Starch graunules from transgenic wheat endosperm (left). Starch graunles from tomato pericarp at 14 days post-anthesis (right).
How the starch granule is assembled to form a semicrystalline supramolecule from glucose residues is enigmatic and fascinating. Plants are the only higher life forms capable of making starch and they provide almost half of all of the calories we eat. In addition, starch is extensively used as a biopolymer in various food and non-food applications. Currently, the market for biodegradable materials is broadening because it has social and political appeal.
Many aspects of starch granule synthesis are unknown but a complex interplay of the activity of starch biosynthetic enzyme isoforms that form functional complexes and, the chemical and physical conditions in the cell, likely plays key roles. We have a strong interest in modulating the fine structure of starch and in investigating its functional properties. We also believe that studying the physico-chemical structure of the granule during synthesis and deconstruction may help us to better understand metabolism.
Tomato plants used for eco-physiological measurements (Photo by Kietsuda Luengwilai)
Consumers value the sugars accumulated in fruit. Elevating tomato fruit sugars without compromising yields and fitness, however, has been a longstanding problem for breeders. To compound matters, fruit are often bred and then subsequently harvested and handled for postharvest longevity, which often decimates sugar content.
Our goal is to understand the cumulative factors (genetic, biochemical and physiological) that determine sugar accumulation in the fruit at market. In our view, this necessitates:
Fig 1. The Sugar Starvation Response in Plants (Dong & Beckles, unpublished)
Environmental stress reduces plant productivity, and poses a growing threat to global food security. Under momentary or sustained stress, leaf carbohydrates may become exhausted. This extreme carbon status is detected; activating signal transduction cascades that alter gene expression and enzyme activity. Biomolecules normally used for critical cellular functions are gradually degraded, replacing the depleted carbohydrates to permit immediate survival. This series of events is called the sugar starvation response (SSR) (Fig 1).
We have a strong interest in understanding how SS changes the way plants use carbon. Specifically, how this limited carbon store is partitioned into various metabolic pathways within a tissue, and how it is allocated from sources to sinks. A detailed investigation of carbon partitioning and allocation in response to SS will broaden our understanding of the events that enable plants to cope with episodic stresses, which could then lead to improved plant stress tolerance.
Fig. 2. Cherry tomato fruit harvested at breaker stage under chilling (2.5 and 5°C) and control (12.5°C) conditions for different lengths of storage. PCI compromises fruit’s ability to develop full color even after rewarming. (Albornoz & Beckles, unpublished data).
Tomato fruit stored at 0-12°C may experience postharvest chilling injury (PCI). This is a complicated disorder that negatively affects fruit quality. Symptoms are usually detected when the fruit is brought back to room temperature, and vary from a lack of flavor and poor texture, to ripening abnormalities (Fig 1), surface pitting, tissue browning and fungal infestation. This leads to consumer dissatisfaction and increased postharvest losses.
Chilling causes dysfunction of the fruit cells. Membrane destabilization occurs, which is accelerated by the production of harmful, cold-induced molecules including reactive oxygen species. Over time, the stressed tissue loses its cellular compartmentation, and a series of downstream metabolic and physiological impairments occur, damaging the fruit and compromising its quality and shelf-life.
We are interested in understanding the early molecular events that lead to PCI in tomato fruit, as there are still gaps in our understanding of the signal transduction pathways. This could help to develop effective solutions to this disorder, which currently limits options for postharvest storage of tomato and other tropical and subtropical fruit.
Aegilops crassa (goat grass) is a wild relative of wheat. For more information, see Uhlmann, N.K. & Beckles, D.M. (2010)
The starch and protein accumulated in wheat endosperm provide 20% of all calories eaten and are an important food source. High seed starch was a prime target of the selection that defined domestication 10,000 years ago and this has led to a smaller proportion of protein-to-starch in modern high yielding seeds.
There have been several attempts to rationally modify wheat grain starch and protein contents; however, it has proved very difficult to accomplish this because the synthesis of these compounds is interrelated with antagonistic effects. Therefore, identifying the regulatory mechanisms that control starch and protein accumulation is necessary for manipulating grain composition.
To study this we have made biochemical, transcriptomic and metabolomic comparisons of wheat grains that are altered in the balance of starch and protein including (i) wild vs. cultivated species (ii) transgenic grain and (iii) developing grain.