Identification and expression of cytokinin signaling and meristem identity genes in sulfur deficient grapevine (Vitis vinifera L.)

In plants, cytokinin (CK) perception and signaling pathway is composed by a histidine kinase receptor (HK) and a response regulator (RR), the signal being mediated by a histidine phosphotransfer (HPt), as described in Arabidopsis, maize and rice. From database searches we identified in grapevine three HKs, three HPs, four A-type RRs and six B-type RRs, suggesting a common mechanism for grapevine. The phylogeneticanalysis of these Vitis genes showed a variable but high degree of homology with Arabidopsis sequences. When sulfate was withdrawn from the culture medium(-S) of in vitro Vitis shoots, we assessed a significant reduction in shoot branching. To ascertain the crosstalk of S status with CK signaling in grapevine, control and -S grown shoots and control, -S and -CK cell suspensions were used as experimental systems. Real-time PCR was elected to quantify the expression of key genes. The expression of CK receptor genes was downregulated in -S cells while not affected in -CK cells. In differentiated shoots no response to -S was observed on those genes. A-type VvRRa4 was downregulated in -S or -CK cells while Vitis B-type RRs did not respond either to CK or S starvation. The results suggest that Vitis CK signaling pathway is affected by -S, although differently according to the model system. Transcription of Vitis apical meristem-identity genes VvWUS, VvCLV and VvSTM and axillary meristem genes VvBRC1, VvBRC2, VvLAS, VvRAX and VvREV was estimated and VvSTM and VvLAS showed to be downregulated in -S. Then, the expression levels of VvSTM and VvLAS make them strong candidates to be associated with the branching pattern of Vitis shoots in -S.