AP2/DREB Transcription Factor RAP2.4 Activates Cuticular Wax Biosynthesis inArabidopsisLeaves Under Drought

Drought is a critical environmental stress that limits growth and development of plants and reduces crop productivity. The aerial part of land plants is covered with cuticular waxes to minimize water loss. To understand the regulatory mechanisms underlying cuticular wax biosynthesis inArabidopsisunder drought stress conditions, we characterized the role of an AP2/DREB type transcription factor, RAP2.4.RAP2.4expression was detected in one-week-old seedlings and rosette leaves, stems, stem epidermis, cauline leaves, buds, flowers, and siliques of 6-week-oldArabidopsis. The levels ofRAP2.4transcripts increased with treatments of abscisic acid (ABA), mannitol, NaCl, and drought stress. Under drought, total wax loads decreased by approximately 11% and 10%, and in particular, the levels of alkanes, which are a major wax component, decreased by approximately 11% and 12% inrap2.4-1andrap2.4-2leaves, respectively, compared with wild type (WT) leaves. Moreover, the transcript levels of cuticular wax biosynthetic genes,KCS2andCER1, decreased by approximately 15-23% and 32-40% inrap2.4-1andrap2.4-2leaves, respectively, relative to WT 4 h after drought treatment, but increased by 2- to 12-fold and 3- to 70-fold, respectively, in three independentRAP2.4OX leaves relative to WT. Epicuticular wax crystals were observed on the leaves ofRAP2.4OX plants, but not on the leaves of WT. Total wax loads increased by 1.5- to 3.3-fold in leaves ofRAP2.4OX plants relative to WT. Cuticular transpiration and chlorophyll leaching occurred slowly in the leaves ofRAP2.4OX plants relative to WT. Transcriptional activation assay in tobacco protoplasts showed that RAP2.4 activates the expression ofKCS2andCER1through the involvement of the consensus CCGAC or GCC motifs present in theKCS2andCER1promoter regions. Overall, our results revealed that RAP2.4 is a transcription factor that activates cuticular wax biosynthesis inArabidopsisleaves under drought stress conditions.