Seed dormancy is an adaptive trait that does not allow the germination of an intact viable seed under favorable environmental conditions. Non-dormant seeds or seeds with low level of dormancy can germinate readily under optimal environmental conditions, and such a trait leads to preharvest sprouting, germination of seeds on the mother plant prior to harvest, which significantly reduces the yield and quality of cereal crops. High level of dormancy, on the other hand, may lead to non-uniform germination and seedling establishment. Therefore, intermediate dormancy is considered to be a desirable trait as it prevents the problems of sprouting and allows uniformity of postharvest germination of seeds. Induction, maintenance, and release of seed dormancy are complex physiological processes that are influenced by a wide range of endogenous and environmental factors. Plant hormones, mainly abscisic acid (ABA) and gibberellin (GA), are the major endogenous factors that act antagonistically in the control of seed dormancy and germination; ABA positively regulates the induction and maintenance of dormancy, while GA enhances germination. Significant progress has been made in recent years in the elucidation of molecular mechanisms regulating ABA/GA balance and thereby dormancy and germination in cereal seeds, and this review summarizes the current state of knowledge on the topic.

Lignin is an important structural component of plant cell wall that confers mechanical strength and tolerance against biotic and abiotic stressors; however it affects the use of biomass such as wheat straw for some industrial applications such as biofuel production. Genetic alteration of lignin quantity and quality has been considered as a viable option to overcome this problem. However, the molecular mechanisms underlying lignin formation in wheat biomass has not been studied. Combining molecular and biochemical approaches, the present study investigated the transcriptional regulation of lignin biosynthesis in two wheat cultivars with varying lodging characteristics and also in response to waterlogging. It also examined the association of lignin level in tissues with that of plant hormones implicated in the control of lignin biosynthesis. Analysis of lignin biosynthesis in the two wheat cultivars revealed a close association of lodging resistance with internode lignin content and expression of 4-coumarate:CoA ligase1 (4CL1), p-coumarate 3-hydroxylase1 (C3H1), cinnamoyl-CoA reductase2 (CCR2), ferulate 5-hydroxylase2 (F5H2) and caffeic acid O-methyltransferase2 (COMT2), which are among the genes highly expressed in wheat tissues, implying the importance of these genes in mediating lignin deposition in wheat stem. Waterlogging of wheat plants reduced internode lignin content, and this effect is accompanied by transcriptional repression of three of the genes characterized as highly expressed in wheat internode including phenylalanine ammonia-lyase6 (PAL6), CCR2 and F5H2, and decreased activity of PAL. Expression of the other genes was, however, induced by waterlogging, suggesting their role in the synthesis of other phenylpropanoid-derived molecules with roles in stress responses. Moreover, difference in internode lignin content between cultivars or change in its level due to waterlogging is associated with the level of cytokinin. Lodging resistance, tolerance against biotic and abiotic stresses and feedstock quality of wheat biomass are closely associated with its lignin content. Therefore, the findings of this study provide important insights into the molecular mechanisms underlying lignin formation in wheat, an important step towards the development of molecular tools that can facilitate the breeding of wheat cultivars for optimized lignin content and enhanced feedstock quality without affecting other lignin-related agronomic benefits.

To gain insights into the molecular mechanisms underlying hormonal regulation in adventitious roots and during their emergence under waterlogged conditions in wheat, the present study investigated transcriptional regulation of genes related to hormone metabolism and transport in the root and stem node tissues. Waterlogging-induced inhibition of axile root elongation and lateral root formation, and promotion of surface adventitious and axile root emergence and aerenchyma formation are associated with enhanced expression levels of ethylene biosynthesis genes, ACS7 and ACO2, in both tissues. Inhibition of axile root elongation is also related to increased root indole acetic acid (IAA) and jasmonate (JA) levels that are associated with up-regulation of specific IAA biosynthesis/transport (TDC, YUC1, and PIN9) and JA metabolism (LOX8, AOS1, AOC1, and JAR1) genes, and transcriptional alteration of gibberellin (GA) metabolism genes (GA3ox2 and GA2ox8). Adventitious root emergence from waterlogged stem nodes is associated with increased levels of IAA and GA but decreased levels of cytokinin and abscisic acid (ABA), which are regulated through the expression of specific IAA biosynthesis/transport (TDC, YUC1, and PIN9), cytokinin metabolism (IPT5-2, LOG1, CKX5, and ZOG2), ABA biosynthesis (NCED1 and NCED2), and GA metabolism (GA3ox2 and GA2ox8) genes. These results enhance our understanding of the molecular mechanisms underlying the adaptive response of wheat to waterlogging.