Barley Grain Development

Toward an Integrative View

Nese Sreenivasulu*; Ljudmilla Borisjuk*; Björn H. Junker*; Hans-Peter Mock*; Hardy Rolletschek*; Udo Seiffert†; Winfriede Weschke*; Ulrich Wobus*    * Leibniz Institute of Plant Genetics and Crop Plant Research, Gatersleben, Germany
† Fraunhofer Institute for Factory Operation and Automation IFF, Magdeburg, Germany

1 Introduction

Seeds are complex structures to aid plant dispersal and hold developmental processes to withstand severe environmental conditions. They are at the same time our most important food and feed mainly derived from cereals. Among the cereals, barley is both an important crop (Baik and Ullrich, 2008) and a model for cereal genetics and genomics (Sreenivasulu et al., 2008a). Due to the economic importance and the central role in plant reproduction, numerous studies have dealt with grain development under controlled and different environmental conditions. Recent rapid development of new techniques from high-throughput genomics and postgenomics technologies to nondestructive imaging provides numerous additional data and paves the way for a much deeper understanding of the developing seed in which parallel or time-shifted interconnected physiological processes occur in different tissues. Especially, the omics technologies became the data-generating workhorses of a more holistic systems approach to seed biology. However, the exploration of cereal seed development with the new technologies is still inadequately advanced; most studies have focused on the model plant Arabidopsis. Nevertheless, we can already start to integrate genetic, molecular, biochemical, physiological, and histological data with the help of various computing tools. One tool for global data integration is the recently developed 3-D morphological grain models, extended to the fourth, the time dimension (Section 3). Such models are especially helpful since a major obstacle in the analysis of grain development is the complex nature of a grain. A detailed histological study of barley caryopsis development between anthesis and early maturation revealed at least 18 different tissues and tissue complexes (Gubatz et al., 2007) but even within a single tissue such as the endosperm biochemical and physiological gradients have been detected (Rolletschek et al., 2004). Therefore, we will place special emphasis on spatial aspects whenever data are available. Manually dissected maternal and filial barley grain tissues (Sreenivasulu et al., 2006) as well as two laser-microdissected transport-related tissues, nucellar projection (NP) and endosperm transfer cells (ETC) (Thiel et al., 2008), revealed not only extensive tissue-specific transcriptome but also biochemical data, and an in silico comparison with Arabidopsis seed tissue-specific transcriptome data (Section 4.7.3) underlined the importance of a spatially differentiated view.

Transcriptome studies unfolded a considerable potential to integrate a multitude of observations due to its global character even if the evidence provided for specific processes and regulatory networks is generally only of correlative nature. Since a single review cannot cover all relevant areas, we had to restrict ourselves to certain aspects, and we chose those that the authors have worked on during recent years with the goal to integrate molecular, biochemical, physiological, histological, and cell biology data. Besides model development, special emphasis is also placed on the molecular mechanisms and regulatory cascades influencing programmed cell death (PCD) events in different grain tissues and on processes involved in seed filling. Furthermore, first attempts to develop predictive models for barley grain metabolism are summarized. Although this review is devoted to barley grain development, we will often refer to studies on other cereals and sometimes also to evolutionary less-related plants if these data significantly broaden our view.

2 Barley Grain Development

The barley grain is a fruit in which pericarp and seed coat (testa) are fused to form a caryopsis. Its development is usually divided into three to four stages: prestorage (or cell division or morphogenesis) phase, storage (or maturation) phase, and desiccation (or late maturation) phase. Based on the massive transcriptional reprograming between prestorage and storage phase, this time span has been defined as a distinct transition or intermediate phase (Sreenivasulu et al., 2004). Gross morphological and histological changes during development of the barley grain have been described (Bethke et al., 2000; Evers and Millar, 2002; Wobus et al., 2005) and are illustrated in Fig. 2.1. In the following, we delineate grain development from fertilization to the early storage phase in some detail to better understand tissue interactions and respective molecular-physiological processes discussed later.

At the time of double fertilization, the diploid zygote together with the triploid nucleus of the central cell, the antipodal cells, and the synergids are surrounded by the maternal nucellus and embedded into the embryo sac demarcated by the inner and outer integument. At anthesis, style and pericarp/testa (for simplicity abbreviated “pericarp”) represent more than 90% of the maternal gametophyte; cells of the style contain high amounts of starch (Weschke et al., 2000). Continuous cell division in the absence of cell wall formation leads to the endosperm coenocyte (Olsen, 2001, 2004). Under defined conditions, the coencytic phase lasts for about 60 h (Engell, 1989). Between anthesis and beginning endosperm cellularization, style volume scales down and pericarp grows in most of its parts by cell division and elongation. In cells surrounding the lateral vascular bundles, storage product accumulation takes place (Weschke et al., 2000). The most endosperm-near parts of the maternal nucellus undergo PCD (D. Weier, unpublished results) followed by cellular disintegration. Only the nucellus parts facing the main vascular tissue do not disintegrate but differentiate into the NP, that maternal tissue releases nutrients into the apoplastic space between the maternal and the filial seed part. At 3 days after flowering(DAF), when cellularization of the endosperm coenocyte starts in the middle of the caryopsis opposite NP (Fig. 2.1B), cells of that region differentiate into the ETC. The process of endosperm cellularization spreads into lateral as well as central parts and is finished at about 4–5 DAF (Fig. 2.1C). Typically, endosperm cellularization is accompanied by pericarp elongation, but at the same time remobilization processes take place in dorsal cell...