Table_5_Comparative functional analyses of PHR1, PHL1, and PHL4 transcription factors in regulating Arabidopsis responses to phosphate starvation.xlsx

To cope with phosphate (Pi) starvation, plants trigger an array of adaptive responses to sustain their growth and development. These responses are largely controlled at transcriptional levels. In Arabidopsis (Arabidopsis thaliana), PHOSPHATE RESPONSE 1 (PHR1) is a key regulator of plant physiological and transcriptional responses to Pi starvation. PHR1 belongs to a MYB-CC-type transcription factor family which contains 15 members. In this PHR1 family, PHR1/PHR1-like 1(PHL1) and PHL2/PHL3 form two distinct modules in regulating plant development and transcriptional responses to Pi starvation. PHL4 is the most closely related member to PHR1. Previously, using the phr1phl4 mutant, we showed that PHL4 is also involved in regulating plant Pi responses. However, the precise roles of PHL1 and PHL4 in regulating plant Pi responses and their functional relationships with PHR1 have not been clearly defined. In this work, we further used the phl1phl4 and phr1phl1phl4 mutants to perform comparative phenotypic and transcriptomic analyses with phr1, phr1phl1, and phr1phl4. The results showed that both PHL1 and PHL4 act redundantly and equally with PHR1 to regulate leaf senescence, Pi starvation induced-inhibition of primary root growth, and accumulation of anthocyanins in shoots. Unlike PHR1 and PHL1, however, the role of PHL4 in maintaining Pi homeostasis is negligible. In regulating transcriptional responses to Pi starvation at genomic levels, both PHL1 and PHL4 play minor roles when acts alone, however, they act synergistically with PHR1. In regulating Pi starvation-responsive genes, PHL4 also function less than PHL1 in terms of the number of the genes it regulates and the magnitude of gene transcription it affects. Furthermore, no synergistic interaction was found between PHL1 and PHL4 in regulating plant response to Pi starvation. Therefore, our results clarified the roles of PHL1 and PHL4 in regulating plant responses to Pi starvation. In addition, this work revealed a new function of these three transcription factors in regulating flowering time.

为应对磷酸盐（Pi）饥饿胁迫，植物会触发一系列适应性响应以维持自身生长发育。这些响应主要在转录水平上受到调控。在拟南芥（Arabidopsis thaliana）中，磷酸盐响应1（PHR1）是植物响应磷酸盐饥饿的生理与转录调控关键调控因子。PHR1属于包含15个成员的MYB-CC型转录因子家族。在该PHR1家族中，PHR1/PHR1类蛋白1（PHL1）与PHL2/PHL3分别形成两个独立模块，调控植物发育及对磷酸盐饥饿的转录响应。PHL4是与PHR1亲缘关系最近的家族成员。此前我们通过phr1phl4突变体的研究发现，PHL4同样参与调控植物的磷酸盐响应。然而，PHL1与PHL4在调控植物磷酸盐响应中的具体功能，以及它们与PHR1的功能关联尚未明确。本研究进一步利用phl1phl4与phr1phl1phl4突变体，并结合phr1、phr1phl1及phr1phl4突变体开展比较表型与转录组分析。结果显示，PHL1与PHL4均可分别与PHR1发挥冗余且等效的调控作用，共同调控叶片衰老、磷酸盐饥饿诱导的主根生长抑制，以及地上部花青素积累。然而与PHR1和PHL1不同的是，PHL4在维持磷酸盐稳态方面的作用可忽略不计。在全基因组层面调控磷酸盐饥饿的转录响应时，PHL1与PHL4单独发挥作用时功能微弱，但可与PHR1协同发挥功能。在调控磷酸盐饥饿响应基因方面，相较于PHL1，PHL4所调控的基因数量更少、对基因转录幅度的影响也更弱。此外，PHL1与PHL4在调控植物磷酸盐饥饿响应的过程中未发现协同互作。综上，本研究明确了PHL1与PHL4在调控植物磷酸盐饥饿响应中的功能。此外，本研究还揭示了这三类转录因子在调控开花时间方面的新功能。