PUB8 modulates early seedling growth of Arabidopsis

In plants, the phytohormone abscisic acid (ABA) plays a vital role in repressing seed germination and subsequent early seedling establishment, and ABA-INSENSITIVE 3 (ABI3) and ABI5 are two essential regulators of these developmental processes. Although remarkable progress has been achieved in recent years, the precise mechanism of how ABA signaling is attenuated still need to be further explored.

In a study published in Plant Physiology, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences found that PLANT U-BOX 8 (PUB8), a U-box E3 ubiquitin ligase, physically interacted with ABI3 and ABI5 and negatively regulated ABA responses during early Arabidopsis (Arabidopsis thaliana) seedling growth.

Loss-of-function pub8 mutants were hypersensitive to ABA-inhibited cotyledon greening, while lines overexpressing PUB8 with low levels of ABI5 protein abundance were insensitive to ABA, according to the researchers.

The researchers conducted genetic analysis and found that the ABA-sensitive phenotype of pub8 was dependent on ABI3 and ABI5, implying that PUB8 functioned upstream of ABI3 and ABI5 to regulate ABA responses. PUB8 negatively mediated ABA signaling to promote early seedling establishment in Arabidopsis by inhibiting the accumulation of endogenous ABI5 protein.

They then carried out biochemical analyses and found that PUB8 can associate with ABI3 and ABI5 for degradation through the ubiquitin-mediated 26S proteasome pathway both in vivo and in vitro.

Further phenotypic analyses showed that PUB8 dampened the function of ABI5 in planta. Disruption of PUB8 enhanced the ABA-hypersensitive phenotype of the transgenic seedlings overexpressing ABI5, whereas the overexpression of PUB8 attenuated the ABA-hypersensitive phenotype of the ABI5-overexpressing seedlings.

"Our results show that a PUB8-mediated ubiquitination proteasome pathway participates in regulating ABA signaling to precisely control early seedling establishment after seed germination," said Hu Yanru of XTBG.