The Interaction of Cytokinin and Strigolactone in controlling Shoot Branching in Arabidopsis thaliana

Abstract

Shoot branching is regulated by auxin, cytokinin (CK) and strigolactone (SL). Cytokinin, being the only promoter of shoot branching, is antagonistic in function to auxin and strigolactone, which inhibit shoot branching. There is a close relationship between auxin and strigolactone, mediating each other to suppress shoot branching. Strigolactone reduces auxin transport from the buds, thus arresting bud outgrowth. On the other hand, auxin increases strigolactone production to control apical dominance. Antagonistic interaction between auxin and cytokinin has been reported as auxin inhibits lateral bud outgrowth by limiting CK supply to axillary buds. Previously, it has been found that levels of tZ-type CKs are extremely low in xylem sap of strigolactone mutants of Arabidopsis and pea.The current research aimed to explore the interaction between cytokinin and strigolactone, especially the regulatory mechanisms behind these low cytokinin levels. It was hypothesized that xylem-CKs may be controlled by strigolactone-mediated regulation of AtIPT genes in Arabidopsis thaliana. For this investigation, atipt double (atipt5,7, atipt3,5 and atipt3,7) and triple (atipt3,5,7) knockout mutants in wild-type and max backgrounds were screened and characterized. The GUS promoter-reporter system was used to study regulation of AtIPT expression by strigolactone. For this purpose, AtIPTs::GUS lines were generated in max SL mutant backgrounds. Further, cytokinin levels were quantified in root and shoot tissues as well as in phloem and xylem sap of atipt mutants in wild-type and max background. Cytokinin biosynthetic genes (AtIPTs) were shown to be regulated by strigolactone and cytokinin synthesis played an important role in the phenotype of max mutants. Loss of AtIPT3 from the SL-deficient mutant, max4, resulted in reduced growth and suppression of shoot branching. However, it was not possible to knockout AtIPT3 from the SL-insensitive mutant, max2, because double mutation of AtIPT3 and MAX2 genes was unexpectedly found to be lethal. Both max2 and max4 showed upregulation of AtIPT3 in phloem and downregulation of AtIPT5 in root and shoot. Application of GR24 (SL synthetic analogue) and NAA (auxin) reversed the regulation in max4. Auxin- and strigolactone-mediated regulation of AtIPT3 and AtIPT5 in root and shoot were strictly MAX2-dependent, apart from auxin upregulation of ATIPT5 in roots, independent of strigolactone. The phloem sap showed elevated levels of the CK metabolite, iPRP, and this was correlated with high expression of AtIPT3 in phloem of max mutants. The increased transport of iPRP to roots did not increase the production of tZ-type CKs in roots of max mutants. Rather the levels of tZ-type CKs in xylem sap of max mutants were highly reduced. The results lead to the conclusionthat SL controls shoot branching partly by mediating CK biosynthesis and iPRP in phloem may function as a feedback signal to modulate translocation of tZ-type CKs through xylem.