After-Ripening Induced Transcriptional Changes of Hormonal Genes in Wheat Seeds: The Cases of Brassinosteroids, Ethylene, Cytokinin and Salicylic Acid

Maintenance and release of seed dormancy is regulated by plant hormones; their levels and seed sensitivity being the critical factors. This study reports transcriptional regulation of brassinosteroids (BR), ethylene (ET), cytokinin (CK) and salicylic acid (SA) related wheat genes by after-ripening, a period of dry storage that decays dormancy. Changes in the expression of hormonal genes due to seed after-ripening did not occur in the anhydrobiotic state but rather in the hydrated state. After-ripening induced dormancy decay appears to be associated with imbibition mediated increase in the synthesis and signalling of BR, via transcriptional activation of de-etiolated2, dwarf4 and brassinosteroid signaling kinase, and repression of brassinosteroid insensitive 2. Our analysis is also suggestive of the significance of increased ET production, as reflected by enhanced transcription of 1-aminocyclopropane-1-carboxylic acid oxidase in after-ripened seeds, and tight regulation of seed response to ET in regulating dormancy decay. Differential transcriptions of lonely guy, zeatin O-glucosyltransferases and cytokinin oxidases, and pseudo-response regulator between dormant and after-ripened seeds implicate CK in the regulation of seed dormancy in wheat. Our analysis also reflects the association of dormancy decay in wheat with seed SA level and NPR independent SA signaling that appear to be regulated transcriptionally by phenylalanine ammonia lyase, and whirly and suppressor of npr1 inducible1 genes, respectively. Co-expression clustering of the hormonal genes implies the significance of synergistic and antagonistic interaction between the different plant hormones in regulating wheat seed dormancy. These results contribute to further our understanding of the molecular features controlling seed dormancy in wheat.

种子休眠的维持与解除受植物激素调控，其中激素水平与种子对激素的敏感性是关键影响因素。本研究阐明了种子后熟作用（即打破休眠的干燥贮藏阶段）对小麦中与油菜素甾醇（BR）、乙烯（ET）、细胞分裂素（CK）及水杨酸（SA）相关基因的转录调控作用。种子后熟作用引发的激素基因表达变化并非发生在脱水存活状态下，而是出现在水合状态中。后熟作用诱导的休眠解除，似乎与吸胀作用介导的BR合成与信号转导增强相关：具体表现为对去黄化2（DET2）、矮秆4（DWF4）及油菜素甾醇信号激酶的转录激活，以及对油菜素甾醇不敏感2（BIN2）的转录抑制。本研究分析还表明，后熟种子中1-氨基环丙烷-1-羧酸氧化酶（ACO）的转录上调反映了乙烯生成量的增加，且种子对乙烯的响应受到严格调控，这两点在休眠解除过程中具有重要意义。休眠种子与后熟种子之间，lonely guy（LOG）基因、玉米素O-葡萄糖基转移酶、细胞分裂素氧化酶及伪响应调节因子的转录差异，暗示细胞分裂素参与调控小麦种子休眠过程。本分析还显示，小麦休眠解除与种子水杨酸（SA）水平及不依赖于NPR的SA信号通路相关，这两类调控过程分别受苯丙氨酸解氨酶、whirly基因以及npr1诱导型抑制因子1（SNI1）的转录调控。对激素基因的共表达聚类分析表明，不同植物激素间的协同与拮抗互作，在调控小麦种子休眠过程中发挥重要作用。本研究结果有助于进一步阐明调控小麦种子休眠的分子机制。