Mycorrhizae enhance drought tolerance of trifoliate orange by regulating circadian clock response patterns

Abstract

Either arbuscular mycorrhizal fungus or circadian clock plays an important role in enhancing the tolerance of plants to soil drought, whereas it is not known whether the change of circadian rhythm is involved in mycorrhiza-enhanced drought tolerance in host plants. The purpose of this study was to investigate effects of an arbuscular mycorrhizal fungus Funneliformis mosseae on biomass production, root morphology, and expressions of circadian clock genes and drought-responsive genes in roots of trifoliate orange plants under drought stress conditions. After 8 weeks of soil drought, root mycorrhizal colonization rate and the length of soil mycelium significantly decreased, together with improved leaf, stem, and root biomass production and root morphology by mycorrhizal fungus. Expressions of six circadian clock genes (PtPRR7, PtCCA1, PtLHY, PtGI, PtPIF3, and PtSRR1) and three drought-responsive genes (PtMn-SOD, PtPOD, and PtCAT1) showed obvious circadian rhythm during the day. Under soil drought, F. mosseae induced the expression of PtPRR7, PtGI, and PtSRR1 at 17:00, PtPIF3 and PtSRR1 at 21:00, and PtPRR7, PtPIF3, and PtSRR1 at 1:00, whereas decreased the expression of PtLHY at all the tested time points. F. mosseae also up-regulated expressions of drought-responsive gene (PtFe-SOD, PtMn-SOD, PtCu/Zn-SOD, PtPOD, and PtLEA) at 17:00, 21:00 and 1:00 under drought conditions. Correlation analysis indicated the significant correlation between circadian clock gene expressions and drought-responsive gene expressions. These results conclude that mycorrhizal fungi regulate the circadian rhythm of trifoliate orange to respond to drought.