Polarly localized Bro1 domain proteins regulate PIN-FORMED abundance and root gravitropic growth in Arabidopsis.

The developmental plasticity of the root system plays an essential role in the adaptation of plants to the environment. Among many other signals, auxin and its directional, intercellular transport are critical in regulating root growth and development. In particular, the PIN-FORMED2 (PIN2) auxin exporter acts as a key regulator of root gravitropic growth. Multiple regulators have been reported to be involved in PIN2-mediated root growth; however, our information remains incomplete. Here, we identified ROWY Bro1-domain proteins as important regulators of PIN2 sorting control. Genetic analysis revealed that Arabidopsis rowy1 single mutants and higher-order rowy1 rowy2 rowy3 triple mutants presented a wavy root growth phenotype. Cell biological experiments revealed that ROWY1 and PIN2 colocalized to the apical side of the plasma membrane in the root epidermis and that ROWYs are required for correct PM targeting of PIN2. In addition, ROWYs also affected PIN3 protein abundance in the stele, suggesting the potential involvement of additional PIN transporters as well as other proteins. A global transcriptome analysis revealed that ROWY genes are involved in the Fe2+ availability perception pathway. This work establishes ROWYs as important novel regulators of root gravitropic growth by connecting micronutrient availability to the proper subcellular targeting of PIN auxin transporters.

Effects of LAZY family genes on shoot gravitropism in Lotus japonicus.

Plant architecture is an important agronomic trait to determine the biomass and sward structure of forage grass. The IGT family plays a pivotal role in plant gravitropism, encompassing both the gravitropic response and the modulation of plant architecture. We have previously shown that LjLAZY3, one of the IGT genes, plays a distinct role in root gravitropism in L. japonicus. However, the function of LAZY proteins on shoot gravitropism in this species is poorly understood. In this study, we identified nine IGT genes in the L. japonicus genome, which have been categorized into four clades based on the phylogenetic relationships of IGT proteins from 18 legumes: LAZY1, NGR (NEGATIVE GRAVITROPIC RESPONSE OF ROOTS), IGT-LIKE, and TAC1. We found that LAZY genes in the first three clades have demonstrated distinct role for modulating plant gravitropism in L. japonicus with specific impacts as follows. Mutation of the LAZY1 gene, LjLAZY1, defected the gravitropic response of hypocotyl without impacting the main stem's branch angle. In contrast, the overexpression of the NGR gene, LjLAZY3, substantially modulated the shoot's gravitropism, leading to narrower lateral branch angles. Additionally, it enhanced the shoots' gravitropic response. The overexpression of another NGR gene, LjLAZY4, specifically reduced the main stem's branch angle and decreased plant stature without affecting the shoot gravitropic response. The phenotype of IGT-LIKE gene LjLAZY2 overexpression is identical to that of LjLAZY4. While overexpression of the IGT-LIKE gene LjLAZY5 did not induce any observable changes in branch angle, plant height, or gravitropic response. Furthermore, the LjLAZYs were selectively interacted with different BRXL and RLD proteins, which should the important factor to determine their different functions in controlling organ architecture in L. japonicus. Our results deepen understanding of the LjLAZY family and its potential for plant architecture improvement in L. japonicus.

YABBY and diverged KNOX1 genes shape nodes and internodes in the stem.

Plant stems comprise nodes and internodes that specialize in solute exchange and elongation. However, their boundaries are not well defined, and how these basic units arise remains elusive. In rice with clear nodes and internodes, we found that one subclade of class I knotted1-like homeobox (KNOX1) genes for shoot meristem indeterminacy restricts node differentiation and allows internode formation by repressing YABBY genes for leaf development and genes from another node-specific KNOX1 subclade. YABBYs promote nodal vascular differentiation and limit stem elongation. YABBY and node-specific KNOX1 genes specify the pulvinus, which further elaborates the nodal structure for gravitropism. Notably, this KNOX1 subclade organization is specific to seed plants. We propose that nodes and internodes are distinct domains specified by YABBY-KNOX1 cross-regulation that diverged in early seed plants.

Molecular mechanism of brassinosteroids involved in root gravity response based on transcriptome analysis.

BACKGROUND: Brassinosteroids (BRs) are a class of phytohormones that regulate a wide range of developmental processes in plants. BR-associated mutants display impaired growth and response to developmental and environmental stimuli. RESULTS: Here, we found that a BR-deficient mutant det2-1 displayed abnormal root gravitropic growth in Arabidopsis, which was not present in other BR mutants. To further elucidate the role of DET2 in gravity, we performed transcriptome sequencing and analysis of det2-1 and bri1-116, bri1 null mutant allele. Expression levels of auxin, gibberellin, cytokinin, and other related genes in the two mutants of det2-1 and bri1-116 were basically the same. However, we only found that a large number of JAZ (JASMONATE ZIM-domain) genes and jasmonate synthesis-related genes were upregulated in det2-1 mutant, suggesting increased levels of endogenous JA. CONCLUSIONS: Our results also suggested that DET2 not only plays a role in BR synthesis but may also be involved in JA regulation. Our study provides a new insight into the molecular mechanism of BRs on the root gravitropism.

Transcription factor OsWRKY72 controls rice leaf angle by regulating LAZY1-mediated shoot gravitropism.

Leaf angle is a major trait of ideal architecture, which is considered to influence rice (Oryza sativa) cultivation and grain yield. Although a few mutants with altered rice leaf inclination angles have been reported, the underlying molecular mechanism remains unclear. In this study, we showed that a WRKY transcription factor gene, OsWRKY72, was highly expressed in the leaf sheath and lamina joint. Phenotypic analyses showed that oswrky72 mutants had smaller leaf angles than the wild type, while OsWRKY72 overexpression lines exhibited an increased leaf angle. This observation suggests that OsWRKY72 functions as a positive regulator, promoting the enlargement of the leaf angle. Our bioinformatics analysis identified LAZY1 as the downstream gene of OsWRKY72. Electrophoretic mobility shift assays and dual-luciferase analysis revealed that OsWRKY72 directly inhibited LAZY1 by binding to its promoter. Moreover, knocking out OsWRKY72 enhanced shoot gravitropism, which contrasted with the phenotype of lazy1 plants. These results imply that OsWRKY72 regulates the leaf angle through gravitropism by reducing the expression of LAZY1. In addition, OsWRKY72 could directly regulate the expression of other leaf angle-related genes such as FLOWERING LOCUS T-LIKE 12 (OsFTL12) and WALL-ASSOCIATED KINASE 11 (OsWAK11). Our study indicates that OsWRKY72 contributes positively to the expansion of the leaf angle by interfering with shoot gravitropism in rice.

Amyloplast is involved in the MIZ1-modulated root hydrotropism.

Roots exhibit hydrotropism in response to moisture gradients, with the hydrotropism-related gene Mizu-kussei1 (MIZ1) playing a role in regulating root hydrotropism in an oblique orientation. However, the mechanisms underlying MIZ1-regulated root hydrotropism are not well understood. In this study, we employed obliquely oriented experimental systems to investigate root hydrotropism in Arabidopsis. We found that the miz1 mutant displays reduced root hydrotropism but increased root gravitropism following hydrostimulation, as compared to wild-type plants. Conversely, overexpression of AtMIZ1 leads to enhanced root hydrotropism but decreased root gravitropism following hydrostimulation, as compared to wild-type plants. Using co-immunoprecipitation followed by mass spectrometry (IP-MS), we explored proteins that interact with AtMIZ1, and we identified PGMC1 co-immunoprecipitated with MIZ1 in vivo. Furthermore, the miz1 mutant exhibited higher expression of the PGMC1 gene and increased phosphoglucomutase (PGM) activity, while AtMIZ1 overexpressors resulted in lower expression of the PGMC1 gene, reduced amyloplast amount, and reduced PGM activity in comparison to wild-type roots. In addition, different Arabidopsis natural accessions having difference in their hydrotropic response demonstrated expression level of PGMC1 was negatively correlated with hydrotropic root curvature and AtMIZ1 expression. Our results provide valuable insights into the role of amyloplast in MIZ1-regulated root hydrotropism.

Defying gravity: WEEP promotes negative gravitropism in peach trees by establishing asymmetric auxin gradients.

Trees with weeping shoot architectures are valued for their beauty and are a resource for understanding how plants regulate posture control. The peach (Prunus persica) weeping phenotype, which has elliptical downward arching branches, is caused by a homozygous mutation in the WEEP gene. Little is known about the function of WEEP despite its high conservation throughout Plantae. Here, we present the results of anatomical, biochemical, biomechanical, physiological, and molecular experiments that provide insight into WEEP function. Our data suggest that weeping peach trees do not have defects in branch structure. Rather, transcriptomes from the adaxial (upper) and abaxial (lower) sides of standard and weeping branch shoot tips revealed flipped expression patterns for genes associated with early auxin response, tissue patterning, cell elongation, and tension wood development. This suggests that WEEP promotes polar auxin transport toward the lower side during shoot gravitropic response, leading to cell elongation and tension wood development. In addition, weeping peach trees exhibited steeper root systems and faster lateral root gravitropic response. This suggests that WEEP moderates root gravitropism and is essential to establishing the set-point angle of lateral roots from the gravity vector. Additionally, size exclusion chromatography indicated that WEEP proteins self-oligomerize, like other proteins with sterile alpha motif domains. Collectively, our results from weeping peach provide insight into polar auxin transport mechanisms associated with gravitropism and lateral shoot and root orientation.

Recovery of root hydrotropism in miz1 mutant by eliminating root gravitropism.

Mizu-kussei1 (MIZ1) plays a crucial role in root hydrotropism, but it is still unclear whether auxin-mediated gravitropism is involved in MIZ1-modulated root hydrotropism. This study aimed to investigate whether the hydrotropism of the Arabidopsis miz1 mutants could be restored through pharmacological inhibition of auxin transport or genetic modification in root gravitropism. Our findings indicate that the hydrotropic defects of miz1 mutant can be partly recovered by using an auxin transport inhibitor. Furthermore, miz1/pin2 double mutants exhibit more pronounced defects in root gravitropism compared to the wild type, while still displaying a normal hydrotropic response similar to the wild type. These results suggest that the elimination of gravitropism enables miz1 roots to become hydrotropically responsive to moisture gradients.

MIZ1 acts downstream of PGM1 in regulating root hydrotropism.

The MIZ1 play an important role in root hydrotropism. However, the relationship between MIZ1-regulated hydrotropism and amyloplast-mediated gravitropism remain largely unclear. Here, we generated the miz1/pgm1 double mutants by crossing the non-hydrotropic miz1 mutant with the amyloplast-defective pgm1 mutant, which lacks gravitropic response. Our results showed that the miz1/pgm1 mutants exhibited a significant reduction in amyloplast and gravitropic bending, while maintaining a similar ahydrotropic phenotype as the miz1 single mutant. These findings suggest that MIZ1 plays a role in hydrotropism downstream of PGM1. Understanding the mechanisms of interaction between hydrotropism and gravitropism is crucial for comprehending the rooting patterns of plants in natural conditions. The counteracting relationship between root hydrotropism and gravitropism in the miz1 mutant should receive attention in this field, particularly considering the interference from gravitropism on Earth.

A single amino acid substitution in MdLAZY1A dominantly impairs shoot gravitropism in Malus.

Plant architecture is 1 of the most important factors that determines crop yield potential and productivity. In apple (Malus domestica), genetic improvement of tree architecture has been challenging due to a long juvenile phase and growth as complex trees composed of a distinct scion and a rootstock. To better understand the genetic control of apple tree architecture, the dominant weeping growth phenotype was investigated. We report the identification of MdLAZY1A (MD13G1122400) as the genetic determinant underpinning the Weeping (W) locus that largely controls weeping growth in Malus. MdLAZY1A is 1 of the 4 paralogs in apple that are most closely related to AtLAZY1 involved in gravitropism in Arabidopsis (Arabidopsis thaliana). The weeping allele (MdLAZY1A-W) contains a single nucleotide mutation c.584T>C that leads to a leucine to proline (L195P) substitution within a predicted transmembrane domain that colocalizes with Region III, 1 of the 5 conserved regions in LAZY1-like proteins. Subcellular localization revealed that MdLAZY1A localizes to the plasma membrane and nucleus in plant cells. Overexpressing the weeping allele in apple cultivar Royal Gala (RG) with standard growth habit impaired its gravitropic response and altered the growth to weeping-like. Suppressing the standard allele (MdLAZY1A-S) by RNA interference (RNAi) in RG similarly changed the branch growth direction to downward. Overall, the L195P mutation in MdLAZY1A is genetically causal for weeping growth, underscoring not only the crucial roles of residue L195 and Region III in MdLAZY1A-mediated gravitropic response but also a potential DNA base editing target for tree architecture improvement in Malus and other crops.

Arabidopsis thaliana B-GATA factors repress starch synthesis and gravitropic growth responses.

Plants perceive the direction of gravity during skotomorphogenic growth, and of gravity and light during photomorphogenic growth. Gravity perception occurs through the sedimentation of starch granules in shoot endodermal and root columella cells. In this study, we demonstrate that the Arabidopsis thaliana GATA factors GNC (GATA, NITRATE-INDUCIBLE, CARBON METABOLISM-INVOLVED) and GNL/CGA1 (GNC-LIKE/CYTOKININ-RESPONSIVE GATA1) repress starch granule growth and amyloplast differentiation in endodermal cells. In our comprehensive study, we analysed gravitropic responses in the shoot, root and hypocotyl. We performed an RNA-seq analysis, used advanced microscopy techniques to examine starch granule size, number and morphology and quantified transitory starch degradation patterns. Using transmission electron microscopy, we examined amyloplast development. Our results indicate that the altered gravitropic responses in hypocotyls, shoots and roots of gnc gnl mutants and GNL overexpressors are due to the differential accumulation of starch granules observed in the GATA genotypes. At the whole-plant level, GNC and GNL play a more complex role in starch synthesis, degradation and starch granule initiation. Our findings suggest that the light-regulated GNC and GNL help balance phototropic and gravitropic growth responses after the transition from skotomorphogenesis to photomorphogenesis by repressing the growth of starch granules.

LAZY3 interacts with LAZY2 to regulate tiller angle by modulating shoot gravity perception in rice.

Starch biosynthesis in gravity-sensing tissues of rice shoot determines the magnitude of rice shoot gravitropism and thus tiller angle. However, the molecular mechanism underlying starch biosynthesis in rice gravity-sensing tissues is still unclear. We characterized a novel tiller angle gene LAZY3 (LA3) in rice through map-based cloning. Biochemical, molecular and genetic studies further demonstrated the essential roles of LA3 in gravity perception of rice shoot and tiller angle control. The shoot gravitropism and lateral auxin transport were defective in la3 mutant upon gravistimulation. We showed that LA3 encodes a chloroplast-localized tryptophan-rich protein associated with starch granules via Tryptophan-rich region (TRR) domain. Moreover, LA3 could interact with the starch biosynthesis regulator LA2, determining starch granule formation in shoot gravity-sensing tissues. LA3 and LA2 negatively regulate tiller angle in the same pathway acting upstream of LA1 to mediate asymmetric distribution of auxin. Our study defined LA3 as an indispensable factor of starch biosynthesis in rice gravity-sensing tissues that greatly broadens current understanding in the molecular mechanisms underlying the starch granule formation in gravity-sensing tissues, and provides new insights into the regulatory mechanism of shoot gravitropism and rice tiller angle.

SlGH3.15, a member of the GH3 gene family, regulates lateral root development and gravitropism response by modulating auxin homeostasis in tomato.

Multiple Gretchen Hagen 3 (GH3) genes have been implicated in a range of processes in plant growth and development through their roles in maintaining hormonal homeostasis. However, there has only been limited study on the functions of GH3 genes in tomato (Solanum lycopersicum). In this work, we investigated the important function of SlGH3.15, a member of the GH3 gene family in tomato. Overexpression of SlGH3.15 led to severe dwarfism in both the above- and below-ground sections of the plant, accompanied by a substantial decrease in free IAA content and reduction in the expression of SlGH3.9, a paralog of SlGH3.15. Exogenous supply of IAA negatively affected the elongation of the primary root and partially restored the gravitropism defects in SlGH3.15-overexpression lines. While no phenotypic change was observed in the SlGH3.15 RNAi lines, double knockout lines of SlGH3.15 and SlGH3.9 were less sensitive to treatments with the auxin polar transport inhibitor. Overall, these findings revealed important roles of SlGH3.15 in IAA homeostasis and as a negative regulator of free IAA accumulation and lateral root formation in tomato.

ENHANCED GRAVITROPISM 2 coordinates molecular adaptations to gravistimulation in the elongation zone of barley roots.

Root gravitropism includes gravity perception in the root cap, signal transduction between root cap and elongation zone, and curvature response in the elongation zone. The barley (Hordeum vulgare) mutant enhanced gravitropism 2 (egt2) displays a hypergravitropic root phenotype. We compared the transcriptomic reprogramming of the root cap, the meristem, and the elongation zone of wild-type (WT) and egt2 seminal roots upon gravistimulation in a time-course experiment and identified direct interaction partners of EGT2 by yeast-two-hybrid screening and bimolecular fluorescence complementation validation. We demonstrated that the elongation zone is subjected to most transcriptomic changes after gravistimulation. Here, 33% of graviregulated genes are also transcriptionally controlled by EGT2, suggesting a central role of this gene in controlling the molecular networks associated with gravitropic bending. Gene co-expression analyses suggested a role of EGT2 in cell wall and reactive oxygen species-related processes, in which direct interaction partners of EGT2 regulated by EGT2 and gravity might be involved. Taken together, this study demonstrated the central role of EGT2 and its interaction partners in the networks controlling root zone-specific transcriptomic reprogramming of barley roots upon gravistimulation. These findings can contribute to the development of novel root idiotypes leading to improved crop performance.

BRXL4-LAZY1 interaction at the plasma membrane controls Arabidopsis branch angle and gravitropism.

Gravitropism guides growth to shape plant architecture above and below ground. Mutations in LAZY1 impair stem gravitropism and cause less upright inflorescence branches (wider angles). The LAZY1 protein resides at the plasma membrane and in the nucleus. The plasma membrane pool is necessary and sufficient for setting branch angles. To investigate the molecular mechanism of LAZY1 function, we screened for LAZY1-interacting proteins in yeast. We identified BRXL4, a shoot-specific protein related to BREVIS RADIX. The BRXL4-LAZY1 interaction occurred at the plasma membrane in plant cells, and not detectably in the nucleus. Mutations in the C-terminus of LAZY1, but not other conserved regions, prevented the interaction. Opposite to lazy1, brxl4 mutants displayed faster gravitropism and more upright branches. Overexpressing BRXL4 produced strong lazy1 phenotypes. The apparent negative regulation of LAZY1 function is consistent with BRXL4 reducing LAZY1 expression or the amount of LAZY1 at the plasma membrane. Measurements indicated that both are true. LAZY1 mRNA was three-fold more abundant in brxl4 mutants and almost undetectable in BRXL4 overexpressors. Plasma membrane LAZY1 was higher and nuclear LAZY1 lower in brxl4 mutants compared with the wild type. To explain these results, we suggest that BRXL4 reduces the amount of LAZY1 at the plasma membrane where it functions in gravity signaling and promotes LAZY1 accumulation in the nucleus where it reduces LAZY1 expression, possibly by suppressing its own transcription. This explanation of how BRXL4 negatively regulates LAZY1 suggests ways to modify shoot system architecture for practical purposes.

Leveraging orthology within maize and Arabidopsis QTL to identify genes affecting natural variation in gravitropism.

Plants typically orient their organs with respect to the Earth's gravity field by a dynamic process called gravitropism. To discover conserved genetic elements affecting seedling root gravitropism, we measured the process in a set of Zea mays (maize) recombinant inbred lines with machine vision and compared the results with those obtained in a similar study of Arabidopsis thaliana. Each of the several quantitative trait loci that we mapped in both species spanned many hundreds of genes, too many to test individually for causality. We reasoned that orthologous genes may be responsible for natural variation in monocot and dicot root gravitropism. If so, pairs of orthologous genes affecting gravitropism may be present within the maize and Arabidopsis QTL intervals. A reciprocal comparison of sequences within the QTL intervals identified seven pairs of such one-to-one orthologs. Analysis of knockout mutants demonstrated a role in gravitropism for four of the seven: CCT2 functions in phosphatidylcholine biosynthesis, ATG5 functions in membrane remodeling during autophagy, UGP2 produces the substrate for cellulose and callose polymer extension, and FAMA is a transcription factor. Automated phenotyping enabled this discovery of four naturally varying components of a conserved process (gravitropism) by making it feasible to conduct the same large-scale experiment in two species.

Diversity of root hydrotropism among natural variants of Arabidopsis thaliana.

Root gravitropism affects root hydrotropism. The interference intensity of root gravitropism with root hydrotropism differs among plant species. However, these differences have not been well compared within a single plant species. In this study, we compared root hydrotropism in various natural variants of Arabidopsis under stationary conditions. As a result, we detected a range of root hydrotropism under stationary conditions among natural Arabidopsis variants. Comparison of root gravitropism and root hydrotropism among several Arabidopsis natural variants classified natural variants that decreased root hydrotropism into two types; namely one type that expresses root gravitropism and root hydrotropism weaker than Col-0, and the other type that expresses weaker root hydrotropism than Col-0 but expresses similar root gravitropism with Col-0. However, root hydrotropism of all examined Arabidopsis natural variants was facilitated by clinorotation. These results suggested that the interference of root gravitropism with root hydrotropism is conserved among Arabidopsis natural variants, although the intensity of root gravitropism interference with root hydrotropism differs.

Blocking Rice Shoot Gravitropism by Altering One Amino Acid in LAZY1.

Tiller angle is an important trait that determines plant architecture and yield in cereal crops. Tiller angle is partially controlled during gravistimulation by the dynamic re-allocation of LAZY1 (LA1) protein between the nucleus and plasma membrane, but the underlying mechanism remains unclear. In this study, we identified and characterized a new allele of LA1 based on analysis of a rice (Oryza sativa L.) spreading-tiller mutant la1G74V, which harbors a non-synonymous mutation in the predicted transmembrane (TM) domain-encoding region of this gene. The mutation causes complete loss of shoot gravitropism, leading to prostrate growth of plants. Our results showed that LA1 localizes not only to the nucleus and plasma membrane but also to the endoplasmic reticulum. Removal of the TM domain in LA1 showed spreading-tiller phenotype of plants similar to la1G74V but did not affect the plasma membrane localization; thus, making it distinct from its ortholog ZmLA1 in Zea mays. Therefore, we propose that the TM domain is indispensable for the biological function of LA1, but this domain does not determine the localization of the protein to the plasma membrane. Our study provides new insights into the LA1-mediated regulation of shoot gravitropism.

The unconventional prefoldin RPB5 interactor mediates the gravitropic response by modulating cytoskeleton organization and auxin transport in Arabidopsis.

Gravity-induced root curvature involves the asymmetric distribution of the phytohormone auxin. This response depends on the concerted activities of the auxin transporters such as PIN-FORMED (PIN) proteins for auxin efflux and AUXIN RESISTANT 1 (AUX1) for auxin influx. However, how the auxin gradient is established remains elusive. Here we identified a new mutant with a short root, strong auxin distribution in the lateral root cap and an impaired gravitropic response. The causal gene encoded an Arabidopsis homolog of the human unconventional prefoldin RPB5 interactor (URI). AtURI interacted with prefoldin 2 (PFD2) and PFD6, two ß-type PFD members that modulate actin and tubulin patterning in roots. The auxin reporter DR5rev :GFP showed that asymmetric auxin redistribution after gravistimulation is disordered in aturi-1 root tips. Treatment with the endomembrane protein trafficking inhibitor brefeldin A indicated that recycling of the auxin transporter PIN2 is disrupted in aturi-1 roots as well as in pfd mutants. We propose that AtURI cooperates with PFDs to recycle PIN2 and modulate auxin distribution.

AtHB40 modulates primary root length and gravitropism involving CYCLINB and auxin transporters.

Gravitropism is a finely regulated tropistic response based on the plant perception of directional cues. Such perception allows them to direct shoot growth upwards, above ground, and root growth downwards, into the soil, anchoring the plant to acquire water and nutrients. Gravity sensing occurs in specialized cells and depends on auxin distribution, regulated by influx/efflux carriers. Here we report that AtHB40, encoding a transcription factor of the homeodomain-leucine zipper I family, was expressed in the columella and the root tip. Athb40 mutants exhibited longer primary roots. Enhanced primary root elongation was in agreement with a higher number of cells in the transition zone and the induction of CYCLINB transcript levels. Moreover, athb40 mutants and AtHB40 overexpressors displayed enhanced and delayed gravitropistic responses, respectively. These phenotypes were associated with altered auxin distribution and deregulated expression of the auxin transporters LAX2, LAX3, and PIN2. Accordingly, lax2 and lax3 mutants also showed an altered gravitropistic response, and LAX3 was identified as a direct target of AtHB40. Furthermore, AtHB40 is induced by AtHB53 when the latter is upregulated by auxin. Altogether, these results indicate that AtHB40 modulates cell division and auxin distribution in the root tip thus altering primary root length and gravitropism.