WUSCHEL-RELATED HOMEOBOX 2 is important for protoderm and suspensor development in the gymnosperm Norway spruce.

Tianqing Zhu,Sara von Arnold,José M. Alvarez,Joel J. Sohlberg,Panagiotis N. Moschou

doi:10.1186/s12870-016-0706-7

Tianqing Zhu, Sara von Arnold + Show 3 more

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https://doi.org/10.1186/s12870-016-0706-7

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Abstract

BackgroundDistinct expression domains of WUSCHEL-RELATED HOMEOBOX (WOX) gene family members are involved in patterning and morphogenesis of the early embryo in Arabidopsis. However, the role of WOX genes in other taxa, including gymnosperms, remains elusive. Here, we use somatic embryos and reverse genetics for studying expression and function of PaWOX2, the corresponding homolog of AtWOX2 in the gymnosperm Picea abies (Pa; Norway spruce).ResultsThe mRNA level of PaWOX2 was transiently up-regulated during early and late embryogeny. PaWOX2 mRNA in early and early late embryos was detected both in the embryonal mass and in the upper part of the suspensor. Down-regulation of PaWOX2 during development of early embryos resulted in aberrant early embryos, which failed to form a proper protoderm. Cells on the surface layer of the embryonal mass became vacuolated, and new embryogenic tissue differentiated from the embryonal mass. In addition, the aberrant early embryos lacked a distinct border between the embryonal mass, and the suspensor and the length of the suspensor cells was reduced. Down-regulation of PaWOX2 in the beginning of embryo development, before late embryos were formed, caused a significant decrease in the yield of mature embryos. On the contrary, down-regulation of PaWOX2 after late embryos were formed had no effect on further embryo development and maturation.ConclusionsOur data suggest an evolutionarily conserved function of WOX2 in protoderm formation early during embryo development among seed plants. In addition, PaWOX2 might exert a unique function in suspensor expansion in gymnosperms.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0706-7) contains supplementary material, which is available to authorized users.

Highlights

Distinct expression domains of WUSCHEL-RELATED HOMEOBOX (WOX) gene family members are involved in patterning and morphogenesis of the early embryo in Arabidopsis
Expression of PaWOX2 during embryo development It has been shown that PaWOX2 is expressed in early somatic embryos of Norway spruce [13, 19]
In order to get a higher resolution of the fluctuations of the mRNA level of PaWOX2 during embryo development, samples from nine sequential developmental stages, spanning all three phases of embryo development, were collected for Quantitative real-time PCR (qRT-PCR) analysis (Fig. 1a)

Summary

Introduction

Distinct expression domains of WUSCHEL-RELATED HOMEOBOX (WOX) gene family members are involved in patterning and morphogenesis of the early embryo in Arabidopsis. The basic plant body pattern is set up during embryogenesis. In seed plants, this body plan has been described as the superimposition of two patterns: an apical-basal and a radial pattern [1]. Embryogenesis in Arabidopsis can be divided into three general phases, described as proembryogeny, early embryogeny (globular-stage to heart-stage transition) and late embryogeny [5]. The apical cell is the founder of the embryo proper, while the basal cell develops into the suspensor. At the beginning of early embryogeny, in the 8-cell embryo proper, a single round of tangential divisions separate the outer layer of eight cells from the eight inner cells [6]. Owing to the periclinal cell division pattern in the protoderm, the protodermal cells remain essentially separated from the inner cells throughout embryogenesis. There is a switch from pattern formation to storage product accumulation

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