Environmental and Developmental Regulation of the Wound-Induced Cell Wall Protein WI12 in the Halophyte Ice Plant

A wounded gene WI12 was used as a marker to examine the interaction between biotic stress (wounding) and abiotic stress (high salt) in the facultative halophyte ice plant (Mesembryanthemum crystallinum). The deduced WI12 amino acid sequence has 68% similarity to WUN1, a known potato (Solanum tuberosum) wound-induced protein. Wounding, methyl jasmonate, and pathogen infection induced local WI12 expression. Upon wounding, the expression of WI12 reached a maximum level after 3 h in 4-week-old juvenile leaves, whereas the maximum expression was after 24 h in 8-week-old adult leaves. The temporal expression of WI12 in salt-stressed juvenile leaves was similar to that of adult leaves. The result suggests that a salt-induced switch from C3 to Crassulacean acid metabolism has a great influence on the ice plant's response to wounding. The expression of WI12 and the accumulation of WI12 protein were constitutively found in phloem and in wounded mesophyll cells. At the reproductive stage, WI12 was constitutively found in petals and styles, and developmentally regulated in the placenta and developing seeds. The histochemical analysis showed that the appearance of WI12 is controlled by both environmental and developmental factors. Immunogold labeling showed WI12 preferentially accumulates in the cell wall, suggesting its role in the reinforcement of cell wall composition after wounding and during plant development.