The Effect of Cytokinin and Abscisic Acid on Iron Stress Response in Tomatoes

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Mark L. Bernards and Dr. Von D. Jolley, Agronomy and Agriculture

Iron is an essential nutrient for healthy plant growth. Dicotyledenous plants respond to insufficient iron with a variety of mechanisms. For example, they may release hydrogen ions or exude reductants (such as caffeic acid) from the roots to acidify the rhizosphere, reduce Fe 3+ to Fe 2+ through enzymes at the root surface, and accumulate organic acids (primarily citrate) in the roots (Brown, 1978). Iron-efficient tomato T3238FER responds to iron stress signals in the roots by activating these mechanisms. Iron-inefficient T3238fer does not activate these mechanisms when subjected to iron stress.

Hormone involvement in these stress response mechanisms has been suggested, but never tested. Hormones influence growth, differentiation, metabolism, gene expression, and plant development. Each hormone exhibits unique characteristics, effects and interactions with other hormones (Gaudin, 1994). Abscisic acid (ABA) and cytokinins are believed to be involved with ion uptake, root transport, and stress responses (Davies, 1995).

In studies conducted by Thorne et al. (1997) using soybeans, ABA seemed to repress iron stress response mechanisms in iron-efficient A7 soybean plants. Thorne also found that low levels of the cytokinin BAP (6-benzylaminopurine) initiated response mechanisms in iron-inefficient T203 soybean plants. They were unable to replicate these results in subsequent trials.

To further test the role of hormones in iron stress response, we used the tomato isolines T3238FER and T3238fer, which differ only in their ability to respond to iron-deficiency stress. We hypothesized that ABA would repress iron stress response in T3238FER and that BAP would initiate iron stress response in T3238fer. By learning which hormones control iron stress response (if hormones do indeed control iron stress response), we hoped to gain a better understanding of how the plant communicates nutritional needs among plant parts.

The tomatoes were grown hydroponically in environmentally controlled growth chambers. Four weeks after germination the seedlings were transferred to an iron-free treatment solution to induce iron deficiency chlorosis. When the plants began to show signs of iron deficiency stress, we treated the iron-efficient T3238FER tomatoes with 0, 4, 40 and 280 mM ABA, and the iron-inefficient T3238fer tomatoes with 0.00, 0.05, 0.01, 0.025, 0.05, and 0.1 mM BAP (cytokinin). To monitor the effects of cytokinin and ABA on iron stress reponse, we took daily measurements of the pH, degree of chlorosis, and reductant release.

The cytokinin BAP had no discernable effect on iron stress response mechanisms in iron-inefficient T3238fer tomatoes (see figures 1-3).

During the course of the experiment we did not believe that ABA repressed the iron stress response mechanisms in iron-efficient T3238FER tomatoes, but after examining our data it appears that there was both a repression and a delay of response in the 4, 40, and 80 mM treatments. The most noticeable effects were higher pH levels and lower reductant release in treated plants (see figures 4- 6). However, we are not sure if the repression of iron stress response is attributable to a signal sent by ABA, or if it was merely the result of slowed growth caused by ABA-triggered stomatal closure and wilting. The day after we added the ABA to the nutrient solution, each of the treatments showed severe wilting. The plants later became turgid, and with the exception of plants in the 80 mM treatment resumed more normal growth, but all treated plants for a time used less water than the control.

It seems that ABA does have some affect on iron-stress response mechanisms. However, because of the questions raised by slowed growth, a second study is needed to measure both plant growth and iron stress response as affected by ABA in iron-efficient T3238FER tomatoes.

References

  • Brown, J. C. 1978. Mechanism of iron-uptake by plants. Plant Cell Environ. 1:249-257.
    Davies, P. J. 1995. Nature, occurrence, and functions. P. 1-11. In P. J. Davies (ed.) Plant hormones: Physiology, biochemistry, and molecular biology. 2nd ed. Kluwer Academic Publishers, Norwell, MA.
  • Gaudin, V., T. Vrain, and L. Jouanin. 1994. Bacterial genes modifying hormonal balances in plants. Plant Physiol. Biochem. 3 2:11-29
  • Thorne, E. T., R. E. Terry, and V. D. Jolley. 1997. Effects of cytokinin and abscisic acid on iron-efficient and iron inefficient soybean. Abstract. Ninth International symposium on Iron Nutrition and Interactions in Plants. Hohenheim, Germany.