IDENTIFICATION AND MANIPULATION OF POSTHARVEST PHYSIOLOGICAL AND MOLECULAR PROCESSES CONTROLLING POTATO NUTRITIONAL AND MARKET QUALITY
Location: Sugarbeet and Potato Research
Title: Chemical inhibition of potato ABA 8'-hydroxylase activity alters in vitro and in vivo ABA metabolism and endogenous ABA levels but does not affect potato microtuber dormancy duration
Submitted to: Journal of Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 20, 2012
Publication Date: September 18, 2012
Citation: Suttle, J.C., Abrams, S.R., De Stefano-Beltran, L., Huckle, L.L. 2012. Chemical inhibition of potato ABA 8'-hydroxylase activity alters in vitro and in vivo ABA metabolism and endogenous ABA levels but does not affect potato microtuber dormancy duration. Journal of Experimental Botany. 63(15):5717-5725.
Interpretive Summary: For an indeterminate period of time following harvest, potatoes will not sprout and are physiologically dormant. Dormancy is gradually lost during postharvest storage and the resultant sprouting is detrimental to the nutritional and processing qualities of potatoes. Because of this, sprouting results in severe financial loss to producers. Currently sprouting is controlled through the use of synthetic sprout inhibitors. The research being conducted in this project is directed towards: 1.) identifying key physiological processes that naturally regulate tuber dormancy and, ultimately, 2.) modifying these processes genetically thereby eliminating the need for artificial sprout suppression. Previous research from this laboratory has demonstrated that the plant hormone abscisic acid (ABA) plays an essential role in the initiation and maintenance of tuber dormancy. The internal content of ABA declines as tuber dormancy weakens but the role that this decline plays in ending dormancy is unknown. In this paper, the biochemical properties of the key enzyme controlling ABA decline together with the effects of a series of chemical inhibitors on enzyme activity are described. The two most potent inhibitors were demonstrated to prevent the decline in ABA content during tuber dormancy progression but had no effects on the length of tuber dormancy. These results suggest that while a decline in ABA content accompanies the loss of tuber dormancy it is not an essential component of the internal processes that control tuber dormancy exit. Therefore, attempts to extend potato tuber dormancy by manipulating ABA content will not be commercially successful.
The effects of azole-type P450 inhibitors and two metabolism-resistant ABA analogs on in vitro ABA 8'-hydroxylase activity, in planta ABA metabolism, endogenous ABA content, and tuber meristem dormancy duration were examined in potato (Solanum tuberosum L. cv. Russet Burbank). When functionally expressed in yeast, three potato CYP707A genes were demonstrated to encode enzymatically active ABA 8'-hydroxylases with micro-molar affinities for (+)-ABA. The in vitro activity of the three enzymes was inhibited by the P450 azole-type inhibitors ancymidol, paclobutrazol, diniconazole, and tetcyclasis and by the 8'-acetylene- and 8'-methylene-ABA analogs with diniconazole and tetcyclasis being the most potent inhibitors. The in planta metabolism of [3H]-(±)-ABA to phaseic acid and dihydrophaseic acid in tuber meristems was inhibited by diniconazole, tetcyclasis and to a lesser extent by 8'-acetylene- and 8'-methylene-ABA. Continuous exposure of in vitro generated microtubers to diniconazole resulted in a two-fold increase in endogenous ABA content and a decline in dihydrophaseic acid content after nine weeks of development. Similar treatment with 8'-acetylene ABA had no effects on the endogenous contents of ABA or phaseic acid but reduced the content of dihydrophaseic acid. Tuber meristem dormancy progression was determined ex vitro in control, DCN-, and 524-treated microtubers following harvest. Sprouting percentages in control microtubers were 10, 17, 40, and 100 after 98 days, 114 days, 128 days, and 141 days, respectively. Continuous exposure to diniconazole during microtuber development had no effects on subsequent sprouting at any time point. Continuous exposure to 8'-acetylene ABA significantly increased the rate of microtuber sprouting. The results indicate that, although a decrease in ABA content is a hallmark of tuber dormancy progression, the decline in ABA levels is not a prerequisite for dormancy exit and the onset of tuber sprouting.