Age-induced loss of wound-healing ability in potato tubers is partly regulated by ABA

Authors: KUMAR, GN - Washington State University; Lulai, Edward; Suttle, Jeffrey; KNOWLES, N - Washington State University

Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/30/2010
Publication Date: 9/10/2010
Citation: Kumar, G.N.M., Lulai, E.C., Suttle, J.C., Knowles, N.R. 2010. Age-induced loss of wound-healing ability in potato tubers is partly regulated by ABA. Planta. 232:1433-1445.

Interpretive Summary: Research was conducted to determine the biochemical factors associated with age-induced loss of potato tuber wound-healing including determination of the regulatory involvement of the plant hormone abscisic acid (ABA). Identification of the wound-healing processes specifically affected by tuber aging and the regulatory mechanisms that undergo diminished control of wound-healing as a result of tuber aging are of great importance to growers, processors and consumers. Potato tubers that are wounded during harvest, handling into storage and upon seed cutting must rapidly heal to avoid water loss/cell desiccation, infection and to maintain food/nutritional quality. Overall wound-healing ability declines with tuber age (storage period). Despite the importance, the mechanism of loss in healing capacity with tuber age is not known. Major components of wound-healing include development of a water vapor barrier, without which would result in cell death and termination of healing, and the induction of a process called suberization where wounded plant tissue develops a new protective skin that resists infection similar to the skin of native periderm found on the surface of healthy tubers. Key biochemical processes involved in wound-healing include activation of biochemical pathways for suberin biosynthesis; the first enzyme involved in these pathways is phenylalanine ammonia lyase (PAL), consequently it is may be as a marker for initiation of suberization. The plant hormone abscisic acid (ABA) is involved in the regulation of PAL formation and the accumulation of the two major biopolymers that comprise the suberized protective surface. Therefore, the roles of ABA and PAL in the age-induced loss of wound-healing ability of tubers were examined. Non-wounded older tubers were found to possess much less ABA than younger tubers and produced much less ABA upon wounding. Wound-induced PAL activity increased more rapidly in younger tubers than older tubers. ABA treatment increased PAL in tissues from both ages of tubers and restored the time-line for PAL formation in older tubers to match that of younger tubers. Moreover, ABA treatment of wounded older tubers enhanced their resistance to water vapor loss. Wound-induced accumulation of suberin was measurably slower in older versus younger tubers. ABA treatment hastened accumulation of the first suberin biopolymer in older tubers to match that in younger tubers and to a much lesser degree enhanced initiation of accumulation of the second suberin biopolymer, i.e. ABA treatment partly restored the wound-healing ability of older tubers by enhancing the accumulation of suberin constituents. Age-induced loss of wound-healing ability is thus partly due to reduced ability to produce ABA and modulate suberin biosynthesis in response to wounding. The results are of practical importance because they: (1) quantitatively demonstrate that aging diminishes wound-induced suberization and (2) identify the plant hormone ABA as a major age-impacted regulator of wound-induced suberization and water vapor losses; all of which directly impact deterioration of food during storage. These results will be useful in the development of future technologies to reduce infections and maintain stored potato quality for consumers.

Technical Abstract: Wounding of potato (Solanum tuberosum L.) tubers induces the development of a suberized closing layer and wound periderm that resists desiccation and microbial invasion. Wound-healing ability declines with tuber age (storage period). The mechanism of loss in healing capacity with age is not known; however, upregulation of superoxide production and phenylalanine ammonia lyase (PAL) activity in response to wounding are processes critical to the development of a suberized closing layer and wound periderm. PAL is induced by abscisic acid (ABA). Therefore, the roles of ABA and PAL in the age-induced loss of wound-healing ability of tubers were examined. Non-wounded older tubers had 86% less ABA (dry matter basis) than younger tubers. PAL transcript increased in younger tubers within 24 h of wounding but transcription was delayed by 5 days in older tubers. Wound-induced PAL activity increased more rapidly in younger tubers than older tubers. ABA treatment ncreased PAL expression and activity in tissue from both ages of tubers and restored the 24 h transcription time-line in older tubers. Moreover, ABA treatment of wounded older tubers enhanced their resistance to water vapor loss following a 6-day wound-healing period. Wound-induced accumulation of suberin poly(phenolic(s)) (SPP) and suberin poly(aliphatic(s)) (SPA) was measurably slower in older versus younger tubers. ABA treatment hastened SPP accumulation in older tubers to match that in younger tubers and enhanced SPA accumulations over the initial 4 days of healing. However, the enhancing effect of ABA on subsequent SPA acumulations diminished by the fifth day regardless of tuber age. Age-induced loss of wound-healing ability is thus partly due to reduced ability to produce ABA and modulate the production of SPP through PAL in response to wounding. ABA treatment partly restored the healing ability of older tubers by enhancing the accumulation of suberin constituents without restoring wound-induced superoxide forming ability, suggesting a minimal role for wound-induced superoxide radicals in the oxidative coupling of phenolic monomers during suberization.