PIF4 enhances the expression of SAUR genes to promote growth in response to nitrate.

Nitrate supply is fundamental to support shoot growth and crop performance, but the associated increase in stem height exacerbates the risks of lodging and yield losses. Despite their significance for agriculture, the mechanisms involved in the promotion of stem growth by nitrate remain poorly understood. Here, we show that the elongation of the hypocotyl of Arabidopsis thaliana, used as a model, responds rapidly and persistently to upshifts in nitrate concentration, rather than to the nitrate level itself. The response occurred even in shoots dissected from their roots and required NITRATE TRANSPORTER 1.1 (NRT1.1) in the phosphorylated state (but not NRT1.1 nitrate transport capacity) and NIN-LIKE PROTEIN 7 (NLP7). Nitrate increased PHYTOCHROME INTERACTING FACTOR 4 (PIF4) nuclear abundance by posttranscriptional mechanisms that depended on NRT1.1 and phytochrome B. In response to nitrate, PIF4 enhanced the expression of numerous SMALL AUXIN-UP RNA (SAUR) genes in the hypocotyl. The growth response to nitrate required PIF4, positive and negative regulators of its activity, including AUXIN RESPONSE FACTORs, and SAURs. PIF4 integrates cues from the soil (nitrate) and aerial (shade) environments adjusting plant stature to facilitate access to light.

SAUR63 stimulates cell growth at the plasma membrane.

In plants, regulated cell expansion determines organ size and shape. Several members of the family of redundantly acting Small Auxin Up RNA (SAUR) proteins can stimulate plasma membrane (PM) H+-ATPase proton pumping activity by inhibiting PM-associated PP2C.D phosphatases, thereby increasing the PM electrochemical potential, acidifying the apoplast, and stimulating cell expansion. Similarly, Arabidopsis thaliana SAUR63 was able to increase growth of various organs, antagonize PP2C.D5 phosphatase, and increase H+-ATPase activity. Using a gain-of-function approach to bypass genetic redundancy, we dissected structural requirements for SAUR63 growth-promoting activity. The divergent N-terminal domain of SAUR63 has a predicted basic amphipathic α-helix and was able to drive partial PM association. Deletion of the N-terminal domain decreased PM association of a SAUR63 fusion protein, as well as decreasing protein level and eliminating growth-promoting activity. Conversely, forced PM association restored ability to promote H+-ATPase activity and cell expansion, indicating that SAUR63 is active when PM-associated. Lipid binding assays and perturbations of PM lipid composition indicate that the N-terminal domain can interact with PM anionic lipids. Mutations in the conserved SAUR domain also reduced PM association in root cells. Thus, both the N-terminal domain and the SAUR domain may cooperatively mediate the SAUR63 PM association required to promote growth.

SAUR proteins and PP2C.D phosphatases regulate H+-ATPases and K+ channels to control stomatal movements.

Activation of plasma membrane (PM) H+-ATPase activity is crucial in guard cells to promote light-stimulated stomatal opening, and in growing organs to promote cell expansion. In growing organs, SMALL AUXIN UP RNA (SAUR) proteins inhibit the PP2C.D2, PP2C.D5, and PP2C.D6 (PP2C.D2/5/6) phosphatases, thereby preventing dephosphorylation of the penultimate phosphothreonine of PM H+-ATPases and trapping them in the activated state to promote cell expansion. To elucidate whether SAUR-PP2C.D regulatory modules also affect reversible cell expansion, we examined stomatal apertures and conductances of Arabidopsis thaliana plants with altered SAUR or PP2C.D activity. Here, we report that the pp2c.d2/5/6 triple knockout mutant plants and plant lines overexpressing SAUR fusion proteins exhibit enhanced stomatal apertures and conductances. Reciprocally, saur56 saur60 double mutants, lacking two SAUR genes normally expressed in guard cells, displayed reduced apertures and conductances, as did plants overexpressing PP2C.D5. Although altered PM H+-ATPase activity contributes to these stomatal phenotypes, voltage clamp analysis showed significant changes also in K+ channel gating in lines with altered SAUR and PP2C.D function. Together, our findings demonstrate that SAUR and PP2C.D proteins act antagonistically to facilitate stomatal movements through a concerted targeting of both ATP-dependent H+ pumping and channel-mediated K+ transport.

miR167 limits anther growth to potentiate anther dehiscence.

In flowering plants, anther dehiscence and pollen release are essential for sexual reproduction. Anthers dehisce after cell wall degradation weakens stomium cell junctions in each anther locule, and desiccation creates mechanical forces that open the locules. Either effect or both together may break stomium cell junctions. The microRNA miR167 negatively regulates ARF6 and ARF8, which encode auxin response transcription factors. Arabidopsis mARF6 or mARF8 plants with mutated miR167 target sites have defective anther dehiscence and ovule development. Null mir167a mutations recapitulated mARF6 and mARF8 anther and ovule phenotypes, indicating that MIR167a is the main miR167 precursor gene that delimits ARF6 and ARF8 expression in these organs. Anthers of mir167a or mARF6/8 plants overexpressed genes encoding cell wall loosening functions associated with cell expansion, and grew larger than wild-type anthers did starting at flower stage 11. Experimental desiccation enabled dehiscence of miR167-deficient anthers, indicating competence to dehisce. Conversely, high humidity conditions delayed anther dehiscence in wild-type flowers. These results support a model in which miR167-mediated anther growth arrest permits anther dehiscence. Without miR167 regulation, excess anther growth delays dehiscence by prolonging desiccation.

Arabidopsis SMALL AUXIN UP RNA63 promotes hypocotyl and stamen filament elongation.

Auxin regulates plant growth and development in part by activating gene expression. Arabidopsis thaliana SMALL AUXIN UP RNAs (SAURs) are a family of early auxin-responsive genes with unknown functionality. Here, we show that transgenic plant lines expressing artificial microRNA constructs (aMIR-SAUR-A or -B) that target a SAUR subfamily (SAUR61-SAUR68 and SAUR75) had slightly reduced hypocotyl and stamen filament elongation. In contrast, transgenic plants expressing SAUR63:GFP or SAUR63:GUS fusions had long hypocotyls, petals and stamen filaments, suggesting that these protein fusions caused a gain of function. SAUR63:GFP and SAUR63:GUS seedlings also accumulated a higher level of basipetally transported auxin in the hypocotyl than did wild-type seedlings, and had wavy hypocotyls and twisted inflorescence stems. Mutations in auxin efflux carriers could partially suppress some SAUR63:GUS phenotypes. In contrast, SAUR63:HA plants had wild-type elongation and auxin transport. SAUR63:GFP protein had a longer half-life than SAUR63:HA. Fluorescence imaging and microsomal fractionation studies revealed that SAUR63:GFP was localized mainly in the plasma membrane, whereas SAUR63:HA was present in both soluble and membrane fractions. Low light conditions increased SAUR63:HA protein turnover rate. These results indicate that membrane-associated Arabidopsis SAUR63 promotes auxin-stimulated organ elongation.

A regulatory network for coordinated flower maturation.

For self-pollinating plants to reproduce, male and female organ development must be coordinated as flowers mature. The Arabidopsis transcription factors AUXIN RESPONSE FACTOR 6 (ARF6) and ARF8 regulate this complex process by promoting petal expansion, stamen filament elongation, anther dehiscence, and gynoecium maturation, thereby ensuring that pollen released from the anthers is deposited on the stigma of a receptive gynoecium. ARF6 and ARF8 induce jasmonate production, which in turn triggers expression of MYB21 and MYB24, encoding R2R3 MYB transcription factors that promote petal and stamen growth. To understand the dynamics of this flower maturation regulatory network, we have characterized morphological, chemical, and global gene expression phenotypes of arf, myb, and jasmonate pathway mutant flowers. We found that MYB21 and MYB24 promoted not only petal and stamen development but also gynoecium growth. As well as regulating reproductive competence, both the ARF and MYB factors promoted nectary development or function and volatile sesquiterpene production, which may attract insect pollinators and/or repel pathogens. Mutants lacking jasmonate synthesis or response had decreased MYB21 expression and stamen and petal growth at the stage when flowers normally open, but had increased MYB21 expression in petals of older flowers, resulting in renewed and persistent petal expansion at later stages. Both auxin response and jasmonate synthesis promoted positive feedbacks that may ensure rapid petal and stamen growth as flowers open. MYB21 also fed back negatively on expression of jasmonate biosynthesis pathway genes to decrease flower jasmonate level, which correlated with termination of growth after flowers have opened. These dynamic feedbacks may promote timely, coordinated, and transient growth of flower organs.

A gain-of-function mutation in IAA18 alters Arabidopsis embryonic apical patterning.

Lateral organ emergence in plant embryos and meristems depends on spatially coordinated auxin transport and auxin response. Here, we report the gain-of-function iaa18-1 mutation in Arabidopsis, which stabilizes the Aux/IAA protein IAA18 and causes aberrant cotyledon placement in embryos. IAA18 was expressed in the apical domain of globular stage embryos, and in the shoot apical meristem and adaxial domain of cotyledons of heart stage embryos. Mutant globular embryos had asymmetric PIN1:GFP expression in the apical domain, indicating that IAA18-1 disrupts auxin transport. Genetic interactions among iaa18-1, loss-of-function mutations in ARF (Auxin response factor) genes and ARF-overexpressing constructs suggest that IAA18-1 inhibits activity of MP/ARF5 and other ARF proteins in the apical domain. The iaa18-1 mutation also increased the frequency of rootless seedlings in mutant backgrounds in which auxin regulation of basal pole development was affected. These results indicate that apical patterning requires Aux/IAA protein turnover, and that apical domain auxin response also influences root formation.

AUXIN RESPONSE FACTOR1 and AUXIN RESPONSE FACTOR2 regulate senescence and floral organ abscission in Arabidopsis thaliana.

In plants, both endogenous mechanisms and environmental signals regulate developmental transitions such as seed germination, induction of flowering, leaf senescence and shedding of senescent organs. Auxin response factors (ARFs) are transcription factors that mediate responses to the plant hormone auxin. We have examined Arabidopsis lines carrying T-DNA insertions in AUXIN RESPONSE FACTOR1 (ARF1) and ARF2 genes. We found that ARF2 promotes transitions between multiple stages of Arabidopsis development. arf2 mutant plants exhibited delays in several processes related to plant aging, including initiation of flowering, rosette leaf senescence, floral organ abscission and silique ripening. ARF2 expression was induced in senescing leaves. ARF2 regulated leaf senescence and floral organ abscission independently of the ethylene and cytokinin response pathways. arf1 mutations enhanced many arf2 phenotypes, indicating that ARF1 acts in a partially redundant manner with ARF2. However, unlike arf2 mutations, an arf1 mutation increased transcription of Aux/IAA genes in Arabidopsis flowers, supporting previous biochemical studies that indicated that ARF1 is a transcriptional repressor. Two other ARF genes, NPH4/ARF7 and ARF19, were also induced by senescence, and mutations in these genes enhanced arf2 phenotypes. NPH4/ARF7 and ARF19 function as transcriptional activators, suggesting that auxin may control senescence in part by activating gene expression.

Auxin response factors ARF6 and ARF8 promote jasmonic acid production and flower maturation.

Pollination in flowering plants requires that anthers release pollen when the gynoecium is competent to support fertilization. We show that in Arabidopsis thaliana, two paralogous auxin response transcription factors, ARF6 and ARF8, regulate both stamen and gynoecium maturation. arf6 arf8 double-null mutant flowers arrested as infertile closed buds with short petals, short stamen filaments, undehisced anthers that did not release pollen and immature gynoecia. Numerous developmentally regulated genes failed to be induced. ARF6 and ARF8 thus coordinate the transition from immature to mature fertile flowers. Jasmonic acid (JA) measurements and JA feeding experiments showed that decreased jasmonate production caused the block in pollen release, but not the gynoecium arrest. The double mutant had altered auxin responsive gene expression. However, whole flower auxin levels did not change during flower maturation, suggesting that auxin might regulate flower maturation only under specific environmental conditions, or in localized organs or tissues of flowers. arf6 and arf8 single mutants and sesquimutants (homozygous for one mutation and heterozygous for the other) had delayed stamen development and decreased fecundity, indicating that ARF6 and ARF8 gene dosage affects timing of flower maturation quantitatively.

Regulation of Arabidopsis SHY2/IAA3 protein turnover.

Auxin/indole acetic acid (Aux/IAA) proteins regulate transcriptional responses to the plant hormone auxin. Gain-of-function mutations in the Arabidopsis SHORT HYPOCOTYL 2 (SHY2/IAA3) gene encoding an Aux/IAA protein increase steady-state levels of SHY2/IAA3 protein and decrease auxin responses, indicating that SHY2/IAA3 negatively regulates auxin signaling. These shy2 mutations also cause ectopic light responses, suggesting that SHY2/IAA3 may promote light signaling. Auxin regulates turnover of the related Auxin-resistant (AXR)2/IAA7 and AXR3/IAA17 proteins by increasing their interaction with the Skp1-Cdc53/cullin-F-box (SCFTIR1) E3 ubiquitin ligase complex. To investigate whether SHY2/IAA3 is regulated similarly, we have used a turnover assay to reveal that axr1 and transport inhibitor resistant (tir)1 mutations affecting SCFTIR1 decrease SHY2/IAA3 turnover. In pull-down assays, SHY2/IAA3 protein interacted with TIR1, the F-box component of SCFTIR1 and with the photoreceptor phytochrome B. Auxin stimulated SHY2/IAA3 interaction with TIR1, whereas the shy2-2 gain-of-function mutation decreased this interaction. Light did not affect the interaction, suggesting that light regulates some other aspect of Aux/IAA gene or protein function. The chemical juglone (5-hydroxy-1,4-naphthoquinone) inhibited the interaction, suggesting that peptidyl-prolyl isomerization may mediate auxin-induced SHY2/IAA3 protein turnover.

A mutation in the Arabidopsis KT2/KUP2 potassium transporter gene affects shoot cell expansion.

Potassium ions (K(+)) are the most abundant cations in plants and are necessary for cell growth. Arabidopsis shy3-1 mutant plants have a short hypocotyl, small leaves, and a short flowering stem, and these defects result from decreased cell expansion. The semidominant shy3-1 mutation changes an amino acid in KT2/KUP2, a K(+) transporter related to the Escherichia coli Kup protein. Second mutations in the KT2/KUP2/SHY3 gene, including presumed null mutations, suppress the shy3-1 phenotypes. Plants with these intragenic suppressor mutations appear similar to wild-type plants, suggesting that KT2/KUP2/SHY3 acts redundantly with other genes. Expression of the shy3-1 mutant version of KT2/KUP2/SHY3 in wild-type plants confers shy3-1-like phenotypes, indicating that shy3-1 probably either causes a gain of function or creates an interfering protein. The shy3-1 mutation does not eliminate the ability of the KT2/KUP2 cDNA to rescue the growth of a potassium transport-deficient E. coli mutant. A P(SHY3)::GUS fusion is expressed in growing portions of the plant. These results suggest that KT2/KUP2/SHY3 mediates K(+)-dependent cell expansion in growing tissues.