Roots allow plants to absorb water and nutrients and are the first part of the plant to experience environmental stresses associated with the soil environment.  We characterize how roots respond to environmental stress focusing on salinity stress, a condition that arises when soils contain high levels of dissolved ions.  Salinity imposes a physiological drought that is combined with ion toxicity when salt ions enter plant cells.  Using the differential display of mRNA technique we have isolated several novel genes that are expressed in salt-stressed tomato and Arabidopsis roots.  Using these genes, our ongoing research has two major objectives.  The first is to establish how gene expression is regulated in salt-stressed roots with a focus on the role played by abscisic acid (ABA) and ethylene (ET), and the second is to determine the function played by the encoded gene products.

This research focuses on ABA and ET, two hormones that play key roles in regulating gene expression in environmentally challenged plants.  Our initial work examined the extent to which salt-responsive gene expression was dependent on root ABA.  We undertook genetic and chemical approaches to manipulate the ABA content in roots and in so doing uncovered a positive role for ET in regulating the expression of several salt-responsive genes.  We are currently dissecting the interaction between ET and ABA in regulating the response to salt stress in roots using a variety of genetic and biochemical approaches.  We know that ABA is a dominant signal in roots as it is capable of suppressing the ET-responsive expression of several genes.

Regulation of root growth in salt-stressed seedlings

We are also exploring the interaction between ABA and ET in controlling root growth in salt-stressed seedlings. ABA inhibits root growth and salt alters the sensitivity of root growth inhibition by ABA, which contributes to the maintenance of root growth in salt-stressed seedlings (Figure 1). Salt and, to a lesser extent, ABA suppress the appearance of lateral roots. Thus, salt-stressed roots emphasize elongation growth, which helps them explore the soil to seek a more optimal environment from which to acquire water and minerals.

What is the function of salt-responsive gene products in roots?

This research is currently focused on two groups of genes in tomato and Arabidopsis.  The first gene family encodes α-dioxygenase-like enzymes that are implicated in generating fatty acid-derived signals due to their ability to catalyse the production of novel fatty acid hydroperoxides and hydroxy-fatty acids.  The second gene family encodes polypeptides with similarity to the Radical-induced Cell Death-1 Arabidopsis protein that belongs to a novel sub-family of proteins involved in ADP-ribose conjugation and protein-protein interactions mediated by a WWE domain. Mutations in RCD-1 confer an ozone-sensitive phenotype implying a role in protection against ozone-derived oxidative damage and lesion propagation.  We use both forward and reverse genetic approaches to explore, in tomato and Arabidopsis, the function of α-dioxygenase and RCD-1-related proteins in salt-stressed roots.

Proposed biochemical pathway describing the action of α-dioxygenase enzymes in generating novel 2-hydroxy fatty acids or one-carbon shortened fatty acids from linolenic acid and proceeding via the formation of an unstable 2-hydroperoxy fatty acid. From: Hamberg, M. et al. J. Biol. Chem. 2003;278:51796-51805

Dissecting the response to drought in Populus

Drought is a major factor that impacts ecosystem viability and economic productivity due to the profound effect that it has on plant survival and productivity. Drought stress causes cellular dehydration and loss of turgor resulting in an immediate reduction in growth. Responses in drought-stressed plants that contribute to tolerance include the synthesis of ABA, which is linked with drought tolerance via ABA-induced closure of stomata and the ABA-responsive expression of stress-inducible genes.

We are screening for drought tolerance in young poplar trees that are responding to a gradual drought stress. Selected drought-resistant and drought-susceptible poplar genotypes are then used to examine the expression of key drought-responsive genes. A major goal of this research is to establish the utility of these drought-responsive genes as candidate molecular markers that correlate with drought resistance and to gain a better understanding of the mechanisms utilized by drought-tolerant trees to cope with drought.  

Water potential of poplar plants maintained under control and drought-stressed conditions.

Defense against Herbivory in Conifers.

Oleoresin constitutes a major defense in conifer trees against a variety of environmental stresses. In many conifers oleoresin is pre-made and housed in specialized anatomical resin ducts in the foliage and stem tissues.  Conifers mount an active defense response that includes the synthesis of new or traumatic resin ducts following a variety of environmental insults including attack by stem-boring and defoliating insects.  Oleoresin is an extremely complex chemical mixture of terpenoids that provides chemical and physical protection against insects and pathogens.

We are assessing whether traumatic resinosis is an important component of tree resistance to insect attack.  To that end we have isolated cDNA representatives for the monoterpenoid, diterpenoid and sesquiterpenoid class of terpene synthase (TPS) enzymes and use these as tools to assess the capacity for an induced resinosis response in insect resistant and susceptible trees.  Our work with conifers has been used to assess whether the induced resinosis response is important for resistance in Sitka spruce (Picea sitchensis) to weevil attack (see below) and to explore the role of monoterpenoids in conferring resistance to deer browsing in Western Red Cedar (Thuja plicata).

Pissodes strobi, the white pine weevil and an example of the type of damage imposed by a successful weevil attack that has resulted in the death of the apical leader.

Histochemical evidence of the induced traumatic resinosis response in a weevil-attacked leader of Sitka spruce. A ring of traumatic resin ducts and resin-soaked parenchyma are embedded in the secondary xylem tissue in the stem cross-section.

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