Regulation of Seed Germination in the Close Arabidopsis RelativeLepidium sativum: A Global Tissue-Specific Transcript Analysis      

Abstract

The completion of germination in Lepidium sativum and other endospermic seeds (e.g. Arabidopsis [Arabidopsis thaliana]) is regulated by two opposing forces, the growth potential of the radicle (RAD) and the resistance to this growth from the micropylar endosperm cap (CAP) surrounding it. We show by puncture force measurement that the CAP progressively weakens during germination, and we have conducted a time-course transcript analysis of RAD and CAP tissues throughout this process. We have also used specific inhibitors to investigate the importance of transcription, translation, and posttranslation levels of regulation of endosperm weakening in isolated CAPs. Although the impact of inhibiting translation is greater, both transcription and translation are required for the completion of endosperm weakening in the whole seed population. The majority of genes expressed during this process occur in both tissues, but where they are uniquely expressed, or significantly differentially expressed between tissues, this relates to the functions of the RAD as growing tissue and the CAP as a regulator of germination through weakening. More detailed analysis showed that putative orthologs of cell wallremodeling genes are expressed in a complex manner during CAP weakening, suggesting distinct roles in the RAD and CAP. Expression patterns are also consistent with the CAP being a receptor for environmental signals influencing germination. Inhibitors of the aspartic, serine, and cysteine proteases reduced the number of isolated CAPs in which weakening developed, and inhibition of the 26S proteasome resulted in its complete cessation. This indicates that targeted protein degradation is a major control point for endosperm weakening.

Germination Potential of Dormant and Nondormant Arabidopsis Seeds Is Driven by Distinct Recruitment of Messenger RNAs to Polysomes

“…Transcripts involved in cell wall modifications were found to be more abundant in the translatome of nondormant seeds than dormant seeds. Overrepresentation of cell wall-related transcripts in the transcriptome was shown in Lepidium sativum (Morris et al, 2011) and Arabidopsis (Endo et al, 2012) germinating seeds and in the translatome of sunflower seeds (Layat et al, 2014). Transcripts for expansins such as EXPA1, EXPA10, EXPA4, and EXPA2 (At1g69530, At1g26770, At2g39700, and At5g05290; Fig.…”

Promotion of Testa Rupture during Garden Cress Germination Involves Seed Compartment-Specific Expression and Activity of Pectin Methylesterases

“…The testa is dead tissue, but cell wall-modifying enzymes could be secreted from the underlying endosperm to cause modifications in the inner testa cell walls, which could lead to sitespecific weakening. A number of cell wall-modifying genes, and the enzyme activities of their products, have been shown to be differentially regulated before ER in the seeds of various endospermic species, such as b-1,3-glucanase in tobacco (Nicotiana tabacum; Leubner-Metzger et al, 1995;Manz et al, 2005), b-1,4-mannanase in tomato (Solanum lycopersicum; Nonogaki et al, 2000), garden cress (Morris et al, 2011), and Arabidopsis (Iglesias-Fernández et al, 2011), and xyloglucan endotransglycosylases/hydrolases in garden cress (Voegele et al, 2011;Graeber et al, 2014) and Arabidopsis (Endo et al, 2012), and species-specific changes in cell wall composition have been observed during the later germination process (Lee et al, 2012), supporting the importance of the cell wall remodeling during seed germination.…”

“…After TR, endosperm resistance decreases through tissue softening, a process called endosperm weakening (Müller et al, 2006;Linkies et al, 2009). Both radicle elongation and endosperm weakening require cell wall modifications (Schopfer, 2006;Müller et al, 2009;Morris et al, 2011).…”

Xyloglucan Metabolism Differentially Impacts the Cell Wall Characteristics of the Endosperm and Embryo during Arabidopsis Seed Germination

“…Radicle protrusion through the micropylar endosperm is also stimulated by ethylene, which has an antagonist action with ABA on endosperm cap weakening (Linkies and LeubnerMetzger, 2012). Microarray analyses highlighted the importance of cell wall remodeling processes during germination in various species (Penfield et al, 2006;Morris et al, 2011;Endo et al, 2012;Martínez-Andújar et al, 2012;Dekkers et al, 2013). These studies provided compelling evidence that the tissue-specific expression of genes encoding cell wall biosynthesis or modification enzymes, and their differential response to hormonal signals in the endosperm and embryo, influences the rate of germination.…”