Silencing ofTaCKX1mediates expression of otherTaCKXgenes to increase grain yield in wheat

TaCKX family genes influence development of wheat plants by specific regulation of cytokinin content in different organs. However, their detailed role is not known. The TaCKX1 , highly and specifically expressed in developing spikes and in seedling roots, was silenced by RNAi-mediated gene silencing via Agrobacterium and the effect of silencing was investigated in 7 DAP spikes of T 1 and T 2 generations. Various levels of TaCKX1 silencing in both generations influence different models of co-expression with other TaCKX genes and parameters of yield-related traits. Only a high level of silencing in T 2 resulted in strong down-regulation of TaCKX11 (3) , up-regulation of TaCKX2.1 , 2.2 , 5 and 9 ( 10 ), and a high yielding phenotype. This phenotype is characterised by higher spike number, grain number and grain yield, as well as slightly higher mass of seedling roots, but lower thousand grain weight (TGW) and slightly lower spike length. Content of most of cytokinin forms in 7 DAP spikes of silenced T 2 lines increased from 40 to 76% compared to the non-silenced control. The CKs cross talk with other phytohormones. Each of the tested yield-related traits is regulated by various up- or down-regulated TaCKX genes and phytohormones. Unexpectedly, increased expression of TaCKX2.1 in silent for TaCKX1 T 2 plants up-regulated trans- and cis-zeatin and trans-zeatin glucosides, determining lower TGW and chlorophyll content in flag leaves but higher grain yield. The coordinated effect of TaCKX1 silencing on expression of other TaCKX genes, phytohormone levels in 7 DAP spikes and yield-related traits in silenced T 2 lines is presented. One-sentence summary Different levels of TaCKX1 silencing influence various models of coordinated expression of TaCKX genes and phytohormone levels in 7 DAP spikes, as well as yield parameters.