Cytokinin signaling promotes salt tolerance by modulating shoot chloride exclusion in maize

Excessive accumulation of chloride(Cl^(-))in the aboveground tissues under saline conditions is harmful to crops.Increasing the exclusion of Cl^(-) from shoots promotes salt tolerance in various crops.However,the underlying molecular mechanisms remain largely unknown.In this study,we demonstrated that a type A response regulator(ZmRR1)modulates Cl^(-) exclusion from shoots and underlies natural variation of salt tolerance in maize.ZmRR1 negatively regulates cytokinin signaling and salt tolerance,likely by interacting with and inhibiting His phosphotransfer(HP)proteins that are key mediators of cytokinin signaling.A naturally occurring non-synonymous SNP variant enhances the interaction between ZmRR1 and ZmHP2,conferring maize plants with a salt-hypersensitive phenotype.We found that ZmRR1 undergoes degradation under saline conditions,leading to the release of ZmHP2 from ZmRR1 inhibition,and subsequently ZmHP2-mediated signaling improves salt tolerance primarily by promoting Cl^(-) exclusion from shoots.Furthermore,we showed that ZmMATE29 is transcriptionally upregulated by ZmHP2-mediated signaling under highly saline conditions and encodes a tonoplast-located Cl^(-) transporter that promotes Cl^(-) exclusion from shoots by compartmentalizing Cl^(-) into the vacuoles of root cortex cells.Collectively,our study provides an important mechanistic understanding of the cytokinin signaling-mediated promotion of Cl^(-) exclusion from shoots and salt tolerance and suggests that genetic modification to promote Cl^(-) exclusion from shoots is a promising route for developing salt-tolerant maize.

Control of rice ratooning ability by a nucleoredoxin that inhibits histidine kinase dimerization to attenuate cytokinin signaling in axillary buds

Rice ratooning,the fast outgrowth of dormant buds on stubble,is an important cropping practice in rice production.However,the low ratooning ability(RA)of most rice varieties restricts the application of this cost-efficient system,and the genetic basis of RA remains unknown.In this study,we dissected the genetic architecture of RA by a genome-wide association study in a natural rice population.Rice ratooning ability 3(RRA3),encoding a hitherto not characterized nucleoredoxin involved in reduction of disulfide bonds,was identified as the causal gene of a major locus controlling RA.Overexpression of RRA3 in rice significantly accelerated leaf senescence and reduced RA,whereas knockout of RRA3 significantly delayed leaf senescence and increased RA and ratoon yield.We demonstrated that RRA3 interacts with Oryza sativa histidine kinase 4(OHK4),a cytokinin receptor,and inhibits the dimerization of OHK4 through disulfide bond reduction.This inhibition ultimately led to decreased cytokinin signaling and reduced RA.In addition,variations in the RRA3 promoter were identified to be associated with RA.Introgression of a superior haplotype with weak expression of RRA3 into the elite rice variety Guichao 2 significantly increased RA and ratoon yield by 23.8%.Collectively,this study not only uncovers an undocumented regulatory mechanism of cytokinin signaling through de-dimerization of a histidine kinase receptor-but also provides an eximious gene with promising value for ratoon rice breeding.

Regulation of Meristem Size by Cytokinin Signaling

The plant meristems possess unique features that involve maintaining the stem cell populations while providing cells for continued development. Although both the primary shoot apical meristem （SAM） and the root apical meristem （RAM） are specified during embryogenesis, post-embryonic tissue proliferation is required for their full establishment and maintenance throughout a plants＇ life. The phytohormone cytokinin （CK） interacts with other systemic signals and is a key regulator of meristem size and functions. The SAM and the RAM respond to CK stimulations in different manners： CK promotes tissue proliferation in the SAM through pathways dominated by homeobox transcription factors, including the class I KNOX genes, ST^P, and WUS; and curiously, it favors proliferation at low levels and differentiation at a slightly higher concentration in the RAM instead. Here we review the current understanding of the molecular mechanisms underlying CK actions in regulating meristematic tissue proliferation.

The Altered Responses of a New Mutant long life span 1 to Cytokinin in Arabidopsis thaliana

Cytokinins are a class of essential plant hormones regulating plant growth and development. Although the two-component phosphorelay pathway of cytokinin has been well characterized, the intact cytokinin responses regulation picture still needs to be fully depicted. Here we report a new mutant, long life span I （llsl）, which displays dwarf stature, curled leaves, numerous axillary branches and nearly 5-month life span. Exogenous cytokinin could not recover the phenotypes of the mutant. Moreover, mutation in Ilsl suppressed the cytokinin-responsive phenotypes, including root and hypocotyl growth inhibition, anthocyanin accumulation, metaxylem promotion in primary root development. The induction of cytokinin-responsive genes, ARR5, AHP5, and CKX3, was also suppressed in llsl. According to quantitative RT-PCR （qRT-PCR） and microarray results, the basal expression of positive factors AHP5, ARR1, and ARRIO were down-regulated, while the negative factors ARR4 and ARR5 were up-regulated. Our results suggested that LLS1 gene might be involved in the regulation of cytokinin signaling. It was mapped to chromosome 4 where no other cytokinin relevant gene has been reported.

Involvement of cytokinins in STOP1-mediated resistance to proton toxicity

STOP1(sensitive to proton rhizotoxicity1)is a master transcription factor that governs the expression of a set of regulatory and structural genes involved in resistance to aluminum and low pH(i.e.,proton)stresses in Arabidopsis.However,the mechanisms and regulatory networks underlying STOP1-mediated resistance to proton stresses are largely unclear.Here,we report that low-pH stresses severely inhibited root growth of the stop1 plants by suppressing root meristem activities.Interestingly,the stop1 plants were less sensitive to exogenous cytokinins at normal and low pHs than the wild type.Significantly,low concentrations of cytokinins promoted root growth of the stop1 mutant under low-pH stresses.Moreover,lateral and adventitious root formation was stimulated in stop1 and by low-pH stresses but suppressed by cytokinins.Further studies of the expression patterns of a cytokinin signaling reporter suggest that both the loss-of-function mutation of STOP1 and low-pH stresses suppressed cytokinin signaling outputs in the root.Furthermore,the expression of critical genes involved in cytokinin biosynthesis,biodegradation,and signaling is altered in the stop1 mutant in response to low-pH stresses.In conclusion,our results reveal a complex network of resistance to low-pH stresses,which involves coordinated actions of STOP1,cytokinins,and an additional low-pH-resistant mechanism for controlling root meristem activities and root growth upon proton stresses.