From Survival to Productivity Mode: Cytokinins Allow Avoiding the Avoidance Strategy under Stress Conditions

Growth retardation and stress-induced premature plant senescence are accompanied by a severe yield reduction and raise a major agro-economic concern. To improve biomass and yield in agricultural crops under mild stress conditions, the survival must be changed to productivity mode. Our previous successful attempts to delay premature senescence and growth inhibition under abiotic stress conditions by autoregulation of cytokinins (CKs) levels constitute a generic technology toward the development of highly productive plants. Since this technology is based on the induction of CKs synthesis during the age-dependent senescence phase by a senescence-specific promoter (SARK), which is not necessarily regulated by abiotic stress conditions, we developed autoregulating transgenic plants expressing the IPT gene specifically under abiotic stress conditions. The Arabidopsis promoter of the stress-induced metallothionein gene (AtMT) was isolated, fused to the IPT gene and transformed into tobacco plants. The MT::IPT transgenic tobacco plants displayed comparable elevated biomass productivity and maintained growth under drought conditions. To decipher the role and the molecular mechanisms of CKs in reverting the survival transcriptional program to a sustainable plant growth program, we performed gene expression analysis of candidate stress-related genes and found unexpectedly clear downregulation in the CK-overproducing plants. We also investigated kinase activity after applying exogenous CKs to tobacco cell suspensions that were grown in salinity stress. In-gel kinase activity analysis demonstrated CK-dependent deactivation of several stress-related kinases including two of the MAPK components, SIPK and WIPK and the NtOSAK, a member of SnRK2 kinase family, a key component of the ABA signaling cascade. A comprehensive phosphoproteomics analysis of tobacco cells, treated with exogenous CKs under salinity-stress conditions indicated that >50% of the identified phosphoproteins involved in stress responses were dephosphorylated by CKs. We hypothesize that upregulation of CK levels under stress conditions desensitize stress signaling cues through deactivation of kinases that are normally activated under stress conditions. CK-dependent desensitization of environmental stimuli is suggested to attenuate various pathways of the avoidance syndrome including the characteristic growth arrest and the premature senescence while allowing normal growth and metabolic maintenance.

植物生长迟缓与胁迫诱导早衰往往伴随严重减产，成为农业生产中亟待解决的重大经济关切问题。为在轻度胁迫条件下提升农作物的生物量与产量，需将植物的生存模式切换为生产模式。此前我们通过自主调控细胞分裂素（cytokinins, CKs）水平，成功延缓了非生物胁迫下的植物早衰与生长抑制，该技术可作为培育高产作物的通用技术手段。由于该技术依赖衰老特异性启动子（SARK）在年龄依赖的衰老阶段诱导细胞分裂素合成，并不受非生物胁迫调控，因此我们开发了仅在非生物胁迫条件下特异性表达IPT基因的自主调控转基因植株。我们分离得到拟南芥胁迫诱导型金属硫蛋白基因（AtMT）的启动子，将其与IPT基因融合后转化烟草。实验结果显示，MT::IPT转基因烟草在干旱条件下可维持正常生长，且生物量产量实现了可观提升。为解析细胞分裂素将植物的生存型转录程序逆转为可持续生长型转录程序的作用与分子机制，我们对候选胁迫相关基因进行了基因表达分析，意外发现细胞分裂素过量合成的植株中这些基因均出现显著下调。我们还对盐胁迫下培养的烟草悬浮细胞施加外源细胞分裂素后，检测了其激酶活性变化。凝胶内激酶活性分析结果表明，细胞分裂素可依赖式灭活多种胁迫相关激酶，包括两类丝裂原活化蛋白激酶（MAPK）组分SIPK、WIPK，以及脱落酸（ABA）信号通路关键成员SnRK2激酶家族的NtOSAK。对盐胁迫下经外源细胞分裂素处理的烟草细胞开展的全面磷酸化蛋白质组学分析显示，超过50%已鉴定的应激响应磷酸化蛋白均可被细胞分裂素去磷酸化。我们提出如下假说：胁迫条件下细胞分裂素水平上调，可通过灭活胁迫条件下正常激活的激酶，使植物胁迫信号通路脱敏。细胞分裂素依赖的环境刺激脱敏作用，可抑制各类避逆综合征通路——包括典型的生长停滞与早衰现象，同时保障植物维持正常生长与代谢稳态。