TWISTED DWARF 1 Associates with BRASSINOSTEROID-INSENSITIVE 1 to Regulate Early Events of the Brassinosteroid Signaling Pathway

A genome-wide screen for mutants showing altered brassinosteroid （BR） sensitivity or bril-like phenotypes resulted in the identification of two new mutant alleles of TWISTED DWARF 1 （TWD1）, twd1-4, and twdl-5. Pre- vious studies indicated that TWD1, also named as ULTRACURVATA 2 or FKBP42, associates with auxin efflux transporters and is essential for their biological functions. Although earlier reports showed that BR signaling is downregulated in twdl, how TWD1 is integrated in BR signaling has not been elucidated. Here, we provide ge- netic and biochemical evidence demonstrating that TWD1 interacts with the BR receptor BRI1 in vivo in a BR- independent manner. Further analyses indicated that TWD1 modulates the BR signal transduction not by altering ER quality control or protein abundance of BRI1; instead, TWD1 appears to be critical in BR- induced interaction of BRI1 and its co-receptor BAK1, as well as BR-induced phosphorylation of these two proteins. These results provide better understanding of the early events of the BR signaling pathway.

Phototropism in land plants： Molecules and mechanism from light perception to response

BACKGROUND： Phototropism is the response a plant exhibits when it is faced with a directional blue light stimulus. Though a seemingly simple differential cell elongation response within a responding tissue that results in organ curvature, phototropism is regulated through a complex set of signal perception and transduction events that move from the plasma membrane to the nucleus. In nature phototropism is one of several plant responses that have evolved to optimize photosynthesis and growth. OBJECTIVE： In the present work we will review the state of the field with respect to the molecules and mechanisms associated with phototropism in land plants. METHODS： A systematic literature search was done to identify relevant advances in the field. Though we tried to focus on literature within the past decade （1998-present）, we have discussed and cited older literature where appropriate because of context or its significant impact to the present field. Several previous review articles are also cited where appropriate and readers should seek those out. RESULTS： A total of 199 articles are cited that fulfill the criteria listed above. CONCLUSIONS： Though important numerous and significant advances have been made in our understanding of the molecular, biochemical, cell biological and physiologic mechanisms underlying phototropism in land plants over the past decade, there are many remaining unanswered questions. The future is indeed bright for researchers in the field and we look forward to the next decade worth of discoveries.

Disruptions in AUXl-Dependent Auxin Influx Alter Hypocotyl Phototropism in Arabidopsis

Phototropism represents a differential growth response by which plant organs can respond adaptively to changes in the direction of incident light to optimize leaf/stem positioning for photosynthetic light capture and root growth orientation for water/nutrient acquisition. Studies over the past few years have identified a number of compo- nents in the signaling pathway（s） leading to development of phototropic curvatures in hypocotyls. These include the pho- totropin photoreceptors （photl and phot2） that perceive directional blue-light （BL） cues and then stimulate signaling, leading to relocalization of the plant hormone auxin, as well as the auxin response factor NPH4/ARF7 that responds to changes in local auxin concentrations to directly mediate expression of genes likely encoding proteins necessary for development of phototropic curvatures, While null mutations in NPH4/ARF7 condition an aphototropic response to unidirectional BL, seedlings carrying the same mutations recover BL-dependent phototropic responsiveness if coirradiated with red light （RL） or pre-treated with either ethylene. In the present study, we identify second-site enhancer mutations in the nph4 background that abrogate these recovery responses. One of these mutations-mapl （modifier of arf7 phenotypes 1）-was found to represent a missense allele of AUX1-a gene encoding a high-affinity auxin influx carrier previously associated with a number of root responses. Pharmocological studies and analyses of additional auxl mutants confirmed that AUX1 functions as a modulator of hypocotyl phototropism. Moreover, we have found that the strength of dependence of hypocotyl phototropism on AUX1-mediated auxin influx is directly related to the auxin responsiveness of the seedling in question.