental processes. Notably, the significant responses to salt and drought stress emphasize the importance of PbrABC genes in mediating adaptive responses. Overall, our study advances the understanding of PbrABC transporter genes in pear, opening avenues for further investigations in plant molecular biology and stress physiology.

Ultraviolet-A1 radiation induced a more favorable light-intercepting leaf-area display than blue light and promoted plant growth

Plants adjust their morphology in response to light environment by sensing an array of light cues. Though the wavelengths of ultraviolet-A1 radiation (UV-A1, 350-400 nm) are close to blue light (B, 400-500 nm) and share same flavoprotein photoreceptors, it remains poorly understood how plant responses to UV-A1 radiation could differ from those to B. We initially grown tomato plants under monochromatic red light (R, 660 nm) as control, subsequently transferred them to four dichromatic light treatments containing ~20 µmol m-2  s-1 of UV-A1 radiation, peaking at 370 nm (UV-A370 ) or 400 nm (V400 ), or B (450 nm, at ~20 or 1.5 µmol m-2  s-1 ), with same total photon irradiance (~200 μmol m-2  s-1 ). We show that UV-A370 radiation was the most effective in inducing light-intercepting leaf-area display formation, resulting in larger leaf area and more shoot biomass, while it triggered weaker and later transcriptome-wide responses than B. Mechanistically, UV-A370 -promoted leaf-area display response was apparent in less than 12 h and appeared as very weakly related to transcriptome level regulation, which likely depended on the auxin transportation and cell wall acidification. This study revealed wavelength-specific responses within UV-A/blue region challenging usual assumptions that the role of UV-A1 radiation function similarly as blue light in mediating plant processes.

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.

Phototropin phosphorylation of ROOT PHOTOTROPISM 2 and its role in mediating phototropism, leaf positioning, and chloroplast accumulation movement in Arabidopsis

Directional movements impact the ability of plants to respond and adjust their growth accordingly to the prevailing light environment. The plasma-membrane associated protein, ROOT PHOTOTROPISM 2 (RPT2) is a key signalling component involved in chloroplast accumulation movement, leaf positioning, and phototropism, all of which are regulated redundantly by the ultraviolet/blue light-activated AGC kinases phototropin 1 and 2 (phot1 and phot2). We recently demonstrated that members of the NON-PHOTOTROPIC HYPOCOTYL 3 (NPH3)/RPT2-like (NRL) family in Arabidopsis thaliana, including RPT2, are directly phosphorylated by phot1. However, whether RPT2 is a substrate for phot2, and the biological significance of phot phosphorylation of RPT2 remains to be determined. Here, we show that RPT2 is phosphorylated by both phot1 and phot2 at a conserved serine residue (S591) within the C-terminal region of the protein. Blue light triggered the association of 14-3-3 proteins with RPT2 consistent with S591 acting as a 14-3-3 binding site. Mutation of S591 had no effect on the plasma membrane localization of RPT2 but reduced its functionality for leaf positioning and phototropism. Moreover, our findings indicate that S591 phosphorylation within the C-terminus of RPT2 is required for chloroplast accumulation movement to low level blue light. Taken together, these findings further highlight the importance of the C-terminal region of NRL proteins and how its phosphorylation contributes to phot receptor signalling in plants.

A mutation in CsABCB19 encoding an ATP-binding cassette auxin transporter leads to erect and compact leaf architecture in cucumber (Cucumis sativus L.)

Leaf architecture, including leaf position and leaf morphology, is a critical component of plant architecture that directly determines plant appearance, photosynthetic utilization, and ultimate productivity. The mechanisms regulating leaf petiole angle and leaf flatness in cucumber remain unclear. In this study, we identified an erect and compact leaf architecture mutant (ecla) from an EMS (ethyl methanesulfonate) -mutagenized cucumber population, which exhibited erect petioles and crinkled leaves. Histological examination revealed significant phenotypic variation in ecla was associated with asymmetric cell expansion. MutMap sequencing combined with genetic mapping revealed that CsaV3_5G037960 is the causative gene for the ecla mutant phenotype. Through protein sequence alignment and Arabidopsis genetic complementation, we identified this gene as a functional direct homolog encoding the ATP-binding cassette transporter AtABCB19, hence named CsABCB19. A nonsynonymous mutation in the eleventh exon of CsABCB19 leads to premature termination of translation. The expression level of CsABCB19 in the ecla mutant was significantly reduced in all tissues compared to the wild type (WT). Transcriptome analysis revealed that auxin and polarity-related genes were significantly differentially expressed in mutant petioles and leaves, compared with those in WT. Auxin assay and exogenous treatment further demonstrated that CsABCB19 regulates leaf architecture by mediating auxin accumulation and transport. Our research is the first report describing the role of the ABCB19 transporter protein in auxin transport controlling cucumber leaf development. Furthermore, this study provides recent insights into the genetic mechanisms conferring morphological diversity and regulation of petiole angle and leaf flattening. DATA AVAILABILITY: The RNA-seq data in this study have been deposited in the NCBI SRA under BioProject accession number PRJNA874548.

Arabidopsis TWISTED DWARF1 regulates stamen elongation by differential activation of ABCB1,19-mediated auxin transport

Despite clear evidence that a local accumulation of auxin is likewise critical for male fertility, much less is known about the components that regulate auxin-controlled stamen development. In this study, we analyzed physiological and morphological parameters in mutants of key players of ABCB-mediated auxin transport, and spatially and temporally dissected their expression on the protein level as well as auxin fluxes in the Arabidopsis stamens. Our analyses revealed that the FKBP42, TWISTED DWARF1 (TWD1), promotes stamen elongation and, to a lesser extent, anther dehiscence, as well as pollen maturation, and thus is required for seed development. Most of the described developmental defects in twd1 are shared with the abcb1 abcb19 mutant, which can be attributed to the fact that TWD1-as a described ABCB chaperone-is a positive regulator of ABCB1- and ABCB19-mediated auxin transport. However, reduced stamen number was dependent on TWD1 but not on investigated ABCBs, suggesting additional players downstream of TWD1. We predict an overall housekeeping function for ABCB1 during earlier stages, while ABCB19 seems to be responsible for the key event of rapid elongation at later stages of stamen development. Our data indicate that TWD1 controls stamen development by differential activation of ABCB1,19-mediated auxin transport in the stamen.

GmPIN1-mediated auxin asymmetry regulates leaf petiole angle and plant architecture in soybean

Crop breeding during the Green Revolution resulted in high yields largely due to the creation of plants with semi-dwarf architectures that could tolerate high-density planting. Although semi-dwarf varieties have been developed in rice, wheat and maize, none was reported in soybean (Glycine max), and few genes controlling plant architecture have been characterized in soybean. Here, we demonstrate that the auxin efflux transporter PINFORMED1 (GmPIN1), which determines polar auxin transport, regulates the leaf petiole angle in soybean. CRISPR-Cas9-induced Gmpin1abc and Gmpin1bc multiple mutants displayed a compact architecture with a smaller petiole angle than wild-type plants. GmPIN1 transcripts and auxin were distributed asymmetrically in the petiole base, with high levels of GmPIN1a/c transcript and auxin in the lower cells, which resulted in asymmetric cell expansion. By contrast, the (iso)flavonoid content was greater in the upper petiole cells than in the lower cells. Our results suggest that (iso)flavonoids inhibit GmPIN1a/c expression to regulate the petiole angle. Overall, our study demonstrates that a signal cascade that integrates (iso)flavonoid biosynthesis, GmPIN1a/c expression, auxin accumulation, and cell expansion in an asymmetric manner creates a desirable petiole curvature in soybean. This study provides a genetic resource for improving soybean plant architecture.

Loss of Multiple ABCB Auxin Transporters Recapitulates the Major twisted dwarf 1 Phenotypes in Arabidopsis thaliana

FK506-BINDING PROTEIN 42/TWISTED DWARF 1 (FKBP42/TWD1) directly regulates cellular trafficking and activation of multiple ATP-BINDING CASSETTE (ABC) transporters from the ABCB and ABCC subfamilies. abcb1 abcb19 double mutants exhibit remarkable phenotypic overlap with twd1 including severe dwarfism, stamen elongation defects, and compact circinate leaves; however, twd1 mutants exhibit greater loss of polar auxin transport and additional helical twisting of roots, inflorescences, and siliques. As abcc1 abcc2 mutants do not exhibit any visible phenotypes and TWD1 does not interact with PIN or AUX1/LAX auxin transporters, loss of function of other ABCB auxin transporters is hypothesized to underly the remaining morphological phenotypes. Here, gene expression, mutant analyses, pharmacological inhibitor studies, auxin transport assays, and direct auxin quantitations were used to determine the relative contributions of loss of other reported ABCB auxin transporters (4, 6, 11, 14, 20, and 21) to twd1 phenotypes. From these analyses, the additional reduction in plant height and the twisted inflorescence, root, and silique phenotypes observed in twd1 compared to abcb1 abcb19 result from loss of ABCB6 and ABCB20 function. Additionally, abcb6 abcb20 root twisting exhibited the same sensitivity to the auxin transport inhibitor 1-napthalthalamic acid as twd1 suggesting they are the primary contributors to these auxin-dependent organ twisting phenotypes. The lack of obvious phenotypes in higher order abcb4 and abcb21 mutants suggests that the functional loss of these transporters does not contribute to twd1 root or shoot twisting. Analyses of ABCB11 and ABCB14 function revealed capacity for auxin transport; however, their activities are readily outcompeted by other substrates, suggesting alternate functions in planta, consistent with a spectrum of relative substrate affinities among ABCB transporters. Overall, the results presented here suggest that the ABCB1/19 and ABCB6/20 pairs represent the primary long-distance ABCB auxin transporters in Arabidopsis and account for all reported twd1 morphological phenotypes. Other ABCB transporters appear to participate in highly localized auxin streams or mobilize alternate transport substrates.

Bradyrhizobium japonicum IRAT FA3 Alters Arabidopsis thaliana Root Architecture via Regulation of Auxin Efflux Transporters PIN2, PIN3, PIN7, and ABCB19

Beneficial rhizobacteria can stimulate changes in plant root development. Although root system growth is mediated by multiple factors, the regulated distribution of the phytohormone auxin within root tissues plays a principal role. Auxin transport facilitators help to generate the auxin gradients and maxima that determine root structure. Here, we show that the plant-growth-promoting rhizobacterial strain Bradyrhizobium japonicum IRAT FA3 influences specific auxin efflux transporters to alter Arabidopsis thaliana root morphology. Gene expression profiling of host transcripts in control and B. japonicum-inoculated roots of the wild-type A. thaliana accession Col-0 confirmed upregulation of PIN2, PIN3, PIN7, and ABCB19 with B. japonicum and identified genes potentially contributing to a diverse array of auxin-related responses. Cocultivation of the bacterium with loss-of-function auxin efflux transport mutants revealed that B. japonicum requires PIN3, PIN7, and ABCB19 to increase lateral root development and utilizes PIN2 to reduce primary root length. Accelerated lateral root primordia production due to B. japonicum was not observed in single pin3, pin7, or abcb19 mutants, suggesting independent roles for PIN3, PIN7, and ABCB19 during the plant-microbe interaction. Our work demonstrates B. japonicum's influence over host transcriptional reprogramming during plant interaction with this beneficial microbe and the subsequent alterations to root system architecture.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Series-Spatial Transcriptome Profiling of Leafy Head Reveals the Key Transition Leaves for Head Formation in Chinese Cabbage

Chinese cabbage is an important leaf heading vegetable crop. At the heading stage, its leaves across inner to outer show significant morphological differentiation. However, the genetic control of this complex leaf morphological differentiation remains unclear. Here, we reported the transcriptome profiling of Chinese cabbage plant at the heading stage using 24 spatially dissected tissues representing different regions of the inner to outer leaves. Genome-wide transcriptome analysis clearly separated the inner leaf tissues from the outer leaf tissues. In particular, we identified the key transition leaf by the spatial expression analysis of key genes for leaf development and sugar metabolism. We observed that the key transition leaves were the first inwardly curved ones. Surprisingly, most of the heading candidate genes identified by domestication selection analysis obviously showed a corresponding expression transition, supporting that key transition leaves are related to leafy head formation. The key transition leaves were controlled by a complex signal network, including not only internal hormones and protein kinases but also external light and other stimuli. Our findings provide new insights and the rich resource to unravel the genetic control of heading traits.

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.