Aintegumenta and Aintegumenta-Like6 regulate auxin-mediated flower development in Arabidopsis

<p>Abstract</p> <p>Background</p> <p>Two related genes encoding AP2/ERF-type transcription factors, <it>AINTEGUMENTA </it>(<it>ANT</it>) and <it>AINTEGUMENTA-LIKE6 </it>(<it>AIL6</it>), are important regulators of floral growth and patterning in Arabidopsis. Evidence suggests that these genes promote several aspects of flower development in response to auxin. To investigate the interplay of <it>ANT</it>, <it>AIL6 </it>and auxin during floral development, I have examined the phenotypic consequences of disrupting polar auxin transport in <it>ant</it>, <it>ail6 </it>and <it>ant ail6 </it>mutants by either genetic or chemical means.</p> <p>Results</p> <p>Plants containing mutations in <it>ANT </it>or <it>AIL6 </it>alone or in both genes together exhibit increased sensitivity to disruptions in polar auxin transport. Both genes promote shoot growth, floral meristem initiation and floral meristem patterning in combination with auxin transport. However, differences in the responses of <it>ant </it>and <it>ail6 </it>single mutants to perturbations in auxin transport suggest that these two genes also have non-overlapping activities in each of these developmental processes.</p> <p>Conclusions</p> <p>The enhanced sensitivity of <it>ant </it>and <it>ail6 </it>mutants to alterations in polar auxin transport suggests that these mutants have defects in some aspect of auxin physiology. The inability of <it>ant ail6 </it>double mutants to initiate flowers in backgrounds disrupted for auxin transport confirm the proposed roles for these two genes in floral meristem initiation.</p

Auxin Response Factor2 (ARF2) and Its Regulated Homeodomain Gene HB33 Mediate Abscisic Acid Response in Arabidopsis

The phytohormone abscisic acid (ABA) is an important regulator of plant development and response to environmental stresses. In this study, we identified two ABA overly sensitive mutant alleles in a gene encoding Auxin Response Factor2 (ARF2). The expression of ARF2 was induced by ABA treatment. The arf2 mutants showed enhanced ABA sensitivity in seed germination and primary root growth. In contrast, the primary root growth and seed germination of transgenic plants over-expressing ARF2 are less inhibited by ABA than that of the wild type. ARF2 negatively regulates the expression of a homeodomain gene HB33, the expression of which is reduced by ABA. Transgenic plants over-expressing HB33 are more sensitive, while transgenic plants reducing HB33 by RNAi are more resistant to ABA in the seed germination and primary root growth than the wild type. ABA treatment altered auxin distribution in the primary root tips and made the relative, but not absolute, auxin accumulation or auxin signal around quiescent centre cells and their surrounding columella stem cells to other cells stronger in arf2-101 than in the wild type. These results indicate that ARF2 and HB33 are novel regulators in the ABA signal pathway, which has crosstalk with auxin signal pathway in regulating plant growth

A Rho Scaffold Integrates the Secretory System with Feedback Mechanisms in Regulation of Auxin Distribution

In plants, auxin distribution and tissue patterning are coordinated via a feedback loop involving the auxin-regulated cell polarity factor ICR1 and the secretory machinery

Role of the Arabidopsis PIN6 auxin transporter in auxin homeostasis and auxin-mediated development

Plant-specific PIN-formed (PIN) efflux transporters for the plant hormone auxin are required for tissue-specific directional auxin transport and cellular auxin homeostasis. The Arabidopsis PIN protein family has been shown to play important roles in developmental processes such as embryogenesis, organogenesis, vascular tissue differentiation, root meristem patterning and tropic growth. Here we analyzed roles of the less characterised Arabidopsis PIN6 auxin transporter. PIN6 is auxin-inducible and is expressed during multiple auxin–regulated developmental processes. Loss of pin6 function interfered with primary root growth and lateral root development. Misexpression of PIN6 affected auxin transport and interfered with auxin homeostasis in other growth processes such as shoot apical dominance, lateral root primordia development, adventitious root formation, root hair outgrowth and root waving. These changes in auxin-regulated growth correlated with a reduction in total auxin transport as well as with an altered activity of DR5-GUS auxin response reporter. Overall, the data indicate that PIN6 regulates auxin homeostasis during plant development.Christopher I. Cazzonelli, Marleen Vanstraelen, Sibu Simon, Kuide Yin, Ashley Carron-Arthur, Nazia Nisar, Gauri Tarle, Abby J. Cuttriss¤, Iain R. Searle, Eva Benkova, Ulrike Mathesius, Josette Masle, Jiří Friml, Barry J. Pogso

Nitric oxide causes root apical meristem defects and growth inhibition while reducing PIN-FORMED 1 (PIN1)-dependent acropetal auxin transport

Nitric oxide (NO) is considered a key regulator of plant developmental processes and defense, although the mechanism and direct targets of NO action remain largely unknown. We used phenotypic, cellular, and genetic analyses in Arabidopsis thaliana to explore the role of NO in regulating primary root growth and auxin transport. Treatment with the NO donors S-nitroso-N-acetylpenicillamine, sodium nitroprusside, and S-nitrosoglutathione reduces cell division, affecting the distribution of mitotic cells and meristem size by reducing cell size and number compared with NO depletion by 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO). Interestingly, genetic backgrounds in which the endogenous NO levels are enhanced [chlorophyll a/b binding protein underexpressed 1/NO overproducer 1 (cue1/nox1) mirror this response, together with an increased cell differentiation phenotype. Because of the importance of auxin distribution in regulating primary root growth, we analyzed auxin-dependent response after altering NO levels. Both elevated NO supply and the NO-overproducing Arabidopsis mutant cue1/nox1 exhibit reduced expression of the auxin reporter markers DR5pro:GUS/GFP. These effects were accompanied by a reduction in auxin transport in primary roots. NO application and the cue1/nox1 mutation caused decreased PIN-FORMED 1 (PIN1)-GFP fluorescence in a proteasome-independent manner. Remarkably, the cue1/nox1-mutant root phenotypes resemble those of pin1 mutants. The use of both chemical treatments and mutants with altered NO levels demonstrates that high levels of NO reduce auxin transport and response by a PIN1-dependent mechanism, and root meristem activity is reduced concomitantly

Phosphorylation and activation of PINOID by the phospholipid signaling kinase 3-phosphoinositide-dependent protein kinase 1 (PDK1) in Arabidopsis

Activity of the serine-threonine protein kinase PINOID (PID) has been implicated in the asymmetrical localization of the membrane-associated PINFORMED (PIN) family of auxin transport facilitators. However, the means by which PID regulates PIN protein distribution is unknown. We have used recombinant PID protein to dissect the regulation of PID activity in vitro. We demonstrate that intramolecular PID autophosphorylation is required for the ability of PID to phosphorylate an exogenous substrate. PID-like mammalian AGC kinases act in a phosphorylation cascade initiated by the phospholipid-associated kinase, 3-phosphoinositide-dependent protein kinase 1 (PDK1), which binds to the C-terminal hydrophobic PDK1-interacting fragment (PIF) domain found in PDK1 substrates. We find that Arabidopsis PDK1 interacts with PID, and that transphosphorylation by PDK1 increases PID autophosphorylation. We show that a PID activation loop serine is required for PDK1-dependent PID phosphorylation. This activation is rapid and requires the PIF domain. Cell extracts from flowers and seedling shoots dramatically increase PID phosphorylation in a tissue-specific manner. A PID protein variant in which the PIF domain was mutated failed to be activated by the seedling shoot extracts. PID immunoprecipitated from Arabidopsis cells in which PDK1 expression was inhibited by RNAi showed a dramatic reduction in transphosphorylation of myelin basic protein substrate. These results indicate that AtPDK1 is a potent enhancer of PID activity and provide evidence that phospholipid signaling may play a role in the signaling processes controlling polar auxin transport