31001:

Ethylene regulation of abscission in drought stressed cotton.

Authors:	Lashbrook, Coralie, C.(A)Klee, Harry, J.(A)
Affiliations:	(A): University of Florida, Horticultural Sciences Department
Presenter:	Lashbrook, Coralie C., lashbrook@gnv.ifas.ufl.edu

Abscission is the process by which plants shed organs such as leaves, flowers and fruit. Cotton exhibits high rates of abscission in response to developmental and environmental cues. Approximately 40% of developing cotton buds routinely abscise prior to fruit set. In response to such abiotic signals as low light, flooding or drought stress, over 80% of developing buds can be shed. Significant changes in ethylene synthesis and perception attend these abscission events. We are studying water stress-induced abscission in cotton in order to elucidate fundamental mechanisms by which ethylene promotes and coordinates plant responses to the environment. In order to initiate organ abscission, cells comprising the separation layer of the abscission zone must become ethylene sensitive. Ethylene production within abscising tissue subsequently induces the de-novo synthesis of specific cell wall hydrolases within the separation layer. In cotton leaves destined to abscise, significant changes in ethylene synthesis and perception in tissues distal to the abscission zone precede organ shed. Thus, the transduction of an abscission stimulus into a detachment response may be dependent upon hormonal signaling within a complex network of plant tissues. We are taking a molecular genetic approach aimed at elucidating how ethylene regulates abscission competence in cotton. Four members of the ACC oxidase (Gh-ACO) gene family were cloned from floral abscission zones. Gh-ACO members exhibit 74-83% and 80-93% sequence identities at the nucleotide and amino acid levels, respectively. Differential expression of Gh-ACO genes is observed in roots, abscission zones and other aerial tissues of plants abscising in response to water stress. Gh-ACO2 mRNA accumulates in wilting roots, shoots and leaf petioles, with mRNA abundance declining upon plant rehydration. In contrast, other Gh-ACO mRNAs are not regulated in roots or shoots; their mRNAs accumulate in abscission zones and petioles of stressed plants upon rehydration. In drought stressed plants, most abscission occurs when plants are rehydrated. Thus, the potential contribution of root to shoot ACC flux and associated ethylene production to the coordination of abscission responses is discussed. The functional significance of Gh-ACO-mediated ethylene biosynthesis in abscising cotton is currently being tested in transgenic plants. Funded by USDA/NRICGP Postdoctoral Fellowship 97-35100-4192 to CCL and a grant from the Monsanto Co. to HJK.