Abstract
Co-ordination of auxin and cytokinin activities determines root meristem size during post-embryonic development. Calcineurin B-like proteins (CBLs) and their interacting protein kinases (CIPKs) constitute signaling modules that relay calcium signals. Here we report that CIPK25 is involved in regulating the root meristem size. Arabidopsis plants lacking CIPK25 expression displayed a short root phenotype and a slower root growth rate with fewer meristem cells. This phenotype was rescued by restoration of CIPK25 expression. CIPK25 interacted with CBL4 and -5, and displayed strong gene expression in the flower and root, except in the cell proliferation domain in the root apical meristem. Its expression in the root was positively and negatively regulated by auxin and cytokinin, respectively. The cipk25 T-DNA insertion line was compromised in auxin transport and auxin-responsive promoter activity. The cipk25 mutant line showed altered expression of auxin efflux carriers (PIN1 and PIN2) and an Aux/IAA family gene SHY2. Decreased PIN1 and PIN2 expression in the cipk25 mutant line was completely restored when combined with a SHY2 loss-of-function mutation, resulting in recovery of root growth. SHY2 and PIN1 expression was partially regulated by cytokinin even in the absence of CIPK25, suggesting a CIPK25-independent cytokinin signaling pathway(s). Our results revealed that CIPK25 plays an important role in the co-ordination of auxin and cytokinin signaling in root meristem development.
Highlights
Roots exhibit indeterminate growth which is regulated by external factors (Schenk and Jackson, 2002; Chapman et al, 2012)
We previously reported that overexpression of the chickpea CIPK25 (CaCIPK25) in tobacco (Nicotiana tabacum) enhanced root growth (Meena et al, 2015)
Our results suggest that CIPK25 negatively regulates SHY2 expression to maintain root meristem size
Summary
Roots exhibit indeterminate growth which is regulated by external factors (Schenk and Jackson, 2002; Chapman et al, 2012). The cells that surround the QC constitute the stem cell niche (SCN) that gives rise to the different cell files of the root by cell proliferation. The proliferation rate of these cells decreases as they move upwards; at one point they show a very low probability of division and begin to elongate rapidly at the elongation zone (EZ). The region between the QC and the first elongated cortex cell was considered as the RAM in several studies. A small domain of the RAM bordering the EZ shows a very low probability of cell division, but is yet to start cell elongation and is referred to as the transition domain (TD).The cells in this domain of the RAM grow at the same relative rate as the cells in the proliferative domain and pass to the EZ. As the rate of cell division is different for each cell type, the boundary of the TD towards the QC cannot be well
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