Sharp, R.E., LeNoble, M.E., Else, M.A., Thorne, E.T., and Gherardi, F. 2000. Endogenous ABA maintains shoot growth in tomato independently of effects on plant water balance: evidence for an interaction with ethylene. Journal of Experimental Botany 51: 1575 – 1584.

The shoot growth of ABA-deficit tomato plants were measured to see if the reduction of growth was caused independently from the water balance of the plant. Flacca and notalilis were grown in controlled humidity to so that the water potentials of the leaves equaled the water potential of the well-watered tomato wild-type. All of the growth parameters of the plants were impaired or significantly reduced. Tests showed that the flacca plant made a considerable recovery once exogenous ABA, this however did not improve water potential. The exogenous ABA restored the leaf expansion by 75%. The results showed that normal levels of endogenous ABA are needed for leaf expansion as well as regular shoot development in well‐watered tomato plants. This was also caused independently from the effects of the plant water balance.

http://jxb.oxfordjournals.org/content/51/350/1575.short

Jess Gibbons

Munns, R. and Sharp, R.E. 1993. Involvement of Abscisic Acid in Controlling Plant Growth in Soil of Low Water Potential. Journal of Plant Physiology 20: 425 – 437.

Hormones are important for stimulation growth (root: shoot) ratio of plants in water deficit soils. Particularly, ABA is well-known for its reaction to water stress in plants. ABA acts differently in plants planted in dry soil versus wet soil. It promotes root growth and shoot inhibition in dry soil to conserve as much water as possible. Whereas, in wet soils it inhibits both root and shoot growth. The role of ABA in long-distance signaling is unclear in regards to the root: shoot growth ratio. The concentration of ABA found in the xylem can affect stomatal conductance but are too low to affect leaf growth. However, in drought conditions, leaf expansion takes precedence over stomatal conductance. This could be due to the fact that ABA is carried through the xylem in a complex form or that there is another factor in the xylem that promotes leaf expansion separately from the effects of ABA.

http://www.publish.csiro.au/?paper=PP9930425

Jess Gibbons

Pellegrineschi, A., Reynolds, M., Pacheco, M., Brito, R.M., Almeraya, R., Yamaguchi-Shinozaki, K., and Hoisington, D. 2004. Stress-induced expression in wheat of the Arabidopsis thaliana DREB1A gene delays water stress symptoms under greenhouse conditions. NRC Research Press 47:493-500.

Many plants such as maize, rice, and wheat are susceptible to water deficit and since water is the limiting factor in plant productivity this creates a huge problem. Genetic engineering is attractive because of its potential to improve abiotic stress tolerance more rapidly. Transcription factors, such as DREB1A, have been shown to produce multiple phenotypic changes, many of which are involved in stress responses like dehydration. DREB1A has been shown in Arabidopsis thaliana to play a crucial role in promoting the expression of drought-tolerance genes. This study placed the A. thaliana DREB1A gene under the control of a stress-inducible promoter and transformed into the genome of bread wheat. The plants expressing the DREB1A showed less damage and water stress than the plants not expressing this gene after 10 days under optimal greenhouse conditions.

http://www.nrcresearchpress.com/doi/abs/10.1139/g03-140#.VSSNXZVFDIU

Jess Gibbons

Mills, D. , Zhangb, G., and Benzionia, A. 2001. Effect of different salts and of ABA on growth and mineral uptake in Jojoba shoots grown in vitro. Journal of Plant Physiology 158: 1031-1039.

ABA and different saline levels were tested in the shoots of jojoba clones. The shoots were exposed to the different salinity levels in vitro to study the shoot elongation, node production, leaf length, and weight. It also looked at the accumulation of Na+, K+, Ca2+, Cl-, and SO4^2-, which resulted in the chloride ion reducing all growth parameters in salt-sensitive clones. The chloride ion had no adverse effects on the salt-insensitive clones. ABA concentrations were lower in salt-sensitive plants than salt-tolerant which correlated with the similar shoot growth of plants in chloride salinities. However, chloride salinity did promote leaf expansion in both tolerant and sensitive plants. The results show that the salt sensitivity of jojoba clones is somehow correlated to the plants sensitivity to ABA.

http://www.sciencedirect.com/science/article/pii/S0176161704701273

Jess Gibbons

Aroca, R., Vernieri, P., and Ruiz-Lozano, J.M. 2008. Mycorrhizal and non-mycorrhizal Lactuca sativa plants exhibit contrasting responses to exogenous ABA during drought stress and recovery. Journal of Experimental Biology 59: 2029-2041.

Abscisic acid seems to promote the gene expression in plants that results in the symbiosis of arbuscular mycorrhizal and plants; this helps the plant deal with water deficit better. The goal of this study was to evaluate the influence of AM symbiosis and exogenous ABA (since it has yet to be studied) on plant development, physiology, and expression of several stress-related genes after both drought and a recovery period. The results showed contrasting data between the exogenous ABA on AM and non-AM plants. The exogenous ABA in AM plants seemed to protect the plant from any physical shoot deficit that would normally be brought on by drought. The AM plants, however had a decrease in ABA content when the exogenous ABA was added, which also led to a decrease in stress marker production of genes Lsp5cs, Lslea, and Lsnced. This was opposite for the non-AM plants that showed an increased ABA content and expression of those stress marker genes in the presence of the exogenous ABA. The results indicate that AM plants regulate their ABA levels better and faster than non-AM plants, allowing a more adequate balance between leaf transpiration and root water movement during drought and recovery.

http://jxb.oxfordjournals.org/content/59/8/2029.short

Jess Gibbons

Jeschke, W., Andreas Peuke, John Pate, and Wolfram Hartung. “Transport, Synthesis and             Catabolism of Abscisic Acid (ABA) in Intact Plants of Castor Bean (Ricinus Communis L.) under Phosphate Deficiency and Moderate Salinity.” Journal of Experimental Botany             48.314 (1997): 1737-747.

This experiment investigated ABA in castor bean plants living in soil with various environmental stresses. It is common that an environment with one stress will have others as well, such as salinity with lower nitrogen. Plants grown under insufficient amounts of phosphate had lower amounts of ABA present in the xylem. However, these plants did not show any change in ABA within the leaves when compared to the control. Plants with added salinity, similar to phosphate deficient plants, had an increased amount of ABA in the roots. Contrary to the plants grown with a lack of phosphate, salinity plants had a greater amount of ABA in the leaves. As shown in this experiment, younger leaves acted as a sink of ABA whereas adult leaves were a source.

http://jxb.oxfordjournals.org/content/48/9/1737.short

Merissa Misiura