Anatomical and Transcriptomic Studies of the Coleorhiza Reveal the Importance of This Tissue in Regulating Dormancy in Barley  

Abstract

The decay of seed dormancy during after-ripening is not well understood, but elucidation of the mechanisms involved may be important for developing strategies for modifying dormancy in crop species and, for example, addressing the problem of preharvest sprouting in cereals. We have studied the germination characteristics of barley (Hordeum vulgare 'Betzes') embryos, including a description of anatomical changes in the coleorhiza and the enclosed seminal roots. The changes that occur correlate with abscisic acid (ABA) contents of embryo tissues. To understand the molecular mechanisms involved in dormancy loss, we compared the transcriptome of dormant and after-ripened barley embryos using a tissue-specific microarray approach. Our results indicate that in the coleorhiza, ABA catabolism is promoted and ABA sensitivity is reduced and that this is associated with differential regulation by after-ripening of ABA 8#-hydroxylase and of the LIPID PHOSPHATE PHOSPHA-TASE gene family and ABI3-INTERACTING PROTEIN2, respectively. We also identified other processes, including jasmonate responses, cell wall modification, nitrate and nitrite reduction, mRNA stability, and blue light sensitivity, that were affected by after-ripening in the coleorhiza that may be downstream of ABA signaling. Based on these results, we propose that the coleorhiza plays a major role in causing dormancy by acting as a barrier to root emergence and that after-ripening potentiates molecular changes related to ABA metabolism and sensitivity that ultimately lead to degradation of the coleorhiza, root emergence, and germination.

DELAY OF GERMINATION 1 mediates a conserved coat-dormancy mechanism for the temperature- and gibberellin-dependent control of seed germination

“…In the mature seed of most angiosperms the embryo is encased by two covering layers ("coats"): the living endosperm tissue and the dead testa (seed coat). Whether a plant undergoes a life-cycle transition by completing seed germination is controlled by the balance of opposing forces: Germination is promoted by the growth potential of the embryo growth zone (the embryonic radicle-lower hypocotyl axis, RAD) and is inhibited by the restraining tissue layers (i.e., coats) covering the RAD (17,19,20). In many angiosperms, including the Brassicaceae Lepidium sativum (garden cress) and A. thaliana, seed germination consists of two sequential steps: Shortly after imbibition, testa rupture (TR) takes place and is followed by endosperm rupture (ER) and radicle emergence, which is the visible completion of germination.…”

Dormancy in cereals (not too much, not so little): about the mechanisms behind this trait

“…In barley, the rapid decline in ABA content observed in imbibing non-dormant (afterripened) grains is likely to be due to increased expression of HvABA8 0 OH1, a gene encoding an ABA catabolic enzyme, while it is not clearly related to the expression of HvNCEDs and other ABA biosynthesis genes Dormancy in cereals 107 (Millar et al, 2006;Barrero et al, 2009). In addition to these changes in ABA metabolism, changes in ABAsignalling genes are also involved in dormancy release by afterripening (Benech-Arnold et al, 2006;Millar et al, 2006;Gubler et al, 2008;Barrero et al, 2009).…”

“…Instead, responsiveness of isolated embryos to exogenous ABA is highly correlated with dormancy expressed in the intact grains from several species, such as wheat (Walker-Simmons, 1987;Morris et al, 1989;Gerjets et al, 2010), barley (Wang et al, 1995;Benech-Arnold et al, 1999Leymarie et al, 2008;Barrero et al, 2009), oat (Corbineau et al, 1991, sorghum (Steinbach et al, 1995) and rice (Gianinetti and Vernieri, 2007). Different embryo sensitivity to ABA can result from differential signalling activity or efficiency of molecular components of the ABA signal transduction pathway, although differential catabolism rates might also affect embryo responsiveness to exogenous ABA (Benech-Arnold et al, 2006).…”

“…This phenomenon, usually termed 'afterripening', results in an increased germination rate at the thermal optimum and widening of the temperature range compatible with good germination Cô me, 1980, 1996;Corbineau et al, 1981;Côme et al, 1984). During dormancy release, grains also become less sensitive to the inhibitory effects of hypoxia (Corbineau and Cô me, 1980;Bradford et al, 2007Bradford et al, , 2008 and light (Chaussat and Zoppolo, 1983;Gubler et al, 2008;Barrero et al, 2009). Using a population-based threshold model, Bradford et al (2007Bradford et al ( , 2008 have quantified the median base O 2 percentage that allows 50% germination; it was estimated to be 2.27% and 0.05%, respectively, for wheat grains before and after ripening, and 36.30% and 0.30%, respectively, for barley grains at harvest and after 9.5 months of storage at 208C.…”

“…In addition to these changes in ABA metabolism, changes in ABAsignalling genes are also involved in dormancy release by afterripening (Benech-Arnold et al, 2006;Millar et al, 2006;Gubler et al, 2008;Barrero et al, 2009). Both hormonal and transcriptional analyses related to ABA metabolism were first carried out in wholeembryo samples (Millar et al, 2006), but more recently the major role of ABA (and the regulation of its metabolism) in dormancy was localized to the coleorhiza (Barrero et al, 2009). The coleorhiza is a non-vascularized multicellular embryonic tissue that covers the seminal roots of grass seeds, and elongates in the early stages of imbibition before root emergence (Barrero et al, 2009).…”