Lisa McDonnell

Ethylene levels are regulated by a plant encoded 1-aminocyclopropane-1-carboxylic acid deaminase

Control of the levels of the plant hormone ethylene is crucial in the regulation of many developmental processes and stress responses. Ethylene production can be controlled by altering endogenous levels of 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor to ethylene or by altering its conversion to ethylene. ACC is known to be irreversibly broken down by bacterial or fungal ACC deaminases (ACDs). Sequence analysis revealed two putative ACD genes encoded for in the genome of Arabidopsis thaliana (A. thaliana) and we detected ACD activity in plant extracts. Expression of one of these A. thaliana genes (AtACD1) in bacteria indicated that it had ACD activity. Moreover, transgenic plants harboring antisense constructs of the gene decreased ACD activity to 70% of wild-type (WT) levels, displayed an increased sensitivity to ACC and produced significantly more ethylene. Taken together, these results show that AtACD1 can act as a regulator of ACC levels in A. thaliana.

Visualization of cellulose synthases in Arabidopsis secondary cell walls.

Secondary cell walls built with speed Plant cell walls provide the cellulose that is integral for wood, cotton fiber, and many biofuels. Cellulose is synthesized outside the cell membrane by cellulose synthase enzymes. Much of the secondary cell wall, responsible for the sturdiness of wood, is formed by xylem cells embedded in the core of the plant. Watanabe et al. leveraged ectopic expression to bring xylem-style cellulose synthase activity to the epidermal surface of the plant (see the Perspective by Schneider and Persson). Combining this improved accessibility with fluorescent tagging showed that secondary cell walls are built faster than primary cell walls, perhaps due to increased velocity and density of cellulose synthase complexes. Science, this issue p. 198, see also p. 156 Cellulose synthesis in secondary plant cell walls is focused in spiral wall thickenings during xylem cell development. [Also see Perspective by Schneider and Perssonn] Cellulose biosynthesis in plant secondary cell walls forms the basis of vascular development in land plants, with xylem tissues constituting the vast majority of terrestrial biomass. We used plant lines that contained an inducible master transcription factor controlling xylem cell fate to quantitatively image fluorescently tagged cellulose synthase enzymes during cellulose deposition in living protoxylem cells. The formation of secondary cell wall thickenings was associated with a redistribution and enrichment of CESA7-containing cellulose synthase complexes (CSCs) into narrow membrane domains. The velocities of secondary cell wall–specific CSCs were faster than those of primary cell wall CSCs during abundant cellulose production. Dynamic intracellular of endomembranes, in combination with increased velocity and high density of CSCs, enables cellulose to be synthesized rapidly in secondary cell walls.

Transgenic Populus Trees for Forest Products, Bioenergy, and Functional Genomics

Species within the genus Populus are among the fastest growing trees in regions with a temperate climate. Not only are they an integral component of ecosystems, but they are also grown commercially for fuel, fiber, and forest products in rural areas of the world. In the late 1970s, they were designated as a bioenergy crop by the U.S. Department of Energy, as a result of research following the oil embargo. Populus species also serve as model trees for plant molecular biology research. In this article, we will review recent progress in the genetic improvement of Populus, considering both classical breeding and genetic engineering for bioenergy, as well as in using transgenics to elucidate gene functionality. A perspective for future improvement of Populus via functional genomics will also be presented.

Plant encoded 1-aminocyclopropane-1-carboxylic acid deaminase activity implicated in different aspects of plant development

Proper plant development is dependent on the coordination and tight control of a wide variety of different signals. In the study of the plant hormone ethylene, control of the immediate biosynthetic precursor 1-aminocyclopropane-1-carboxylic acid (ACC) is of interest as the level of ethylene can either help or hinder plant growth during times of stress. It is known that ACC can be reversibly removed from the biosynthesis pathway through conjugation into other compounds. We recently reported that plants can also irreversibly remove ACC from ethylene production through the activity of a plant encoded ACC deaminase. Heretofore only found in bacteria, we showed that there was ACC deaminase activity in both Arabidopsis and in developing wood of poplar. Here we extend this original work and show that there is also ACC deaminase activity in tomato plants, and that this activity is regulated during tomato fruit development. Further, using an antisense construct of AtACD1 in Arabidopsis, we investigate the role of ACC deamination during salt stress. Together these studies shed light on a new level of control during ethylene production in a wide variety of plant species and during different plant developmental stages.

Concepts First, Jargon Second Improves Student Articulation of Understanding.

In this experiment, students in a large undergraduate biology course were first exposed to the concepts without new technical vocabulary (“jargon”) in a pre‐class reading assignment. Their learning of the concepts and jargon was compared with that of an equivalent group of students in another section of the same course, whose pre‐class reading presented both the jargon and concepts together in the traditional manner. Both groups had the same active‐learning classes with the same instructor, and then completed the same post‐test. Although the two groups performed the same on the multiple choice questions of the post‐test, the group exposed to concepts first and jargon second included 1.5 times and 2.5 times more correct arguments on two free‐response questions about the concepts. The correct use of jargon between the two groups was similar, with the exception of one jargon term that the control group used more often. These results suggest that modest instructional changes whereby new concepts are introduced in a concepts‐first, jargon‐second manner can increase student learning, as demonstrated by their ability to articulate their understanding of new concepts.

A Guide for Graduate Students Interested in Postdoctoral Positions in Biology Education Research

Intended as a resource for life sciences graduate students, this essay discusses the diversity of postdoctoral positions in biology education and the careers to which they lead. The authors also provide advice to help graduate students develop the skills necessary to obtain a biology education research postdoctoral position.