Biological differences that distinguish the two major stages of wound healing in potato tubers

Authors: Lulai, Edward; Campbell, Larry; Fugate, Karen; McCue, Kent

Submitted to: Plant Signaling and Behavior
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/27/2016
Publication Date: 11/10/2016
Citation: Lulai, E.C., Campbell, L.G., Fugate, K.K., McCue, K.F. 2016. Biological differences that distinguish the two major stages of wound healing in potato tubers. Plant Signaling and Behavior. 11(12):e31256531. doi:10.1080/15592324.2016.1256531.

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 (WH) 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 $320 million/year; this does not include losses that are more difficult to quantify from problems such as tuber pink eye, pink rot or losses incurred by the grocer and consumer. Despite the economic and nutritional importance of tuber WH, it has incorrectly been treated as a single event with a continuum of biological processes. However, we have shown that wound-healing involves biosynthesis and assembly of protective suberin biopolymers in two different stages on walls of cells adjoining the damaged area. Little has been determined or discussed regarding the coordinate induction of the two stages of wound-induce suberization or the biological differences distinguishing these two stages. These two stages are: Closing layer formation (CLF)- the first stage where existing cells at the wound surface are suberized, and wound periderm formation (WPF)- the second stage where organized stacks of new protective cells are formed and suberized beneath the closing layer. Our objective was to determine critical biological differences, similarities, and processes that couple these two important stages of wound healing. The two stages of WH have critical biological differences. Earlier, we showed that the first stage, CLF, involves early induction of DNA synthesis as a necessity for cell division during WH. However, we also determined that cell division does not occur until five days after induction of DNA synthesis, i.e. during the second stage, WPF; this clearly shows differences in the two stages, but it also links them. The transition from CLF to the second stage is marked by a transient decrease in the expression of genes specifically required for suberin biosynthesis. The second stage, WPF, involves cell division for the creation of a new periderm consisting of several layers of new and physically durable suberized cells. Certain hormones, i.e. cytokinins (CK) and indole acetic acid (IAA), are required for these cell divisional processes. However, wounding did not induce the formation of biologically active CKs in quantifiable amounts, but the presence of precursor and catabolic breakdown products of CKs were present indicating that trace amounts of biologically active CKs were produced; this occurred in conjunction with increased IAA to collectively provide necessary signals for cell division during WPF. The requirement for only trace levels of bioactive CK was further demonstrated by exogenous supplementation of CKs (dihydrozeatin and cis zeatin) to potato tuber tissues which resulted in no effect during CLF, a time period where there were no cell divisional processes, but inhibition of healing when small concentrations (1 µM) of these CKs were added during WPF when cell division is a major process. These and other research results provide evidence for separate biological processes in signaling and biologically distinguishing the two stages of WH. The biological processes that distinguish differences and those that link the two stages are important because of the roles CLF and WPF respectively play in rapid and long term protection of the tuber from disease and other challenges. This information is mechanistically important in the development of new technologies to hasten all aspects of suberization during WH and in turn reducing associated economic and nutritional losses in stored potatoes.

Technical Abstract: The two stages of wound healing have critical biological differences. The first stage, closing layer formation (CLF), involves early induction of DNA synthesis and nuclear division in the absence of cell division. The transition phase from CLF to the second stage is marked by a transient decrease in expression of suberin-specific genes. The second stage, wound periderm formation (WPF) involves cell division. Biologically active cytokinins (CKs) are not present in quantifiable amounts, but the presence of precursor and catabolic products suggest the presence of trace amounts of active CKs in conjunction with increased auxin, indole acetic acid, providing necessary signals for meristematic activity. We discuss evidence for separate biological processes and signals as distinguishing the two stages of wound healing, and provide data suggesting that augmenting the low concentrations of biologically active CK inhibits WPF.