The circadian-controlled PIF8-BBX28 module regulates petal senescence in rose flowers by governing mitochondrial ROS homeostasis at night

Authors: ZHANG, YI - China Agricultural University; WU, ZHICHENG - China Agricultural University; FENG, MING - China Agricultural University; CHEN, JIWEI - China Agricultural University; QIN, MEIZHU - China Agricultural University; WANG, WENRAN - China Agricultural University; BAO, YING - China Agricultural University; XU, QIAN - China Agricultural University; YE, YING - China Agricultural University; MA, CHAO - China Agricultural University; Jiang, Cai-Zhong; GAN, SU-SHENG - Cornell University; ZHOU, HOUGAO - China Agricultural University; CAI, YOUMING - China Agricultural University; HONG, BO - China Agricultural University; GAO, JUNPING - China Agricultural University; MA, NAN - China Agricultural University

Submitted to: The Plant Cell
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/19/2021
Publication Date: 5/27/2021
Citation: Zhang, Y., Wu, Z., Feng, M., Chen, J., Qin, M., Wang, W., Bao, Y., Xu, Q., Ye, Y., Ma, C., Jiang, C., Gan, S., Zhou, H., Cai, Y., Hong, B., Gao, J., Ma, N. 2021. The circadian-controlled PIF8-BBX28 module regulates petal senescence in rose flowers by governing mitochondrial ROS homeostasis at night. The Plant Cell. 33(8):2716-2735. https://doi.org/10.1093/plcell/koab152.
DOI: https://doi.org/10.1093/plcell/koab152

Interpretive Summary: Senescence is the final stage in the development of a plant organ and usually leads to programmed death of all constituent cells. Accordingly, senescence is a tightly controlled process during which plants remobilize nutrients from senescing leaves or petals to other developing organs. This important and unique process is coordinately regulated by various internal and external factors, including developmental status, reactive oxygen species (ROS) signaling, phytohormones, the circadian clock and light. ROS play a key role in regulating plant organ senescence. ROS are generated as byproducts of various metabolic processes that occur in organelles with a highly oxidizing metabolic activity or an intense rate of electron flow, such as chloroplasts, mitochondria, peroxisomes and microbodies in plant cells. ROS are commonly toxic to cells, although some, such as H2O2, are important regulatory molecules that are broadly involved in multiple cell signaling pathways. Therefore, cellular ROS homeostasis must be tightly controlled to protect cells from dysfunction and death. The main sources of ROS generation in petals are most likely the mitochondria, mainly through electron transfer from components of the respiratory chain, representing a process fundamentally different from that in leaves. Respiration rate continues to increase until flowers wilt, implying that respiration-produced ROS might be involved in petal senescence in rose. Although ROS is considered to play an important role in petal senescence, the regulatory mechanism behind ROS homeostasis in petals has been largely unknown. Here, we show that H2O2 levels in rose (Rosa hybrida) petals follow a circadian rhythm and peak at night, which is consistent with the pattern of respiration rate in petals. Our combined physiological, biochemical and genetic approaches revealed that a circadian regulatory module consisting of a phytochrome-interacting factor (RhPIF8) and a B-box gene (RhBBX28) play critical roles in H2O2 homeostasis in petals and consequent petal senescence. Furthermore, we demonstrated that RhBBX28 directly repressed the expression of RhSDH1 (SUCCINATE DEHYDROGENASE 1), which encodes the central subunit of mitochondrial respiratory complex II. Together, our results establish a regulatory pathway for the circadian-regulated mitochondrial ROS homeostasis in petals, thus providing a better understanding of the underlying mechanism of ROS homeostasis in non-photosynthetic organs.

Technical Abstract: Reactive oxygen species (ROS) are unstable reactive molecules that are toxic to cells. Regulation of ROS homeostasis is crucial to protect cells from dysfunction, senescence and death. In plant leaves, ROS are mainly generated from chloroplasts and are tightly temporally restricted by the circadian clock. However, little is known about how ROS homeostasis is regulated in non-photosynthetic organs, such as petals. Here, we showed that H2O2 levels exhibit typical circadian rhythmicity in rose (Rosa hybrida) petals, consistent with the measured respiratory rate. RNA-seq and functional screening identified a B-box gene, RhBBX28, whose expression was associated with H2O2 rhythms. Silencing RhBBX28 accelerated flower senescence and promoted H2O2 accumulation at night in petals, while overexpression of RhBBX28 had the opposite effects. RhBBX28 influenced the expression of various genes related to respiratory metabolism, including the TCA cycle and glycolysis, and directly repressed the expression of RhSDH1 (SUCCINATE DEHYDROGENASE1), which plays a central role in mitochondrial ROS homeostasis. We also found that RhPIF8 (PHYTOCHROME INTERACTING FACTOR8) could activate RhBBX28 expression to control H2O2 levels in petals and thus flower senescence. Our results indicate that the circadian- controlled RhPIF8-RhBBX28 module is a critical player that controls flower senescence by governing mitochondrial ROS homeostasis in rose.