Functions and signaling mechanisms of a non-proteinogenic amino acid ACC: the case for a novel plant hormone

植物产生激素，如乙烯，共同控制植物生物学的所有方面。该项目提出了一个新的假设，即植物用于制造乙烯的小分子本身作为一种新型植物激素发挥作用，影响植物的生长和发育，并进一步研究了激素在植物进化过程中如何在分子水平上发挥作用。该项目有可能通过改变现有的教条，通过要求重新评估几十年的乙烯文献，并通过影响未来乙烯实验的方式来改变植物激素领域。从这个项目中获得的知识可以为未来的战略作出贡献，为社会的利益修改作物植物。该项目扩大了代表性不足的学生的参与，每年提供两到三个学生在主要研究员的实验室实习。实习涉及与霍华德大学（一所历史上的黑人学院或大学）和埃莉诺罗斯福高中（一所当地公立学校，其中70%以上的学生是代表性不足的少数民族）的伙伴关系。实习生执行项目的各个方面，同时接受科学方法、数据分析和交流方面的宝贵培训。该项目还为两名博士后科学家、两名研究生和两名马里兰州大学本科生提供培训和职业准备。为了进一步扩大参与，首席研究员共同教授霍华德大学本科生的夏季研究讲习班。乙烯生物合成途径中众所周知的乙烯前体是1-氨基环丙烷-1-羧酸（ACC），一种非蛋白质氨基酸。令人信服的新数据表明，ACC本身可能是一个重要的信号分子，在进化上早于高等陆地植物有效地将ACC转化为乙烯的能力。特别地，ACC抑制地钱中的细胞分化和生长。该项目通过分析ACC功能（包括ACC信号机制的研究）来验证ACC是一种新型植物激素的假设。深入了解ACC功能的ACC合成突变体和ACC反应地钱，并在进化相关的物种在植物谱系（衣藻reinhardtii，水绵，小立碗藓和拟南芥）的研究，从而解决ACC功能的保护，同时利用较低的基因拷贝数在基础植物。在拟南芥中，初步的研究结果表明，ACC可以诱导花粉管生长伴随着Ca 2+内流，可以刺激初生根的生长。ACC激活拟南芥根原生质体中的Ca 2+电流，并且值得注意的是，这种激活依赖于谷氨酸受体样（GLR）离子通道。该项目使用分子遗传学，膜片钳，离子特异性振动探针和Ca 2+的实时成像的组合来测试ACC是GLR配体的假设。遗传证据的基础上寻求glr突变体和ACC合成突变体的植物物种的范围内的表型比较。ACC激活GLR，导致调控花粉管生长和/或根生长中的Ca 2+信号传导，将是理解ACC信号传导机制的突破，以及GLR信号传导，并将代表配体操作的Ca 2+调节系统，目前在植物中不存在一致的系统。在地钱中发现的引人注目的ACC反应提供了ACC信号突变体的遗传筛选，然后基于T-DNA标记或全基因组测序进行基因克隆。

Plants produce hormones such as ethylene that collectively control essentially all aspects of plant biology. This project advances a new hypothesis that a small molecule, which plants use to make ethylene, functions itself as a novel plant hormone that impacts plant growth and development, and further investigates how the hormone functions at the molecular level throughout the evolutionary breadth of plants. The project has the potential to transform the plant hormone field by altering existing dogma, by requiring the reevaluation of several decades of ethylene literature and by impacting how future ethylene experiments are carried out. The knowledge gained from this project could contribute to future strategies for modifying crop plants for the benefit of society. The project broadens participation of underrepresented students by providing two to three student internships per year in the lab of the principal investigator. The internships involve partnerships with Howard University (an Historically Black College or University) and Eleanor Roosevelt High School (a local public school in which over 70% of the students are underrepresented minorities). The interns carry out aspects of the project while receiving valuable training in scientific methods, data analysis and communication. The project also provides training and career preparation for two postdoctoral scientists, two graduate students and two University of Maryland undergraduates. To further broaden participation, the principal investigator co-teaches summer research workshops for Howard University undergraduates. The well-known ethylene precursor in the ethylene biosynthesis pathway is 1-aminocyclopropane-1-carboxylic acid (ACC), a non-proteinogenic amino acid. Compelling new data suggest that ACC itself may be an important signaling molecule that evolutionarily predated the ability of higher land plants to efficiently convert ACC to ethylene. In particular, ACC inhibits cellular differentiation and growth in the liverwort Marchantia polymorpha. This project tests the hypothesis that ACC is a novel plant hormone through analyses of ACC function, including an investigation of ACC signaling mechanisms. Insight into ACC function is provided by studies of ACC synthesis mutants and ACC responses in Marchantia, and in evolutionarily relevant species in the plant lineage (Chlamydomonas reinhardtii, Spirogyra pratensis, Physcomitrella patens and Arabidopsis thaliana), thus addressing the conservation of ACC function, while taking advantage of lower gene copy number in basal plants. In Arabidopsis, preliminary findings indicate that ACC can induce pollen tube growth concomitant with Ca2+ influx and can stimulate primary root growth. ACC activates Ca2+ currents in Arabidopsis root protoplasts, and notably, this activation is dependent on glutamate receptor-like (GLR) ionotropic channels. The project tests the hypothesis that ACC is a GLR ligand using a combination of molecular genetics, patch-clamping, ion-specific vibrating probes and live imaging of Ca2+. Genetic evidence is sought based on phenotypic comparisons of glr mutants and ACC synthesis mutants in the range of plant species. The activation of GLRs by ACC, leading to the regulation of Ca2+ signaling in pollen tube growth and/or root growth, would be a breakthrough in understanding mechanisms of ACC signaling, as well as GLR signaling, and would represent a ligand-operated system of Ca2+ regulation for which no consensual system currently exists in plants. The striking ACC response uncovered in Marchantia provides for a genetic screen for ACC signaling mutants, followed by gene cloning based on T-DNA tagging or whole genome sequencing.