A female in vivo haploid-induction system via mutagenesis of egg cell-specific peptidases

Crop breeding schemes can be significantly accelerated by using(doubled)haploid plants.In vivo haploid induction has been applied in plant breeding for decades but is still not available for all crops and genotypes,and haploidization rates are generally very low.Therefore,methodological improvements to and new concepts for haploidization are required.Here,we report a novel system for the induction of haploid plants by mutating genes encoding egg cell-specific aspartic endopeptidases(ECSs).We show that after successful sperm–egg cell fusion,ECSs play a critical role to ensure male and female nucleus fusion after fertilization.The ecs1 ecs2 double mutant can induce haploids by both selfing and hybridization in Arabidopsis and ECS mutation is also capable of producing haploids in rice.In summary,our study develops a novel approach for maternal haploidization and provides new insights into the molecular basis of fertilization.

Do egg cell-secreted aspartic proteases promote gamete attachment?

During the double fertilization process of flowering plants(angiosperms),pollen tubes transport a pair of immobile sperm cells toward and inside the embryo sac for fusion with egg and central cell,respectively.Male–female gamete interactions during fertilization are critical for reproductive success and seed development,and thus have interested biologists for decades.Compared to animals,little is known about gamete interaction mechanisms in angiosperms.

A VPE-like protease NtTPE8 exclusively expresses in the integumentary tapetum and is involved in seed development

Programmed cell death(PCD)is an essential process for development,and shows conserved cytological features in both plants and animals.Caspases are well-known critical components of the PCD machinery in animals.However,currently few typical counterparts have been identified in plants and only several caspase-like proteases are known to be involved in plant PCD,indicating the existence of great challenge for confirming new caspase-like proteases and elucidating the mechanisms regulating plant PCD.Here,we report a novel cysteine protease,NtTPE8,which was extracted from tobacco seeds and confirmed as a new caspase-like protease.Recombinant NtTPE8 exhibited legumain and caspase-like proteolytic activities,both of which could be inhibited by the pan-caspase inhibitor(Z-VAD-FMK).Notably,NtTPE8 possessed several caspase activities and the capacity to cleave the cathepsin H substrate FVR,indicating a unique character of NtTPE8.NtTPE8 was exclusively expressed in the integumentary tapetum and thus,is the first specific molecular marker reported to date for this cell type.Downregulation of NtTPE8 caused seed abortion,via disturbing early embryogenesis,indicating its critical role in embryogenesis and seed development.In conclusion,we identified a novel caspase-like cysteine protease,NtTPE8,exclusively expressed in the integumentary tapetum that is involved in seed development.

OsGCD1,a novel player in rice intine construction

In angiosperms,pollen development is an intricate indispensable process for successful sexual reproduction,which involves accurately and tightly controlled regulations and requires numerous genes expressed in both sporophytic and gametophytic tissues of the anther(McCormick,2004).Understanding the molecular mechanism underlying pollen development is of great significance for the construction of male sterile lines and crop breeding.However,many critical issues concerning pollen development still need to be elucidated.Recently,targeted editing of pollen development-related genes mediated by CRISPR/Cas9 system has provided powerful tools to facilitate the relevant studies in crops to figure out these issues(Li et al., 2016,2017).

The Maternal-to-Zygotic Transition in Higher Plants

During early embryogenesis in mammals and higher plants, the maternal- to-zygotic transition （MZT） marks the turnover of developmental control from maternal products to de novo zygotic genome transcripts. Intensive studies in animals indicate that early embryonic development is largely maternally controlled. In recent years, the MZT has drawn the attention of botanists, as it is important for understanding the mechanism of embryogenesis and hybrid vigor. In this study, we present a brief overview of some aspects of the MZT in flowering plants. Based on what we have learned from Nicotiana tabacum, we hypothesize that the MZT occurs before zygotic cell division and that the development of the fertilized egg cell in flowering plants can be divided into two phases： the zygote stage, which is mainly controlled maternally, and the one-celled proembryo stage, in which zygotic genome activation （ZGA） occurs and is required for zygote division.

Pollen tube,a one-way special train for special passengers

Double fertilization is a specific characteristic of angiosperms,in which two sperm cells fuse with the egg and central cell respectively[1].Different from that of animals,the two sperm cells of angiosperms have lost their motility.Therefore,to fulfill the double fertilization,the two immobile sperm cells have to be sent to the female games by a specific structure,the pollen tube.Upon landing on the stigma of the pistil,the pollen grain germinates and devel-

New insights into cell-cell communications during seed development in flowering plants

The evolution of seeds is a major reason why flowering plants are a dominant life form on Earth.The developing seed is composed of two fertilization products,the embryo and endosperm,which are surrounded by a maternally derived seed coat.Accumulating evidence indicates that efficient communication among all three seed components is required to ensure coordinated seed development.Cell communication within plant seeds has drawn much attention in recent years.In this study,we review current knowledge of cross-talk among the endosperm,embryo,and seed coat during seed development,and highlight recent advances in this field.

Embryogenesis,a process of pattern formation

Plant embryogenesis is traditionally defined as a developmental process from zygote to mature embryo,which has the potential to form a complete plant(Bhojwani,1974;Hu,2005).In dicotyledonous species,the fertilized egg or zygote usually divides according to a stereotyped pattern and gives rise to an embryo that consists of an embryonic shoot,cotyledons,hypocotyls,and an embryonic root.Thus,the basic body plan of the plant is established during the embryogenesis.Interestingly,the shoot-leaf-stem structure,not including the root,is repeatedly photocopied as a basic unit throughout plant vegetative growth(Wolpert et al.,2002).In this case,embryogenesis is a seemingly endless process and the epitome of plant body-building.However,recent studies on the molecular mechanism of embryogenesis revealed that the basic plant body plan is laid down as the establishment of the radial and apical-basal axes.During this process,both the radial concentric tissue layers and the apicalbasal sections of the plant body are generated,especially the root and shoot stem cell pools,which are essential for postembryonic growth(De Smet and Jürgens,2007;De Smet et al.,2010).All further development and morphogenesis are governed or guided by this basic pattern.Thus,embryogenesis is,in fact,a process of pattern formation.

Plant reproduction: Recent discoveries from China

Sexual reproduction in higher plants consists of several major biological processes which include mega- and micro-gametogenesis, male-female communications, embryogenesis and endosperm development. Male-female communications include male gametophyte-sporophytic tissues （stigma/transmitting tract） interaction and male-female gametophyte communication, during which various signaling pathways are involved.