Cytokinin membrane receptors modulate transcript accumulation of plastid encoded genes

Abstract

Cytokinins (CKs) have a strong influence on plastid development. We used the Arabidopsis thaliana (L.) Heynh. loss-of function insertion mutants ahk2, ahk3 and ahk4 alone or in different combinations to examine the effect of individual membrane cytokinin (CK) receptors on the expression of plastid genes and nuclear genes for the plastid transcription machinery. Quantitative real-time PCR-analyses demonstrate that the CK membrane receptors have overlapping functions in regulating the levels of plastid house-keeping genes in 7-day-old seedlings, relative to the nuclear UBQ10 gene. In contrast, photosynthesis genes responded differently to AHK3 and AHK2 receptor inactivation and the results show a dominant role of the AHK2/AHK3 receptor combination in the CK responses. These differences could be, at least in part, mediated through the regulation by the nuclear-encoded RPOTp and RPOTmp genes for plastid phage-type RNA polymerases. CK positively regulated the transcript abundance of the nuclear encoded SIG2 and SIG6, and negatively that of SIG4 and SIG5 for sigma factors of the plastid transcription apparatus. We propose that these two sigma factors may affect the CK-controlled plastid gene expression.