Formation and excretion of autophagic plastids (plastolysomes) in Brassica napus embryogenic microspores.

Verã³Nica Parra-Vega,Alba Rivas-Sendra,Jose M Seguã­-Simarro,Patricia Corral-Martã­Nez

doi:10.3389/fpls.2015.00094

Verã³Nica Parra-Vega, Alba Rivas-Sendra + Show 2 more

Open Access

https://doi.org/10.3389/fpls.2015.00094

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Abstract

The change in developmental fate of microspores reprogrammed toward embryogenesis is a complex but fascinating experimental system where microspores undergo dramatic changes derived from the developmental switch. After 40 years of study of the ultrastructural changes undergone by the induced microspores, many questions are still open. In this work, we analyzed the architecture of DNA-containing organelles such as plastids and mitochondria in samples of B. napus isolated microspore cultures covering the different stages before, during, and after the developmental switch. Mitochondria presented a conventional oval or sausage-like morphology for all cell types studied, similar to that found in vivo in other cell types from vegetative parts. Similarly, plastids of microspores before induction and of non-induced cells showed conventional architectures. However, approximately 40% of the plastids of embryogenic microspores presented atypical features such as curved profiles, protrusions, and internal compartments filled with cytoplasm. Three-dimensional reconstructions confirmed that these plastids actually engulf cytoplasm regions, isolating them from the rest of the cell. Acid phosphatase activity was found in them, confirming the lytic activity of these organelles. In addition, digested plastid-like structures were found excreted to the apoplast. All these phenomena seemed transient, since microspore-derived embryos (MDEs) showed conventional plastids. Together, these results strongly suggested that under special circumstances, such as those of the androgenic switch, plastids of embryogenic microspores behave as autophagic plastids (plastolysomes), engulfing cytoplasm for digestion, and then are excreted out of the cytoplasm as part of a cleaning program necessary for microspores to become embryos.

Highlights

Microspore embryogenesis is the most efficient biotechnological approach to obtain haploid individuals and doubled haploid (DH) lines
We focused on the analysis of DNA-containing organelles such as plastids and mitochondria, and analyzed their ultrastructure and development during the process of embryogenesis induction and further microspore-derived embryos (MDEs) development
Our results demonstrate that many plastids of embryogenic microspores undergo dramatic structural changes as a consequence of embryogenesis induction

Summary

Introduction

Microspore embryogenesis is the most efficient biotechnological approach to obtain haploid individuals and doubled haploid (DH) lines. DHs are used as pure lines in breeding programs to produce hybrid seeds. Microspore embryogenesis consists of the reprogramming of the microspores (the pollen precursors) toward embryogenesis This developmental switch ( known as androgenesis) is generally induced through the application of stress. They undergo multiple changes to readapt themselves to the new developmental scenario. These changes include, among others, a profound remodeling of gene expression, the triggering of a (stress) response as a consequence of the inductive (stressing) treatment, the suppression of the ongoing gametophytic program, and the initiation of embryogenesis (Maraschin et al, 2005; SeguíSimarro and Nuez, 2008; Dunwell, 2010). There is an extensive remodeling of cell ultrastructure, including a displacement of the nucleus to the center of the cell, a rearrangement of the cytokinetic machinery, a switch from an asymmetric to a symmetric division pattern, and a reduction in the number of plastids (Zaki and Dickinson, 1991; Hause et al, 1993; Telmer et al, 1995; Testillano et al, 2000; Shariatpanahi et al, 2006; Makowska and Oleszczuk, 2014)

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