A new component of ethylene signaling in regulation of plant root growth in Arabidopsis

Ph.D.Plant roots provide structural support and help plants absorb water and nutrients for growth and development. The plant root system is not only shaped by external environmental signals, but also modulated by internal hormone cues. The gaseous phytohormone ethylene regulates plant root growth and development by inhibiting cell elongation, promoting root hair differentiation, as well as inhibiting cell proliferation of the root meristem.The ethylene signaling pathway has been well studied in the model plant Arabidopsis thaliana. But currently, there is no evidence about physical interaction between the upstream ER-localized ethylene receptors/CTR1/EIN2 complex and the downstream EIN3/EIL1 transcription factors. In this project, we characterized a new gene, Long Primary Root, playing a positive role in ethylene inhibition of plant root growth. The loss-of-function T-DNA insertional mutant lpr showed a long primary root phenotype with fewer root hairs compared to the wild type plant, while the transgenic LPR overexpression lines have shorter primary roots. LPR is mainly expressed in the root elongation zone and differentiation zone at the young seedling stage. We observed that the lpr mutant was insensitive to the phytohormone ethylene but only the roots were insensitive to treatment with ACC, the precursor of ethylene. However, both the roots and shoots have normal responses to jasmonic acid (JA). Later, we demonstrated that LPR interacts with the ethylene receptors (Ethylene Response Sensor 1 (ERS1) and ERS2) and the Jasmonate-Zim-domain proteins (JAZ6 and JAZ8) both in vitro and in vivo. The lpr mutant phenotypes can be rescued by either mutations of JAZ6 or JAZ8, or overexpression of EIN3 or MYC2, two key transcription factors that regulate plant root growth and development by acting downstream of JAZ proteins. Interestingly, LPR appears to promote the proteasomal degradation of JAZ proteins, probably through interacting with its two paralogous E3 ligases, Plant U-Box protein 32 (PUB32) and PUB33. All these data indicate that LPR plays a positive role in ethylene inhibition of plant root growth and development by promoting the degradation of the transcription repressors JAZ proteins. Our results from this project provide a link between the upstream ethylene receptors/CTR1/EIN2 complex and the downstream EIN3/EIL1 transcription factors, which may establish a new signaling pathway between the ethylene receptors and core ethylene transcription factors in ethylene regulation of plant root growth and development.植物根係為植物的生長和發育提供結構支持，吸收水分和養分。植物根係的生長發育不僅受外部環境因素的影響，還受內部激素信號的調節。氣態植物激素乙烯通過抑制細胞伸長，促進根毛分化以及抑制根分生組織的細胞增殖來調節植物根的生長和發育。乙烯信號傳導途徑已在模式植物擬南芥中進行了充分研究。但目前沒有證據表明上游位於內質網的乙烯受體/CTR1/EIN2複合物與下游的EIN3/EIL1轉錄因子之間存在直接的相互作用。在這個課題中，我們闡述了一個新基因LPR在乙烯抑制植物根系生長中起正調節作用。與野生型植物相比，功能缺失型TDNA插入突變體lpr具有比較長的主根和較少的根毛，而轉基因LPR過表達株系則具有較短的主根。在幼苗期LPR蛋白主要在根的伸長區和分化區表達。我們觀察到lpr突變體對植物激素乙烯不敏感，但只有根對用乙烯前體ACC處理不敏感。然而，根和芽都對JA有正常反應。之后我們证实了LPR在體外和體內都可以與乙烯受體ERS1和ERS2）和茉莉酸ZIM结构域蛋白（JAZ6和JAZ8）相互作用。並且lpr突變體表型可以被JAZ6或JAZ8的突變或EIN3或MYC2的過表達來恢復，EIN3和MYC2是植物根生長和發育調節中位於JAZ蛋白下游的兩個關鍵轉錄因子。有趣的是，LPR似乎可能通過與其兩種旁系同源E3連接酶，植物UBox蛋白32PUB32和PUB33相互作用來促進JAZ蛋白的蛋白酶體降解。所有這些數據表明LPR通過促進轉錄抑制子JAZ蛋白的降解在乙烯抑制植物根生長和發育中起正調節作用。我們的這個項目的結果提供了上游乙烯受體/CTR1/EIN2複合物和下游EIN3/EIL1轉錄因子的鏈接，因而此課題可能在內質網定位的乙烯受體和核定位的乙烯轉錄因子之間，建立一條新的乙烯調節植物根生長和發育的信號轉導途徑。Yang, Wei.Thesis Ph.D. Chinese University of Hong Kong 2019.Includes bibliographical references (leaves 99-107).Abstracts also in Chinese.Title from PDF title page (viewed on 20, May 2021)