Figure. (a) An embryo from wildtype. (b) An arrested zygote fromzyg1mutant. (c) CYCB1-GUS was over-accumulated in aborted ovules of aZYG1/zyg1silique. (d) The differential ratios of ovules accumulating CYCB1-GUS from the wildtype orZYG1/zyg1plants.(e) and (f) GUS staining of ovules from wildtype orZYG1/zyg1transgenic plants withCYCB1-GUS.

Zygote activation is the critical event of starting a new life in both plants and animals. Through double fertilization, two sperm cells delivered by the pollen tube fuse with the egg and the central cell, to produce an embryo and an endosperm, respectively. Although many genes involved in zygotic embryogenesis have been identified in plants, themechanismunderlying zygote activation in plants is largely unknown. Recently, through genetic analyses of zygote-arrest mutants, Chun-Ming Liu’s group discovered thatZYGOTE-ARREST 1(ZYG1) gene that encodes the APC11 subunit of the anaphase promoting complex/cyclosome (APC/C) is critical for the initiation of the zygote division.ZYG1is primarily expressed in mitosis-active tissues such as embryo,endosperm, root andshootapical meristems. Mutation ofZYG1led to zygote-lethal and CYCB1-GUS over-accumulation phenotypes. Biochemical studies revealed that ZYG1,together with E1 and E2, constitutes an active E3 ligase that is able to ubiquitinate CYCB1;1, leading to the degradation of the CYCB1;1.Expression ofCYCB1;1with a mutated D-box under an embryo-specific promoter also showed zygote-lethal phenotype similar tozyg1. It was also showed that mutations of several other APC subunits led to zygote lethal phenotype as well. These results demonstrated that APC/C initiates zygote activation in plants through promoting CYCB1;1degradation.This work is publishedinPlant Journalon April 6^th, 2016. Two PhD students, Guo Lei and Jiang Li, are co-first authored the paper.This study was supported by grants from the National Basic Research Program of China, andtheSino-Dutch International Collaboration Project.