Cytokinin plays a critical role in bitter gourd rootstock-induced thermotolerance of cucumber

Abstract

Plants, as sessile in nature, are constantly confronted with diverse biotic and abiotic stresses throughout their life cycle in the changing environment. As a result, plants evolved root-shoot communications to optimize plant growth and development, and regulate responses to environmental stresses. Here, we examined the roles of root-sourced cytokinin (CTK) response to heat stress in grafted cucumber seedlings. Cucumber plants grafted onto cucumber roots and bitter gourd (<i>Momordica charantia</i>) roots were exposed to heat to examine their heat tolerance by assessing the levels of photosynthetic capacity, CTK contents, chlorophyll-a/b-binding protein (Lhcb2), ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and its activating enzyme (RCA) content, and the enzyme activity of Rubisco. Bitter gourd rootstock enhanced cucumber scions heat stress tolerance. This enhancement was positively correlated with a higher content of CTK in both leaf and root parts, chlorophyll contents, and Rubisco abundance and activity. In addition, the higher level of CTK and Rubisco content in bitter gourd grafted plants shoots than in cucumber self-gafted plants shoots were attributed to an increase in CTK transport from roots in grafted plants under high-temperature conditions. These results indicated that CTK transfer from bitter gourd rootstock to scion and triggered the accumulation of Rubisco in leaf, thus improving the heat resistance of bitter gourd-grafted plants.