ArabidopsisCytokinin Receptor Mutants Reveal Functions in Shoot Growth, Leaf Senescence, Seed Size, Germination, Root Development, and Cytokinin Metabolism

Abstract

We used loss-of-function mutants to study three Arabidopsis thaliana sensor histidine kinases, AHK2, AHK3, and CRE1/ AHK4, known to be cytokinin receptors. Mutant seeds had more rapid germination, reduced requirement for light, and decreased far-red light sensitivity, unraveling cytokinin functions in seed germination control. Triple mutant seeds were more than twice as large as wild-type seeds. Genetic analysis indicated a cytokinin-dependent endospermal and/or maternal control of embryo size. Unchanged red light sensitivity of mutant hypocotyl elongation suggests that previously reported modulation of red light signaling by A-type response regulators may not depend on cytokinin. Combined loss of AHK2 and AHK3 led to the most prominent changes during vegetative development. Leaves of ahk2 ahk3 mutants formed fewer cells, had reduced chlorophyll content, and lacked the cytokinin-dependent inhibition of dark-induced chlorophyll loss, indicating a prominent role of AHK2 and, particularly, AHK3 in the control of leaf development. ahk2 ahk3 double mutants developed a strongly enhanced root system through faster growth of the primary root and, more importantly, increased branching. This result supports a negative regulatory role for cytokinin in root growth regulation. Increased cytokinin content of receptor mutants indicates a homeostatic control of steady state cytokinin levels through signaling. Together, the analyses reveal partially redundant functions of the cytokinin receptors and prominent roles for the AHK2/ AHK3 receptor combination in quantitative control of organ growth in plants, with opposite regulatory functions in roots and shoots.

Sugar-Hormone Cross-Talk in Anthocyanin Biosynthesis

“…CK signaling, which involves a phosphorelay mechanism similar to the bacterial two-component system (Ferreira and Kieber, 2005), is mediated by sensor His kinases (AHKs), Hiscontaining phosphotransfer proteins (AHPs), and type-B and -A response regulators (ARRs) (Hwang and Sheen, 2001). AHKs are positive and functionally overlapping regulators of CK signaling (Higuchi et al, 2004;Nishimura et al, 2004;Riefler et al, 2006). Several Arabidopsis CK signaling pathway mutants have been used to investigate the role of CK in the regulation of anthocyanin accumulation.…”

Arabidopsis ABCG14 protein controls the acropetal translocation of root-synthesized cytokinins

“…These morphological alterations are reminiscent of the symptoms of cytokinin deficiency in the cytokinin receptor mutant lines ahk2/ahk3 (ref. 21) and in plants overexpressing Arabidopsis CYTOKININ OXIDASE (AtCKX) 22 , implying that both mutants might have defective cytokinin signalling pathways or responses. Transformation with a genomic DNA sequence comprising the native AtABCG14 promoter, exons and introns completely restored wild-type morphology in these mutant lines ( Supplementary Fig.…”

Isolation and initial characterization of GW5, a major QTL associated with rice grain width and weight

“…The mutant protein encoded by the da1-1 allele has negative activity toward DA1 and a DA1-related protein, and overexpression of da1-1 cDNA dramatically increases the seed and organ size of wildtype plants [15]. In addition, plant hormones and their receptors [16][17][18][19], as well as the nutritional synthesis pathway [20,21], have also been implicated in the development of grain in crops and other plant species. For example, the rice dwarf11 mutant has a defect in a novel cytochrome P450, which shares homology with enzymes involved in brassinosteroid biosynthesis, and bears seeds of reduced length [17].…”