Wounding induces changes in cytokinin and auxin content in potato tuber, but does not induce formation of gibberellins

Authors: Lulai, Edward; Suttle, Jeffrey; Young, Linda; Neubauer, Jonathan; Campbell, Larry; CAMPBELL, MICHAEL - Pennsylvania State University

Submitted to: Journal of Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/18/2015
Publication Date: 2/1/2016
Citation: Lulai, E.C., Suttle, J.C., Olson, L.L., Neubauer, J.D., Campbell, L.G., Campbell, M.A. 2016. Wounding induces changes in cytokinin and auxin content in potato tuber, but does not induce formation of gibberellins. Journal of Plant Physiology. 191:22-28.

Interpretive Summary: Potato tuber wounds incurred during harvest, handling into and out of storage and upon seed cutting require rapid suberization as a major part of the wound-healing processes that prevent infection, desiccation, defect development and loss of nutritional value. The induction and hormonal regulation of suberization is of great economic and nutritional importance. Wound related losses cost growers, processors and storage operators well over $300 million/year; this does not include losses that are difficult to quantify from problems such as tuber pink eye, pink rot or losses incurred by the grocer and consumer. Despite the large economic and nutritional importance of tuber wound-healing, little is known about the induction of hormones that regulate molecular mechanisms and related genes controlling these processes and how they are expressed during the two major stages of wound-induced suberization, i.e. closing layer formation followed by wound periderm formation. Although the involvement of some plant hormones (abscisic acid, ethylene and jasmonic acid) in the regulation of wound-induced suberization has been determined, quantitative information on the remaining classical plant hormones (cytokinin, auxin and gibberellin content) in resting and wound-responding potato tuber is lacking. Consequently, the coordination and possible networking of these classical hormones in the regulation of wound-healing processes are poorly understood. The ability to hasten wound-healing processes through manipulation of these regulatory plant hormones has great potential to improve economic and nutritional issues for the industry and consumers. The objective of this study was to determined content and, where possible, associated expression of genes limiting biosynthesis and catabolism of some of these hormones in potato tuber for coordination with established wound-healing processes. These processes include formation of the protective closing layer (the first stage where existing cells at the wound surface are suberized) and wound periderm formation (the second stage where new protective cells are formed and suberization beneath the closing layer) as well as associated supporting processes such as the defined period of wound-induced DNA synthesis. Consistently quantifiable amounts of biologically active cytokinins (Z, zeatin; DZ, dihydrozeatin and IP, isopentenyladenine), which putatively regulate cell division during wound-healing, were not detected in resting or wound-responding tubers. However, the precursor isopentenyladenosine (IPA) and catabolic product (cis) zeatin-O-glucoside (c-ZOG) were found in small amounts in resting and wound-responding tubers. Wound-induced activation of cytokinin biosynthesis was suggested by the appearance of precursors (trans) zeatin riboside and (cis) zeatin riboside (t-ZR and c-ZR respectively) at 0.5 d and large increases in IPA and c-ZR by 3 d and throughout 7 d after wounding. Three days of wounding healing represents a period of significant advancement toward completion of closing layer formation (ca. 5 d) and 7 d represents a time period after completion of the closing layer and where wound periderm formation is fully initiated and underway. The expression of key genes involved in cytokinin metabolism followed similar profiles with transcripts decreasing through 3 d and then increasing at 5 – 7 d after wounding; this is consistent with the increases in cytokinin content and induction of cells for subsequent meristematic activity during wound healing. Auxins are known to induce cell elongation in some tissues and promote cell division in other tissues. Both active (free) and inactive auxins (IAA, indole-3-acetic acid and IAA-Asp, N-(indole-3-yl-acetyl)-aspartic acid respectively) were present in resting tubers. While IAA-Asp was no longer present by 3 d after wounding, IAA content nearly doub

Technical Abstract: Cytokinin, auxin and gibberellin content in resting and wound-responding potato tuber are not clearly defined. Consequently, the coordination and possible networking of these classical hormones in the regulation of wound-healing processes are poorly understood. Using a well-defined tuber wound-healing model system, the content and associated gene expressions of these three hormones were determined following physical damage. Consistently quantifiable amounts of biologically active cytokinins (Z, DZ and IP) were not detected in resting or wound-responding tubers. However, the precursor IPA and catabolic product c-ZOG were found in small amounts in resting and wound-responding tubers. Wound-induced activation of cytokinin biosynthesis was suggested by an increase in t-ZR and c-ZR content at 0.5 d and large increases in IPA and c-ZR content by 3 d and throughout 7 d after wounding. The expression of key genes involved in cytokinin metabolism followed similar profiles with transcripts decreasing through 3 d and then increasing at 5 – 7 d after wounding. Both free IAA and IAA-Asp were present in resting tubers. While IAA-Asp was no longer present by 3 d after wounding, IAA content nearly doubled by 5 d and was nearly 5-fold greater at 7 d compared to that in resting tuber (0 d). Gibberellins were not present in consistently quantifiable amounts in resting or wound-responding tubers. Collectively, these results suggest that bio-active cytokinins are wound-induced, but their residency is very temporal and highly regulated. The highly regulated and transient presence of active cytokinins in conjunction with corresponding increases in IAA content strongly suggest the involvement of these two hormones in the regulation of phellogen mediated meristematic activity as part of wound periderm development; while the absence of gibberellins indicates that they are not a regulatory component of wound-healing processes.