ARR5 and ARR6 Mediate Tissue Specific Cross-talk between Auxin and Cytokinin in <i>Arabidopsis</i>

Auxin and cytokinin interaction plays an essential role in a wide range of plant growth and developmental processes. The interaction consequence of the two hormones is highly tissue specific. The molecular mechanisms underlying the tissue specificity are largely unknown. Here we show that the cytokinin signaling pathway key components ARR5 and ARR6 respond to auxin with a highly tissue specific and contrasted pattern in Arabidopsis seedlings and calli in the presence of cytokinin. Our results suggest that the two highly homologous but functionally distinct genes, ARR5 and ARR6, play a critical role in mediating tissue specific interaction between auxin and cytokinin.

Genome-wide comparative analysis of type-A Arabidopsis response regulator genes by overexpression studies reveals their diverse roles and regulatory mechanisms in cytokinin signaling

Cytokinin is a critical growth regulator for various aspects of plant growth and development. In Arabidopsis, cytokinin signaling is mediated by a two-component system-based phosphorelay that transmits a signal from the receptors, through histidine phosphotransfer proteins, to the downstream response regulators （ARRs）. Of these ARRs, type-A ARR genes, whose transcription can be rapidly induced by cytokinin, act as negative regulators of eytokinin signaling. However, because of functional redundancy, the function of type-A ARR genes in plant growth and development is not well understood by analyzing loss-of-function mutants. In this study, we performed a comparative functional study on all ten type-A ARR genes by analyzing transgenic plants overexpressing these ARR genes fused to a MYC epitope tag. Overexpression of ARR genes results in a variety of cytokinin-associated phenotypes. Notably, overexpression of different ARR transgenes causes diverse phenotypes, even between phylogenetically closely-related gene pairs, such as within the ARR3-ARR4 and ARR5-ARR6 pairs. We found that the accumulation of a subset of ARR proteins （ARR3, ARR5, ARR7, ARR16 and ARR17; possibly ARR8 and ARR15） is increased by MG132, a specific proteasomal inhibitor, indicating that stability of these proteins is regulated by proteasomal degradation. Moreover, similar to that of previously characterized ARR5, ARR6 and ARR7, stability of ARR16 and ARR17, possibly including ARR8 and ARR15, is regulated by cytokinin. These results suggest that type-A ARR proteins are regulated by a combinatorial mechanism involving both the cytokinin and proteasome pathways, thereby executing distinctive functions in plant growth and development.

The Antagonistic Action of Abscisic Acid and Cytokinin Signaling Mediates Drought Stress Response in Arabidopsis

As sessile organisms, plants encounter a variety of environmental stresses and must optimize their growth for survival. Abscisic acid （ABA） and cytokinin antagonistically regulate many developmental processes and environmental stress responses in plants. However, the molecular mechanism underlying this antag- onism remains poorly defined. In this study, we demonstrated that Sucrose nonfermentingl-related kinases SnRK2.2, SnRK2.3, and SnRK2.6, the key kinases of the ABA signaling pathway, directly interact with and phosphorylate type-A response regulator 5 （ARR5）, a negative regulator of cytokinin signaling. The phosphorylation of ARR5 Ser residues by SnRK2s enhanced ARR5 protein stability. Accord- ingly, plants overexpressing ARR5 showed ABA hypersensitivity and drought tolerance, and these pheno- types could not be recapitulated by overexpressing a non-phosphorylated ARR5 mimic. Moreover, the type-B ARRs, ARR1, ARR11 and ARR12, physically interacted with SnRK2s and repressed the kinase activ- ity of SnRK2.6. The arrl, 11,12 triple mutant exhibited hypersensitivity to ABA. Genetic analysis demon- strated that SnRK2s act upstream of ARR5 but downstream of ARR1, ARR11 and ARR12 in mediating ABA response and drought tolerance. Taken together, this study unravels the antagonistic actions of several molecular components of the ABA and cytokinin signaling pathways in mediates drought stress response, providing significant insights into how plants coordinate growth and drought stress response by integrating multiple hormone pathways.