Hormonal responses during early embryogenesis in maize

Abstract

Plant hormones have been shown to regulate key processes during embryogenesis in the model plant Arabidopsis thaliana, but the mechanisms that determine the peculiar embryo pattern formation of monocots are largely unknown. Using the auxin and cytokinin response markers DR5 and TCSv2 (two-component system, cytokinin-responsive promoter version #2), as well as the auxin efflux carrier protein PIN1a (PINFORMED1a), we have studied the hormonal response during early embryogenesis (zygote towards transition stage) in the model and crop plant maize. Compared with the hormonal response in Arabidopsis, we found that detectable hormone activities inside the developing maize embryo appeared much later. Our observations indicate further an important role of auxin, PIN1a and cytokinin in endosperm formation shortly after fertilization. Apparent auxin signals within adaxial endosperm cells and cytokinin responses in the basal endosperm transfer layer as well as chalazal endosperm are characteristic for early seed development in maize. Moreover, auxin signalling in endosperm cells is likely to be involved in exogenous embryo patterning as auxin responses in the endosperm located around the embryo proper correlate with adaxial embryo differentiation and outgrowth. Overall, the comparison between Arabidopsis and maize hormone response and flux suggests intriguing mechanisms in monocots that are used to direct their embryo patterning, which is significantly different from that of eudicots.

The role of auxin transporters in monocots development

“…ZmPIN1a , -b , -c are ubiquitously expressed but differentially modulated in maize vegetative and reproductive tissues and during kernel development. ZmPIN1 plays an important role during embryogenesis, where detectable hormone activity inside the developing maize embryo appears much later than in Arabidopsis (Forestan et al, 2010; Chen et al, 2014). In situ hybridization showed that ZmPIN1a localizes in the root apical meristem (RAM) and the calyptrogen, which is a specialized layer of meristematic cells that continuously generate replacements for the root cap cells that die during primary root growth.…”

The Yin-Yang of Hormones: Cytokinin and Auxin Interactions in Plant Development

“…This is also the point when cytokinin activity becomes clearly associated with the SAM, which as we discuss below is a key element in maintaining WUS expression (Zürcher et al, 2013). The association of cytokinin activity with the embryonic SAM also occurs in monocots, even though monocots differ from dicots in the cell division patterns that occur during embryogenesis (Chen et al, 2014). Maintenance of a stem cell population in the SAM thereafter relies on a non-cell-autonomous feedback loop involving WUS and the CLAVATA (CLV) signaling pathway ( Figure 5C).…”

Zygotic Genome Activation Occurs Shortly after Fertilization in Maize

“…We therefore analyzed the expression of auxin biosynthesis, transport, and response genes, as shown in Figure 7 (Supplemental Data Set 3D; important gene families). In maize, the earliest ZmPIN1a localization was observed at 6 DAP at the adaxial side of the embryo proper (Chen et al, 2014). To identify potential PIN proteins that function at the earliest stage of embryo patterning, we analyzed the expression of all PIN family genes.…”