Brassinosteroid and Cytokinin Synergistically Increase Ovule Initiation and Seed Number Per Silique in <i>Arabidopsis</i>

Abstract

The main determinant of crop yield is seed yield. In Arabidopsi, many of the genes that regulate seed number are involved in gametophyte and zygote development, and their overexpression cannot increase seed number. However, several mutants with enhanced brassinosteroid (BR) and cytokinin (CK) signals show increased seed number per silique (SNS). Here, we studied the crosstalk between BR and CK signals in regulating SNS through crossing BR- and CK-related mutants. Compared with the mutants ckx3 ckx5 and bzr1-1D , the triple mutant ckx3 ckx5 bzr1-1D with enhanced BR and CK signals formed elongated placentae and showed increased ovule initiation and SNS, indicating that BR and CK synergistically up-regulate SNS. Although BR enhanced the CK signal and CK enhanced the BR signal, while CK partially rescued the phenotypes of BR deficient- and -insensitive mutants, BR could not rescue the phenotypes of CK-deficient mutants. This suggested that CK functions downstream of BR in regulating SNS and that BR regulates SNS through CK-mediated or CK-independent pathways. Comparative transcriptome analyses of ckx3 ckx5 , bzr1-1D , and ckx3 ckx5 bzr1-1D identified known and new genes involved in ovule initiation. The BR-induced transcription factor BZR1 was found to directly interact with the B-type regulator of CK, ARR1, and this interaction enhanced the ability of ARR1 to target and induce genes related to ovule development. Our results demonstrate that BR and CK synergistically increase SNS through co-regulating downstream genes involved in ovule identity and initiation. The genes involved in this process have potential applications in enhancing crop yield.