Robust increase of leaf size by Arabidopsis thaliana GRF3-like transcription factors under different growth conditions

CLAVATA3 Signaling Buffers Arabidopsis Shoot Apical Meristem Activity in Response to Photoperiod

Land plants grow throughout their life cycle via the continuous activity of stem cell reservoirs contained within their apical meristems. The shoot apical meristem (SAM) of Arabidopsis and other land plants responds to a variety of environmental cues, yet little is known about the response of meristems to seasonal changes in day length, or photoperiod. Here, the vegetative and reproductive growth of Arabidopsis wild-type and clavata3 (clv3) plants in different photoperiod conditions was analyzed. It was found that SAM size in wild-type Arabidopsis plants grown in long-day (LD) conditions gradually increased from embryonic to reproductive development. clv3 plants produced significantly more leaves as well as larger inflorescence meristems and more floral buds than wild-type plants in LD and short-day (SD) conditions, demonstrating that CLV3 signaling limits vegetative and inflorescence meristem activity in both photoperiods. The clv3 phenotypes were more severe in SDs, indicating a greater requirement for CLV3 restriction of SAM function when the days are short. In contrast, clv3 floral meristem size and carpel number were unchanged between LD and SD conditions, which shows that the photoperiod does not affect the regulation of floral meristem activity through the CLV3 pathway. This study reveals that CLV3 signaling specifically restricts vegetative and inflorescence meristem activity in both LD and SD photoperiods but plays a more prominent role during short days.

Genome-Wide Identification and Expression Analysis of Growth-Regulating Factor Family in Sweet Potato and Its Two Relatives

Growth-regulating factor (GRF) is a multi-gene family that plays an important role in plant growth and development and is widely present in plants. Currently, GRF gene members have been reported in many plants, but the GRF gene family has not been found in sweet potato. In this study, ten GRF genes were identified in sweet potato (Ipomoea batatas), twelve and twelve were identified in its two diploid relatives (Ipomoea trifida) and (Ipomoea triloba), which were unevenly distributed on nine different chromosomes. Subcellular localization analysis showed that GRF genes of sweet potato, I. trifida, and I. triloba were all located in the nucleus. The expression analysis showed that the expression of IbGRFs was diverse in different sweet potato parts, and most of the genes were upregulated and even had the highest expression in the vigorous growth buds. These findings provide molecular characterization of sweet potato and its two diploid relatives, the GRF families, further supporting functional characterization.

Genome-wide identification and expression profiling of growth‑regulating factor (GRF) and GRF‑interacting factor (GIF) gene families in chickpea and pigeonpea

The growth-regulating factor (GRF) and GRF-interacting factor (GIF) families encode plant-specific transcription factors and play vital roles in plant development and stress response processes. Although GRF and GIF genes have been identified in various plant species, there have been no reports of the analysis and identification of the GRF and GIF transcription factor families in chickpea (Cicer arietinum) and pigeonpea (Cajanus cajan). The present study identified seven CaGRFs, eleven CcGRFs, four CaGIFs, and four CcGIFs. The identified proteins were grouped into eight and three clades for GRFs and GIFs, respectively based on their phylogenetic relationships. A comprehensive in-silico analysis was performed to determine chromosomal location, sub-cellular localization, and types of regulatory elements present in the putative promoter region. Synteny analysis revealed that GRF and GIF genes showed diploid-polyploid topology in pigeonpea, but not in chickpea. Tissue-specific expression data at the vegetative and reproductive stages of the plant showed that GRFs and GIFs were strongly expressed in tissues like embryos, pods, and seeds, indicating that GRFs and GIFs play vital roles in plant growth and development. This research characterized GRF and GIF families and hints at their primary roles in the chickpea and pigeonpea growth and developmental process. Our findings provide potential gene resources and vital information on GRF and GIF gene families in chickpea and pigeonpea, which will help further understand the regulatory role of these gene families in plant growth and development.

Molecular mechanisms regulating GROWTH-REGULATING FACTORS activity in plant growth, development, and environmental responses

Plants rely on complex regulatory mechanisms to ensure proper growth and development. As plants are sessile organisms, these mechanisms must be flexible enough to adapt to changes in the environment. GROWTH-REGULATING FACTORS (GRFs) are plant-specific transcription factors that act as a central hub controlling plant growth and development, which offer promising biotechnological applications to enhance plant performance. Here, we analyze the complex molecular mechanisms that regulate GRFs activity, and how their natural and synthetic variants can impact on plant growth and development. We describe the biological roles of the GRFs and examine how they regulate gene expression and contribute to the control of organ growth and plant responses to a changing environment. This review focuses on the premise that unlocking the full biotechnological potential of GRFs requires a thorough understanding of the various regulatory layers governing GRF activity, the functional divergence among GRF family members, and the gene networks that they regulate.

After silencing suppression: miRNA targets strike back

Within the continuous tug-of-war between plants and microbes, RNA silencing stands out as a key battleground. Pathogens, in their quest to colonize host plants, have evolved a diverse arsenal of silencing suppressors as a common strategy to undermine the host's RNA silencing-based defenses. When RNA silencing malfunctions in the host, genes that are usually targeted and silenced by microRNAs (miRNAs) become active and can contribute to the reprogramming of host cells, providing an additional defense mechanism. A growing body of evidence suggests that miRNAs may act as intracellular sensors to enable a rapid response to pathogen threats. Herein we review how plant miRNA targets play a crucial role in immune responses against different pathogens.

CsRAXs negatively regulate leaf size and fruiting ability through auxin glycosylation in cucumber

Leaves are the main photosynthesis organ that directly determines crop yield and biomass. Dissecting the regulatory mechanism of leaf development is crucial for food security and ecosystem turn-over. Here, we identified the novel function of R2R3-MYB transcription factors CsRAXs in regulating cucumber leaf size and fruiting ability. Csrax5 single mutant exhibited enlarged leaf size and stem diameter, and Csrax1/2/5 triple mutant displayed further enlargement phenotype. Overexpression of CsRAX1 or CsRAX5 gave rise to smaller leaf and thinner stem. The fruiting ability of Csrax1/2/5 plants was significantly enhanced, while that of CsRAX5 overexpression lines was greatly weakened. Similarly, cell number and free auxin level were elevated in mutant plants while decreased in overexpression lines. Biochemical data indicated that CsRAX1/5 directly promoted the expression of auxin glucosyltransferase gene CsUGT74E2. Therefore, our data suggested that CsRAXs function as repressors for leaf size development by promoting auxin glycosylation to decrease free auxin level and cell division in cucumber. Our findings provide new gene targets for cucumber breeding with increased leaf size and crop yield.

Genome-wide characterization and function analysis of ginger (Zingiber officinale Roscoe) ZoGRFs in responding to adverse stresses

Growth-regulating factors (GRFs) play crucial roles in plant growth, development, hormone signaling, and stress response. Despite their significance, the roles of GRFs in ginger remain largely unknown. Herein, 31 ginger ZoGRFs were identified and designated as ZoGRF1-ZoGRF31 according to their phylogenetic relationships. All ZoGRFs were characterized as unstable, hydrophilic proteins, with 29 predicted to be located in the nucleus. Functional cis-elements related to growth and development were enriched in ZoGRF's promoter regions. RNA-seq and RT-qPCR analysis revealed that ZoGRF12, ZoGRF24, and ZoGRF28 were highly induced in various growth and development stages, displaying differential regulation under waterlogging, chilling, drought, and salt stresses, indicating diverse expression patterns of ZoGRFs. Transient expression analysis in Nicotiana benthamiana indicated that overexpressing ZoGRF28 regulated the transcription levels of salicylic acid, jasmonic acid, and pattern-triggered immunity-related genes, increased chlorophyll content and contributed to reduced disease lesions and an increased net photosynthetic rate. This research lays the foundation for further understanding the biological roles of ZoGRFs.

Genome-wide molecular evolution analysis of the GRF and GIF gene families in Plantae (Archaeplastida)

BACKGROUND

Plant growth-regulating factors (GRFs) and GRF-interacting factors (GIFs) interact with each other and collectively have important regulatory roles in plant growth, development, and stress responses. Therefore, it is of great significance to explore the systematic evolution of GRF and GIF gene families. However, our knowledge and understanding of the role of GRF and GIF genes during plant evolution has been fragmentary.

RESULTS

In this study, a large number of genomic and transcriptomic datasets of algae, mosses, ferns, gymnosperms and angiosperms were used to systematically analyze the evolution of GRF and GIF genes during the evolution of plants. The results showed that GRF gene first appeared in the charophyte Klebsormidium nitens, whereas the GIF genes originated relatively early, and these two gene families were mainly expanded by segmental duplication events after plant terrestrialization. During the process of evolution, the protein sequences and functions of GRF and GIF family genes are relatively conservative. As cooperative partner, GRF and GIF genes contain the similar types of cis-acting elements in their promoter regions, which enables them to have similar transcriptional response patterns, and both show higher levels of expression in reproductive organs and tissues and organs with strong capacity for cell division. Based on protein-protein interaction analysis and verification, we found that the GRF-GIF protein partnership began to be established in pteridophytes and is highly conserved across different terrestrial plants.

CONCLUSIONS

These results provide a foundation for further exploration of the molecular evolution and biological functions of GRF and GIF genes.

Genome-Wide Analysis of Flax (Linum usitatissimum L.) Growth-Regulating Factor (GRF) Transcription Factors

Flax is an important cash crop globally with a variety of commercial uses. It has been widely used for fiber, oil, nutrition, feed and in composite materials. Growth regulatory factor (GRF) is a transcription factor family unique to plants, and is involved in regulating many processes of growth and development. Bioinformatics analysis of the GRF family in flax predicted 17 LuGRF genes, which all contained the characteristic QLQ and WRC domains. Equally, 15 of 17 LuGRFs (88%) are predicted to be regulated by lus-miR396 miRNA. Phylogenetic analysis of GRFs from flax and several other well-characterized species defined five clades; LuGRF genes were found in four clades. Most LuGRF gene promoters contained cis-regulatory elements known to be responsive to hormones and stress. The chromosomal locations and collinearity of LuGRF genes were also analyzed. The three-dimensional structure of LuGRF proteins was predicted using homology modeling. The transcript expression data indicated that most LuGRF family members were highly expressed in flax fruit and embryos, whereas LuGRF3, LuGRF12 and LuGRF16 were enriched in response to salt stress. Real-time quantitative fluorescent PCR (qRT-PCR) showed that both LuGRF1 and LuGRF11 were up-regulated under ABA and MeJA stimuli, indicating that these genes were involved in defense. LuGRF1 was demonstrated to be localized to the nucleus as expected for a transcription factor. These results provide a basis for further exploration of the molecular mechanism of LuGRF gene function and obtaining improved flax breeding lines.

Natural variation in the SVP contributes to the pleiotropic adaption of Arabidopsis thaliana across contrasted habitats

Coordinated plant adaptation involves the interplay of multiple traits driven by habitat-specific selection pressures. Pleiotropic effects, wherein genetic variants of a single gene control multiple traits, can expedite such adaptations. Until present, only a limited number of genes have been reported to exhibit pleiotropy. Here, we create a recombinant inbred line (RIL) population derived from two Arabidopsis thaliana (A. thaliana) ecotypes originating from divergent habitats. Using this RIL population, we identify an allelic variation in a MADS-box transcription factor, SHORT VEGETATIVE PHASE (SVP), which exerts a pleiotropic effect on leaf size and drought-versus-humidity tolerance. Further investigation reveals that a natural null variant of the SVP protein disrupts its normal regulatory interactions with target genes, including GRF3, CYP707A1/3, and AtBG1, leading to increased leaf size, enhanced tolerance to humid conditions, and changes in flowering time of humid conditions in A. thaliana. Remarkably, polymorphic variations in this gene have been traced back to early A. thaliana populations, providing a genetic foundation and plasticity for subsequent colonization of diverse habitats by influencing multiple traits. These findings advance our understanding of how plants rapidly adapt to changing environments by virtue of the pleiotropic effects of individual genes on multiple trait alterations.

Molecular Mechanisms of the miR396b-GRF1 Module Underlying Rooting Regulation in Acer rubrum L

Rooting and root development in Acer rubrum have important effects on overall growth. A. rubrum does not take root easily in natural conditions. In this study, the mechanisms of the miR396b-GRF1 module underlying rooting regulation in A. rubrum were studied. The subcellular localization and transcriptional activation of miR396b and its target gene growth regulating factor 1 (GRF1) were investigated. These experiments showed that GRF1 was localized in the nucleus and had transcriptional activation activity. Functional validation experiments in transgenic plants demonstrated that overexpression of Ar-miR396b inhibited adventitious root growth, whereas overexpression of ArGRF1 increased adventitious root growth. These results help clarify the molecular regulatory mechanisms underlying adventitious root growth in A. rubrum and provide some new insights into the rooting rate in this species.

The SOD7/DPA4-GIF1 module coordinates organ growth and iron uptake in Arabidopsis

Organ growth is controlled by both intrinsic genetic factors and external environmental signals. However, the molecular mechanisms that coordinate plant organ growth and nutrient supply remain largely unknown. We have previously reported that the B3 domain transcriptional repressor SOD7 (NGAL2) and its closest homologue DPA4 (NGAL3) act redundantly to limit organ and seed growth in Arabidopsis. Here we report that SOD7 represses the interaction between the transcriptional coactivator GRF-INTERACTING FACTOR 1 (GIF1) and growth-regulating factors (GRFs) by competitively interacting with GIF1, thereby limiting organ and seed growth. We further reveal that GIF1 physically interacts with FER-LIKE IRON DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT), which acts as a central regulator of iron uptake and homeostasis. SOD7 can competitively repress the interaction of GIF1 with FIT to influence iron uptake and responses. The sod7-2 dpa4-3 mutant enhances the expression of genes involved in iron uptake and displays high iron accumulation. Genetic analyses support that GIF1 functions downstream of SOD7 to regulate organ and seed growth as well as iron uptake and responses. Thus, our findings define a previously unrecognized mechanism that the SOD7/DPA4-GIF1 module coordinates organ growth and iron uptake by targeting key regulators of growth and iron uptake.

Geographic-genomic and geographic-phenotypic differentiation of the Aquilegia viridiflora complex

How species diverge into different lineages is a central issue in evolutionary biology. Despite the increasing evidence indicating that such divergences do not need geographic isolation, the correlation between lineage divergence and the adaptive ecological divergence of phenotype corresponding to distribution is still unknown. In addition, gene flow has been widely detected during and through such diverging processes. We used one widely distributed Aquilegia viridiflora complex as a model system to examine genomic differentiation and corresponding phenotypic variations along geographic gradients. Our phenotypic analyses of 20 populations from northwest to northeast China identified two phenotypic groups along the geographic cline. All examined traits are distinct from each other, although a few intermediate individuals occur in their contacting regions. We further sequenced the genomes of representative individuals of each population. However, four distinct genetic lineages were detected based on nuclear genomes. In particular, we recovered numerous genetic hybrids in the contact regions of four lineages. Gene flow is widespread and continuous between four lineages but much higher between contacting lineages than geographically isolated lineages. Gene flow and natural selection might result in inconsistency between heredity and phenotype. Moreover, many genes with fast lineage-specific mutations were identified to be involved in local adaptation. Our results suggest that both geographic isolation and local selection exerted by the environment and pollinators may together create geographic distributions of phenotypic variations as well as the underlying genomic divergences in numerous lineages.

Enhancing alfalfa resistance to Spodoptera herbivory by sequestering microRNA396 expression

KEY MESSAGE

Sequestering microRNA396 by overexpression of MIM396 enhanced alfalfa resistance to Spodoptera litura larvae, which may be due to increased lignin content and enhanced low-molecular weight flavonoids and glucosinolates biosynthesis. Alfalfa (Medicago sativa), the most important leguminous forage crop, suffers from the outbreak of defoliator insects, especially Spodoptera litura, resulting in heavy losses in yield and forage quality. Here, we found that the expression of alfalfa microRNA396 (miR396) precursor genes and mature miR396 was significantly up-regulated in wounding treatment that simulates feeding injury by defoliator insects. To verify the function of miR396 in alfalfa resistance to insect, we generated MIM396 transgenic alfalfa plants with significantly down-regulated miR396 expression by Agrobacterium-mediated genetic transformation. The MIM396 transgenic alfalfa plants exhibited improved resistance to Spodoptera litura larvae with increased lignin content but decreased JA accumulation. Most of the miR396 putative target GRF genes were up-regulated in MIM396 transgenic lines, and responded to the wounding treatment. By RNA sequencing analysis, we found that the differentially expressed genes related to insect resistance between WT and MIM396 transgenic plants mainly clustered in biosynthesis pathways in lignin, flavonoids and glucosinolates. In addition to the phenotype of enhanced insect resistance, MIM396 transgenic plants also displayed reduced biomass yield and forage quality. Our results broaden the function of miR396 in alfalfa and provide genetic resources for studying alfalfa insect resistance.

Interplay among ZF-HD and GRF transcription factors during Arabidopsis leaf development

The growth-regulating factor (GRF) family of transcriptional factors are involved in the control of leaf size and senescence, inflorescence and root growth, grain size, and plant regeneration. However, there is limited information about the genes regulated by these transcriptional factors, which are in turn responsible for their functions. Using a meta-analysis approach, we identified genes encoding Arabidopsis (Arabidopsis thaliana) zinc-finger homeodomain (ZF-HD) transcriptional factors, as potential targets of the GRFs. We further showed that GRF3 binds to the promoter of one of the members of the ZF-HD family, HOMEOBOX PROTEIN 33 (HB33), and activates its transcription. Increased levels of HB33 led to different modifications in leaf cell number and size that were dependent on its expression levels. Furthermore, we found that expression of HB33 for an extended period during leaf development increased leaf longevity. To cope with the functional redundancy among ZF-HD family members, we generated a dominant repressor version of HB33, HB33-SRDX. Expression of HB33-SRDX from HB33 regulatory regions was seedling-lethal, revealing the importance of the ZF-HD family in plant development. Misexpression of HB33-SRDX in early leaf development caused a reduction in both cell size and number. Interestingly, the loss-of-function of HB33 in lines carrying a GRF3 allele insensitive to miR396 reverted the delay in leaf senescence characteristic of these plants. Our results revealed functions for ZF-HDs in leaf development and linked them to the GRF pathway.

An optimized pipeline for live imaging whole Arabidopsis leaves at cellular resolution

BACKGROUND

Live imaging is the gold standard for determining how cells give rise to organs. However, tracking many cells across whole organs over large developmental time windows is extremely challenging. In this work, we provide a comparably simple method for confocal live imaging entire Arabidopsis thaliana first leaves across early development. Our imaging method works for both wild-type leaves and the complex curved leaves of the jaw-1D mutant.

RESULTS

We find that dissecting the cotyledons, affixing a coverslip above the samples and mounting samples with perfluorodecalin yields optimal imaging series for robust cellular and organ level analysis. We provide details of our complementary image processing steps in MorphoGraphX software for segmenting, tracking lineages, and measuring a suite of cellular properties. We also provide MorphoGraphX image processing scripts we developed to automate analysis of segmented images and data presentation.

CONCLUSIONS

Our imaging techniques and processing steps combine into a robust imaging pipeline. With this pipeline we are able to examine important nuances in the cellular growth and differentiation of jaw-D versus WT leaves that have not been demonstrated before. Our pipeline is approachable and easy to use for leaf development live imaging.

Genome-wide analysis of growth-regulating factor genes in grape (Vitis vinifera L.): identification, characterization and their responsive expression to osmotic stress

Identification, characterization and osmotic stress responsive expression of growth-regulating factor genes in grape. The growth and fruit production of grape vine are severely affected by adverse environmental conditions. Growth-regulating factors (GRFs) play a vital role in the regulation of plant growth, reproduction and stress tolerance. However, their biological functions in fruit vine crops are still largely unknown. In the present study, a total number of nine VvGRFs were identified in the grape genome. Phylogenetic and collinear relationship analysis revealed that they formed seven subfamilies, and have gone through three segmental duplication events. All VvGRFs were predicted to be nucleic localized and contained both the conserved QLQ and WRC domains at their N-terminals, one of the typical structural features of GRF proteins. Quantitative real-time PCR analyses demonstrated that all VvGRFs, with a predominant expression of VvGRF7, were constitutively expressed in roots, leaves and stems of grape plants, and showed responsive expression to osmotic stress. Further growth phenotypic analysis demonstrated that ectopic expression of VvGRF7 promoted the growth and sensitivity of transgenic Arabidopsis plants to osmotic stress. Our findings provide important information for the future study of VvGRF gene functions, and potential gene resources for the genetic breeding of new fruit vine varieties with improved fruit yield and stress tolerance.

Genome-wide identification of GRF gene family and their contribution to abiotic stress response in pitaya (Hylocereus polyrhizus)

Growth-regulating factors (GRFs) are plant-specific transcription factors identified in many land plants. Recently, their indispensable roles in stress response are highlighted. In present work, 11 HpGRFs were cloned in pitaya. Segmental duplication is considered essential for the expansion of HpGRFs. A phylogenetic tree suggested that GRFs could be divided into eight categories, among which G-I was a Caryophyllales-specific one. The categorization was further evidenced by differences in the gene structure, collinearity, protein domain of HpGRFs. Five miR396 hairpins giving rise to two types of matured miR396s were identified in pitaya via sRNA-Seq in combination with bioinformatic analysis. Parallel analysis of RNA ends proved that HpGRFs except HpGRF5 were degraded by miR396-directed cleavages at the regions which code the conserved WRC motifs of HpGRFs. Multiple cis-regulatory elements were discovered in the promoters of HpGRFs. Among the elements, most are involved in stress and phytohormone response as well as plant growth, indicating a crosstalk between them. Expression analysis showed the responsive patterns of the miR396-GRF module under abiotic stresses. To conclude, our work systematically identified the miR396-targeted HpGRFs in pitaya and confirmed their involvement in stress response, providing novel insights into the comprehensive understanding of the stress resistance of pitaya.

Identification of growth regulators using cross-species network analysis in plants

With the need to increase plant productivity, one of the challenges plant scientists are facing is to identify genes that play a role in beneficial plant traits. Moreover, even when such genes are found, it is generally not trivial to transfer this knowledge about gene function across species to identify functional orthologs. Here, we focused on the leaf to study plant growth. First, we built leaf growth transcriptional networks in Arabidopsis (Arabidopsis thaliana), maize (Zea mays), and aspen (Populus tremula). Next, known growth regulators, here defined as genes that when mutated or ectopically expressed alter plant growth, together with cross-species conserved networks, were used as guides to predict novel Arabidopsis growth regulators. Using an in-depth literature screening, 34 out of 100 top predicted growth regulators were confirmed to affect leaf phenotype when mutated or overexpressed and thus represent novel potential growth regulators. Globally, these growth regulators were involved in cell cycle, plant defense responses, gibberellin, auxin, and brassinosteroid signaling. Phenotypic characterization of loss-of-function lines confirmed two predicted growth regulators to be involved in leaf growth (NPF6.4 and LATE MERISTEM IDENTITY2). In conclusion, the presented network approach offers an integrative cross-species strategy to identify genes involved in plant growth and development.

Genome-wide identification of growth-regulating factor transcription factor family related to leaf and stem development in alfalfa

Growth-regulating factors (GRFs) play crucial roles in plant growth and stress response. To date, there have been no reports of the analysis and identification of the GRF transcription factor family in alfalfa. In this study, we identified 27 GRF family members from alfalfa (Medicago sativa L.) "Xinjiang Daye", and analyzed their physicochemical properties. Based on phylogenetic analysis, these MsGRFs were divided into five subgroups, each with a similar gene structure and conserved motifs. MsGRFs genes are distributed on 23 chromosomes, and all contain QLQ and WRC conserved domains. The results of the collinearity analysis showed that all MsGRFs are involved in gene duplication, including multiple whole-genome duplication or segmental duplication and a set of tandem duplication, indicating that large-scale duplication is important for the expansion of the GRF family in alfalfa. Several hormone-related and stress-related cis-acting elements have been found in the promoter regions of MsGRFs. Some MsGRFs were highly expressed in young leaves and stems, and their expression decreased during development. In addition, the leaf size of different varieties was found to vary, and MsGRF1 to 4, MsGRF18 to 20, and MsGRF22 to 23 were differentially expressed in large and small leaf alfalfa varieties, suggesting that they are critical in the regulation of leaf size. The results of this study can benefit further exploration of the regulatory functions of MsGRFs in growth and development, and can identify candidate genes that control leaf size development.

Spatial control of cell division by GA-OsGRF7/8 module in a leaf explaining the leaf length variation between cultivated and wild rice

Cellular and genetic understanding of the rice leaf size regulation is limited, despite rice being the staple food of more than half of the global population. We investigated the mechanism controlling the rice leaf length using cultivated and wild rice accessions that remarkably differed for leaf size. Comparative transcriptomics, gibberellic acid (GA) quantification and leaf kinematics of the contrasting accessions suggested the involvement of GA, cell cycle and growth-regulating factors (GRFs) in the rice leaf size regulation. Zone-specific expression analysis and VIGS established the functions of specific GRFs in the process. The leaf length of the selected accessions was strongly correlated with GA levels. Higher GA content in wild rice accessions with longer leaves and GA-induced increase in the leaf length via an increase in cell division confirmed a GA-mediated regulation of division zone in rice. Downstream to GA, OsGRF7 and OsGRF8 function for controlling cell division to determine the rice leaf length. Spatial control of cell division to determine the division zone size mediated by GA and downstream OsGRF7 and OsGRF8 explains the leaf length differences between the cultivated and wild rice. This mechanism to control the rice leaf length might have contributed to optimizing leaf size during domestication.

Integration of genomics, transcriptomics and metabolomics identifies candidate loci underlying fruit weight in loquat

Fruit weight is an integral part of fruit-quality traits and directly influences commodity values and economic returns of fruit crops. Despite its importance, the molecular mechanisms underlying fruit weight remain understudied, especially for perennial fruit tree crops such as cultivated loquat (Eriobotrya japonica Lindl.). Auxin is known to regulate fruit development, whereas its role and metabolism in fruit development remain obscure in loquat. In this study, we applied a multi-omics approach, integrating whole-genome resequencing-based quantitative trait locus (QTL) mapping with an F1 population, population genomics analysis using germplasm accessions, transcriptome analysis, and metabolic profiling to identify the genomic regions potentially associated with fruit weight in loquat. We identified three major loci associated with fruit weight, supported by both QTL mapping and comparative genomic analysis between small- and big-fruited loquat cultivars. Comparison between two genotypes with contrasting fruit weight performance through transcriptomic and metabolic profiling revealed an important role of auxin in regulating fruit development, especially at the fruit enlarging stage. The multi-omics approach identified two homologs of ETHYLENE INSENSITIVE 4 (EjEIN4) and TORNADO 1 (EjTRN1) as promising candidates controlling fruit weight. Moreover, three single nucleotide polymorphism (SNP) markers were closely associated with fruit weight. Results from this study provided insights from multiple perspectives into the genetic and metabolic controls of fruit weight in loquat. The candidate genomic regions, genes, and sequence variants will facilitate understanding the molecular basis of fruit weight and lay a foundation for future breeding and manipulation of fruit weight in loquat.

PIF7 controls leaf cell proliferation through an AN3 substitution repression mechanism

Phytochrome photoreceptors can markedly alter leaf blade growth in response to far-red (FR) rich neighbor shade, yet we have a limited understanding of how this is accomplished. This study identifies ANGUSTIFOLIA3 (AN3) as a central component in phytochrome promotion of leaf cell proliferation and PHYTOCHROME-INTERACTING FACTOR 7 (PIF7) as a potent repressor. AN3 and PIF7 impose opposing regulation on a shared suite of genes through common cis-acting promoter elements. In response to FR light, activated PIF7 blocks AN3 action by evicting and substituting for AN3 at target promoters. This molecular switch module provides a mechanism through which changes in external light quality can dynamically manipulate gene expression, cell division, and leaf size.

Plants are agile, plastic organisms able to adapt to everchanging circumstances. Responding to far-red (FR) wavelengths from nearby vegetation, shade-intolerant species elicit the adaptive shade-avoidance syndrome (SAS), characterized by elongated petioles, leaf hyponasty, and smaller leaves. We utilized end-of-day FR (EODFR) treatments to interrogate molecular processes that underlie the SAS leaf response. Genetic analysis established that PHYTOCHROME-INTERACTING FACTOR 7 (PIF7) is required for EODFR-mediated constraint of leaf blade cell division, while EODFR messenger RNA sequencing data identified ANGUSTIFOLIA3 (AN3) as a potential PIF7 target. We show that PIF7 can suppress AN3 transcription by directly interacting with and sequestering AN3. We also establish that PIF7 and AN3 impose antagonistic control of gene expression via common cis-acting promoter motifs in several cell-cycle regulator genes. EODFR triggers the molecular substitution of AN3 to PIF7 at G-box/PBE-box promoter regions and a switch from promotion to repression of gene expression.

Molecular Manipulation of the miR396 and miR399 Expression Modules Alters the Response of Arabidopsis thaliana to Phosphate Stress

In plant cells, the molecular and metabolic processes of nucleic acid synthesis, phospholipid production, coenzyme activation and the generation of the vast amount of chemical energy required to drive these processes relies on an adequate supply of the essential macronutrient, phosphorous (P). The requirement of an appropriate level of P in plant cells is evidenced by the intricately linked molecular mechanisms of P sensing, signaling and transport. One such mechanism is the posttranscriptional regulation of the P response pathway by the highly conserved plant microRNA (miRNA), miR399. In addition to miR399, numerous other plant miRNAs are also required to respond to environmental stress, including miR396. Here, we exposed Arabidopsis thaliana (Arabidopsis) transformant lines which harbor molecular modifications to the miR396 and miR399 expression modules to phosphate (PO₄) starvation. We show that molecular alteration of either miR396 or miR399 abundance afforded the Arabidopsis transformant lines different degrees of tolerance to PO₄ starvation. Furthermore, RT-qPCR assessment of PO₄-starved miR396 and miR399 transformants revealed that the tolerance displayed by these plant lines to this form of abiotic stress most likely stemmed from the altered expression of the target genes of these two miRNAs. Therefore, this study forms an early step towards the future development of molecularly modified plant lines which possess a degree of tolerance to growth in a PO₄ deficient environment.

Leaf Size Development Differences and Comparative Trancriptome Analyses of Two Poplar Genotypes

The plant leaf, the main organ of photosynthesis, is an important regulator of growth. To explore the difference between leaf size of Populusdeltoides ‘Danhong’ (Pd) and Populus simonii ‘Tongliao1’ (Ps), we investigated the leaf length, leaf width, leaf thickness, leaf area, leaf mass per area (LMA), and cell size of leaves from two genotypes and profiled the transcriptome-wide gene expression patterns through RNA sequencing. Our results show that the leaf area of Pd was significantly larger than that of Ps, but the epidermal cell area was significantly smaller than that of Ps. The difference of leaf size was caused by cell numbers. Transcriptome analysis also revealed that genes related to chromosome replication and DNA repair were highly expressed in Pd, while genes such as the EXPANSIN (EXPA) family which promoted cell expansion were highly expressed in Ps. Further, we revealed that the growth-regulating factors (GRFs) played a key role in the difference of leaf size between two genotypes through regulation of cell proliferation. These data provide a valuable resource for understanding the leaf development of the Populus genus.

Small RNA profiling for identification of microRNAs involved in regulation of seed development and lipid biosynthesis in yellowhorn

BACKGROUND

Yellowhorn (Xanthoceras sorbifolium), an endemic woody oil-bearing tree, has become economically important and is widely cultivated in northern China for bioactive oil production. However, the regulatory mechanisms of seed development and lipid biosynthesis affecting oil production in yellowhorn are still elusive. MicroRNAs (miRNAs) play crucial roles in diverse aspects of biological and metabolic processes in seeds, especially in seed development and lipid metabolism. It is still unknown how the miRNAs regulate the seed development and lipid biosynthesis in yellowhorn.

RESULTS

Here, based on investigations of differences in the seed growth tendency and embryo oil content between high-oil-content and low-oil-content lines, we constructed small RNA libraries from yellowhorn embryos at four seed development stages of the two lines and then profiled small RNA expression using high-throughput sequencing. A total of 249 known miRNAs from 46 families and 88 novel miRNAs were identified. Furthermore, by pairwise comparisons among the four seed development stages in each line, we found that 64 miRNAs (53 known and 11 novel miRNAs) were differentially expressed in the two lines. Across the two lines, 15, 11, 10, and 7 differentially expressed miRNAs were detected at 40, 54, 68, and 81 days after anthesis, respectively. Bioinformatic analysis was used to predict a total of 2654 target genes for 141 differentially expressed miRNAs (120 known and 21 novel miRNAs). Most of these genes were involved in the fatty acid biosynthetic process, regulation of transcription, nucleus, and response to auxin. Using quantitative real-time PCR and an integrated analysis of miRNA and mRNA expression, miRNA-target regulatory modules that may be involved in yellowhorn seed size, weight, and lipid biosynthesis were identified, such as miR172b-ARF2 (auxin response factor 2), miR7760-p3_1-AGL61 (AGAMOUS-LIKE 61), miR319p_1-FAD2-2 (omega-6 fatty acid desaturase 2-2), miR5647-p3_1-DGAT1 (diacylglycerol acyltransferase 1), and miR7760-p5_1-MED15A (Mediator subunit 15a).

CONCLUSIONS

This study provides new insights into the important regulatory roles of miRNAs in the seed development and lipid biosynthesis in yellowhorn. Our results will be valuable for dissecting the post-transcriptional and transcriptional regulation of seed development and lipid biosynthesis, as well as improving yellowhorn in northern China.

Uncovering Transcriptional Responses to Fractional Gravity in Arabidopsis Roots

Although many reports characterize the transcriptional response of Arabidopsis seedlings to microgravity, few investigate the effect of partial or fractional gravity on gene expression. Understanding plant responses to fractional gravity is relevant for plant growth on lunar and Martian surfaces. The plant signaling flight experiment utilized the European Modular Cultivation System (EMCS) onboard the International Space Station (ISS). The EMCS consisted of two rotors within a controlled chamber allowing for two experimental conditions, microgravity (stationary rotor) and simulated gravity in space. Seedlings were grown for 5 days under continuous light in seed cassettes. The arrangement of the seed cassettes within each experimental container results in a gradient of fractional g (in the spinning rotor). To investigate whether gene expression patterns are sensitive to fractional g, we carried out transcriptional profiling of root samples exposed to microgravity or partial g (ranging from 0.53 to 0.88 g). Data were analyzed using DESeq2 with fractional g as a continuous variable in the design model in order to query gene expression across the gravity continuum. We identified a subset of genes whose expression correlates with changes in fractional g. Interestingly, the most responsive genes include those encoding transcription factors, defense, and cell wall-related proteins and heat shock proteins.

OsGRF6 interacts with SLR1 to regulate OsGA2ox1 expression for coordinating chilling tolerance and growth in rice

Low temperature is one of the abiotic stressors that affect growth and productivity of rice. The plant hormone gibberellin not only regulates growth and development but is also involved in stress defense. Our rice seedling experiments demonstrated that overexpression of SLR1, a gene that encodes the rice DELLA protein, enhanced chilling tolerance. In contrast, overexpression of the active GA synthesis gene OsGA20ox1 reduced chilling tolerance, indicating that weakening GA signaling promoted plant defense against cold stress. CoIP-MS and BiFC assays showed that SLR1 physically interacted with OsGRF6. After cold treatment and recovery, the survival rates of OsGRF6-overexpression lines and an osgrf6 mutant and its complementary lines indicated that OsGRF6 is a negative regulator of chilling tolerance in rice. The yeast one-hybrid, qRT-PCR, and transactivation assays showed that both SLR1 and OsGRF6 can bind to the promoter of the active GA catabolic gene OsGA2ox1, where SLR1 promoted and OsGRF6 suppressed OsGA2ox1 expression. At normal temperature, OsGRF6 was responsible for maintaining active GA levels by inhibiting OsGA2ox1. When rice seedlings were subjected to chilling stress, the repressive effect of OsGRF6 on OsGA2ox1 was released by cold-induced SLR1, which activated OsGA2ox1 expression to decrease the active GA levels, enhancing chilling tolerance. These results suggest that OsGRF6 is an important regulator in the balance between growth and chilling tolerance in rice.

ARF2 represses expression of plant GRF transcription factors in a complementary mechanism to microRNA miR396

Members of the GROWTH REGULATING FACTOR (GRF) family of transcription factors play key roles in the promotion of plant growth and development. Many GRFs are post-transcriptionally repressed by microRNA (miRNA) miR396, an evolutionarily conserved small RNA, which restricts their expression to proliferative tissue. We performed a comprehensive analysis of the GRF family in eudicot plants and found that in many species all the GRFs have a miR396-binding site. Yet, we also identified GRFs with mutations in the sequence recognized by miR396, suggesting a partial or complete release of their post-transcriptional repression. Interestingly, Brassicaceae species share a group of GRFs that lack miR396 regulation, including Arabidopsis GRF5 and GRF6. We show that instead of miR396-mediated post-transcriptional regulation, the spatiotemporal control of GRF5 is achieved through evolutionarily conserved promoter sequences, and that AUXIN RESPONSE FACTOR 2 (ARF2) binds to such conserved sequences to repress GRF5 expression. Furthermore, we demonstrate that the unchecked expression of GRF5 in arf2 mutants is responsible for the increased cell number of arf2 leaves. The results describe a switch in the repression mechanisms that control the expression of GRFs and mechanistically link the control of leaf growth by miR396, GRFs, and ARF2 transcription factors.

The Molecular Basis of Kale Domestication: Transcriptional Profiling of Developing Leaves Provides New Insights Into the Evolution of a Brassica oleracea Vegetative Morphotype

Morphotypes of Brassica oleracea are the result of a dynamic interaction between genes that regulate the transition between vegetative and reproductive stages and those that regulate leaf morphology and plant architecture. In kales, ornate leaves, extended vegetative phase, and nutritional quality are some of the characters potentially selected by humans during domestication. We used a combination of developmental studies and transcriptomics to understand the vegetative domestication syndrome of kale. To identify candidate genes that are responsible for the evolution of domestic kale, we searched for transcriptome-wide differences among three vegetative B. oleracea morphotypes. RNA-seq experiments were used to understand the global pattern of expressed genes during a mixture of stages at one time in kale, cabbage, and the rapid cycling kale line TO1000. We identified gene expression patterns that differ among morphotypes and estimate the contribution of morphotype-specific gene expression that sets kale apart (3958 differentially expressed genes). Differentially expressed genes that regulate the vegetative to reproductive transition were abundant in all morphotypes. Genes involved in leaf morphology, plant architecture, defense, and nutrition were differentially expressed in kale. This allowed us to identify a set of candidate genes we suggest may be important in the kale domestication syndrome. Understanding candidate genes responsible for kale domestication is of importance to ultimately improve Cole crop production.

Accumulation of the Auxin Precursor Indole-3-Acetamide Curtails Growth through the Repression of Ribosome-Biogenesis and Development-Related Transcriptional Networks

The major auxin, indole-3-acetic acid (IAA), is associated with a plethora of growth and developmental processes including embryo development, expansion growth, cambial activity, and the induction of lateral root growth. Accumulation of the auxin precursor indole-3-acetamide (IAM) induces stress related processes by stimulating abscisic acid (ABA) biosynthesis. How IAM signaling is controlled is, at present, unclear. Here, we characterize the ami1rooty double mutant, that we initially generated to study the metabolic and phenotypic consequences of a simultaneous genetic blockade of the indole glucosinolate and IAM pathways in Arabidopsisthaliana. Our mass spectrometric analyses of the mutant revealed that the combination of the two mutations is not sufficient to fully prevent the conversion of IAM to IAA. The detected strong accumulation of IAM was, however, recognized to substantially impair seed development. We further show by genome-wide expression studies that the double mutant is broadly affected in its translational capacity, and that a small number of plant growth regulating transcriptional circuits are repressed by the high IAM content in the seed. In accordance with the previously described growth reduction in response to elevated IAM levels, our data support the hypothesis that IAM is a growth repressing counterpart to IAA.

MicroRNA Techniques: Valuable Tools for Agronomic Trait Analyses and Breeding in Rice

MicroRNAs (miRNAs) are a class of small noncoding RNAs that regulate gene expression at the post-transcriptional level. Extensive studies have revealed that miRNAs have critical functions in plant growth, development, and stress responses and may provide valuable genetic resources for plant breeding research. We herein reviewed the development, mechanisms, and characteristics of miRNA techniques while highlighting widely used approaches, namely, the short tandem target mimic (STTM) approach. We described STTM-based advances in plant science, especially in the model crop rice, and introduced the CRISPR-based transgene-free crop breeding. Finally, we discussed the challenges and unique opportunities related to combining STTM and CRISPR technology for crop improvement and agriculture.

The Rice miR396-GRF-GIF-SWI/SNF Module: A Player in GA Signaling

Rice Growth-Regulating Factors (GRFs) were originally identified to be gibberellin (GA)-induced, but the nature of GA induction has remained unknown because most reports thereafter focused on revealing their roles in growth-promoting activities. GRFs have the WRC (Trp, Arg, Cys) domain to target DNA and contain the QLQ (Gln, Leu, Gln) domain to interact with GRF-Interacting Factor (GIF), which recruits ATP-dependent DNA translocase Switch/Sucrose Non-fermenting (SWI/SNF) for chromatin remodeling. Both GRFs and GIFs exhibit transcriptional activities but GIFs lack a DNA-binding domain. So, GRFs act like a navigator in the GRF-GIF-SWI/SNF complex, determining when and where the complex should work on. The levels of most rice GRFs can be sensitively regulated by miR396, which responds to many developmental and environmental factors. Recent clues from several studies highlight the original question of how GRFs participate in GA signaling. DELLA (contain DELLA motif) protein plays dual roles in controlling the level of GRFs by regulating the level of miR396 and interacting with GRFs. Here we address the question of why this complex plays an essential role in controlling plant growth focusing on the action of GA signaling pivot, DELLA.

A Multi-OMICs Approach Sheds Light on the Higher Yield Phenotype and Enhanced Abiotic Stress Tolerance in Tobacco Lines Expressing the Carrot lycopene β-cyclase1 Gene

Recently, we published a set of tobacco lines expressing the Daucus carota (carrot) DcLCYB1 gene with accelerated development, increased carotenoid content, photosynthetic efficiency, and yield. Because of this development, DcLCYB1 expression might be of general interest in crop species as a strategy to accelerate development and increase biomass production under field conditions. However, to follow this path, a better understanding of the molecular basis of this phenotype is essential. Here, we combine OMICs (RNAseq, proteomics, and metabolomics) approaches to advance our understanding of the broader effect of LCYB expression on the tobacco transcriptome and metabolism. Upon DcLCYB1 expression, the tobacco transcriptome (~2,000 genes), proteome (~700 proteins), and metabolome (26 metabolites) showed a high number of changes in the genes involved in metabolic processes related to cell wall, lipids, glycolysis, and secondary metabolism. Gene and protein networks revealed clusters of interacting genes and proteins mainly involved in ribosome and RNA metabolism and translation. In addition, abiotic stress-related genes and proteins were mainly upregulated in the transgenic lines. This was well in line with an enhanced stress (high light, salt, and H₂O₂) tolerance response in all the transgenic lines compared with the wild type. Altogether, our results show an extended and coordinated response beyond the chloroplast (nucleus and cytosol) at the transcriptome, proteome, and metabolome levels, supporting enhanced plant growth under normal and stress conditions. This final evidence completes the set of benefits conferred by the expression of the DcLCYB1 gene, making it a very promising bioengineering tool to generate super crops.

Altering Plant Architecture to Improve Performance and Resistance

High-stress resistance and yield are major goals in crop cultivation, which can be addressed by modifying plant architecture. Significant progress has been made in recent years to understand how plant architecture is controlled under various growth conditions, recognizing the central role phytohormones play in response to environmental stresses. miRNAs, transcription factors, and other associated proteins regulate plant architecture, mainly via the modulation of hormone homeostasis and signaling. To generate crop plants of ideal architecture, we propose simultaneous editing of multiple genes involved in the regulatory networks associated with plant architecture as a feasible strategy. This strategy can help to address the need to increase grain yield and/or stress resistance under the pressures of the ever-increasing world population and climate change.

Differential Expression of Maize and Teosinte microRNAs under Submergence, Drought, and Alternated Stress

Submergence and drought stresses are the main constraints to crop production worldwide. MicroRNAs (miRNAs) are known to play a major role in plant response to various stresses. In this study, we analyzed the expression of maize and teosinte miRNAs by high-throughput sequencing of small RNA libraries in maize and its ancestor teosinte (Zea mays ssp. parviglumis), under submergence, drought, and alternated stress. We found that the expression patterns of 67 miRNA sequences representing 23 miRNA families in maize and other plants were regulated by submergence or drought. miR159a, miR166b, miR167c, and miR169c were downregulated by submergence in both plants but more severely in maize. miR156k and miR164e were upregulated by drought in teosinte but downregulated in maize. Small RNA profiling of teosinte subject to alternate treatments with drought and submergence revealed that submergence as the first stress attenuated the response to drought, while drought being the first stress did not alter the response to submergence. The miRNAs identified herein, and their potential targets, indicate that control of development, growth, and response to oxidative stress could be crucial for adaptation and that there exists evolutionary divergence between these two subspecies in miRNA response to abiotic stresses.

GROWTH-REGULATING FACTORS Interact with DELLAs and Regulate Growth in Cold Stress

DELLA proteins are repressors of the gibberellin (GA) hormone signaling pathway that act mainly by regulating transcription factor activities in plants. GAs induce DELLA repressor protein degradation and thereby control a number of critical developmental processes as well as responses to stresses such as cold. The strong effect of cold temperatures on many physiological processes has rendered it difficult to assess, based on phenotypic criteria, the role of GA and DELLAs in plant growth during cold stress. Here, we uncover substantial differences in the GA transcriptomes between plants grown at ambient temperature (21°C) and plants exposed to cold stress (4°C) in Arabidopsis (Arabidopsis thaliana). We further identify over 250, to the largest extent previously unknown, DELLA-transcription factor interactions using the yeast two-hybrid system. By integrating both data sets, we reveal that most members of the nine-member GRF (GROWTH REGULATORY FACTOR) transcription factor family are DELLA interactors and, at the same time, that several GRF genes are targets of DELLA-modulated transcription after exposure to cold stress. We find that plants with altered GRF dosage are differentially sensitive to the manipulation of GA and hence DELLA levels, also after cold stress, and identify a subset of cold stress-responsive genes that qualify as targets of this DELLA-GRF regulatory module.

A role for Arabidopsis growth-regulating factors 1 and 3 in growth-stress antagonism

Growth-regulating factors (GRFs) belong to a small family of transcription factors that are highly conserved in plants. GRFs regulate many developmental processes and plant responses to biotic and abiotic stimuli. Despite the importance of GRFs, a detailed mechanistic understanding of their regulatory functions is still lacking. In this study, we used ChIP sequencing (ChIP-seq) to identify genome-wide binding sites of Arabidopsis GRF1 and GRF3, and correspondingly their direct downstream target genes. RNA-sequencing (RNA-seq) analysis revealed that GRF1 and GRF3 regulate the expression of a significant number of the identified direct targets. The target genes unveiled broad regulatory functions of GRF1 and GRF3 in plant growth and development, phytohormone biosynthesis and signaling, and the cell cycle. Our analyses also revealed that clock core genes and genes with stress- and defense-related functions are most predominant among the GRF1- and GRF3-bound targets, providing insights into a possible role for these transcription factors in mediating growth-defense antagonism and integrating environmental stimuli into developmental programs. Additionally, GRF1 and GRF3 target molecular nodes of growth-defense antagonism and modulate the levels of defense- and development-related hormones in opposite directions. Taken together, our results point to GRF1 and GRF3 as potential key determinants of plant fitness under stress conditions.

MicroRNA miR396, GRF transcription factors and GIF co-regulators: a conserved plant growth regulatory module with potential for breeding and biotechnology

Multicellular life relies on complex regulatory mechanisms ensuring proper growth and development. In plants, these mechanisms construct a body plan that is both reproducible, and highly flexible for adaptation to different environmental conditions. A crucial regulatory module - consisting of microRNA miR396, GROWTH REGULATING FACTORS (GRFs) and GRF-INTERACTING FACTORS (GIFs) - has been shown to control growth of multiple tissues and organs in a variety of species. Especially in the last few years, research has expanded our knowledge of miR396-GRF/GIF function to crops, where it affects agronomically important traits, and highlighted its role in coordinating growth with endogenous and environmental factors. Special properties make the miR396-GRF/GIF system highly efficient in growth regulation and a promising target for improving plant yield.

Biological roles and an evolutionary sketch of the GRF-GIF transcriptional complex in plants

GROWTH-REGULATING FACTORs (GRFs) are sequence-specific DNA-binding transcription factors that regulate various aspects of plant growth and development. GRF proteins interact with a transcription cofactor, GRF-INTERACTING FACTOR (GIF), to form a functional transcriptional complex. For its activities, the GRF-GIF duo requires the SWITCH2/SUCROSE NONFERMENTING2 chromatin remodeling complex. One of the most conspicuous roles of the duo is conferring the meristematic potential on the proliferative and formative cells during organogenesis. GRF expression is post-transcriptionally down-regulated by microRNA396 (miR396), thus constructing the GRF-GIF-miR396 module and fine-tuning the duo’s action. Since the last comprehensive review articles were published over three years ago, many studies have added further insight into its action and elucidated new biological roles. The current review highlights recent advances in our understanding of how the GRF-GIF-miR396 module regulates plant growth and development. In addition, I revise the previous view on the evolutionary origin of the GRF gene family.

Conserved miR396b-GRF Regulation Is Involved in Abiotic Stress Responses in Pitaya (Hylocereus polyrhizus)

MicroRNA396 (miR396) is a conserved microRNA family that targets growth-regulating factors (GRFs), which play significant roles in plant growth and stress responses. Available evidence justifies the idea that miR396-targeted GRFs have important functions in many plant species; however, no genome-wide analysis of the pitaya (Hylocereus polyrhizus) miR396 gene has yet been reported. Further, its biological functions remain elusive. To uncover the regulatory roles of miR396 and its targets, the hairpin sequence of pitaya miR396b and the open reading frame (ORF) of its target, HpGRF6, were isolated from pitaya. Phylogenetic analysis showed that the precursor miR396b (MIR396b) gene of plants might be clustered into three major groups, and, generally, a more recent evolutionary relationship in the intra-family has been demonstrated. The sequence analysis indicated that the binding site of hpo-miR396b in HpGRF6 is located at the conserved motif which codes the conserved "RSRKPVE" amino acid in the Trp-Arg-Cys (WRC) region. In addition, degradome sequencing analysis confirmed that four GRFs (GRF1, c56908.graph_c0; GRF4, c52862.graph_c0; GRF6, c39378.graph_c0 and GRF9, c54658.graph_c0) are hpo-miR396b targets that are regulated by specific cleavage at the binding site between the 10th and 11th nucleotides from the 5' terminus of hpo-miR396b. Furthermore, quantitative real-time polymerase chain reaction (qRT-PCR) analysis showed that hpo-miR396b is down-regulated when confronted with drought stress (15% polyethylene glycol, PEG), and its expression fluctuates under other abiotic stresses, i.e., low temperature (4 ± 1 °C), high temperature (42 ± 1 °C), NaCl (100 mM), and abscisic acid (ABA; 0.38 mM). Conversely, the expression of HpGRF6 showed the opposite trend to exposure to these abiotic stresses. Taken together, hpo-miR396b plays a regulatory role in the control of HpGRF6, which might influence the abiotic stress response of pitaya. This is the first documentation of this role in pitaya and improves the understanding of the molecular mechanisms underlying the tolerance to drought stress in this fruit.