Atypical response regulators expressed in the maize endosperm transfer cells link canonical two component systems and seed biology

Abstract

BackgroundTwo component systems (TCS) are phosphotransfer-based signal transduction pathways first discovered in bacteria, where they perform most of the sensing tasks. They present a highly modular structure, comprising a receptor with histidine kinase activity and a response regulator which regulates gene expression or interacts with other cell components. A more complex framework is usually found in plants and fungi, in which a third component transfers the phosphate group from the receptor to the response regulator. They play a central role in cytokinin mediated functions in plants, affecting processes such as meristem growth, phyllotaxy, seed development, leaf senescence or tissue differentiation. We have previously reported the expression and cellular localization of a type A response regulator, ZmTCRR-1, in the transfer cells of the maize seed, a tissue critical for seed filling and development, and described its regulation by a tissue specific transcription factor. In this work we investigate the expression and localization of other components of the TCS signalling routes in the maize seed and initiate the characterization of their interactions.ResultsThe discovery of a new type A response regulator, ZmTCRR-2, specifically expressed in the transfer cells and controlled by a tissue specific transcription factor suggests a previously unknown role for TCS in the biology of transfer cells. We have characterized other canonical TCS molecules, including 6 histidine kinases and 3 phosphotransfer proteins, potentially involved in the atypical transduction pathway defined by ZmTCRR-1 and 2. We have identified potential upstream interactors for both proteins and shown that they both move into the developing endosperm. Furthermore, ZmTCRR-1 expression in an heterologous system (Arabidopsis thaliana) is directed to xylem parenchyma cells, probably involved in transport processes, one of the major roles attributed to the transfer cell layer.ConclusionsOur data prove the expression of the effector elements of a TCS route operating in the transfer cells under developmental control. Its possible role in integrating external signals with seed developmental processes is discussed.

RNA Sequencing of Laser-Capture Microdissected Compartments of the Maize Kernel Identifies Regulatory Modules Associated with Endosperm Cell Differentiation

“…Furthermore, the BETLspecific TF gene MRP-1, previously shown to be involved in regulation of BETL differentiation (Gómez et al, 2002(Gómez et al, , 2009, was also detected in M18 with a high MM score (1.00; Supplemental Data Set 4). This is consistent with its role as an activator of many BETLspecific genes, including BETL-1, BETL-9, BETL-10, BAP-2, MEG-1, TCRR-1, and TCRR-2 (Gómez et al, 2002(Gómez et al, , 2009Gutiérrez-Marcos et al, 2004;Muñiz et al, 2006Muñiz et al, , 2010. Therefore, these data suggest that MRP-1 acts as a major regulator of a subset of genes in the M18 coexpression module.…”

“…These genes showed CS scores in the given compartments ranging from 0.47 to 0.99. Third, we summarized previously reported in situ hybridization or promoter activity data for cell-specific genes from the literature (Hueros et al, 1995(Hueros et al, , 1999Magnard et al, 2000Magnard et al, , 2003Serna et al, 2001;Woo et al, 2001;Gómez et al, 2002Gómez et al, , 2009Gutiérrez-Marcos et al, 2004;Wisniewski and Rogowsky, 2004;Balandín et al, 2005;Massonneau et al, 2005;Muñiz et al, 2006Muñiz et al, , 2010Royo et al, 2014) and also found these genes to have a relatively high range of CS scores (0.51 to 0.99). Altogether, these comprise 44 genes showing cell-specific expression in the endosperm (Supplemental Table 6).…”

“…The M18 module contained nearly all of the previously identified BETL-expressed genes (Hueros et al, 1995(Hueros et al, , 1999Cheng et al, 1996;Doan et al, 1996;Serna et al, 2001;Magnard et al, 2003;Gutiérrez-Marcos et al, 2004;Massonneau et al, 2005;Gruis et al, 2006;Muñiz et al, 2006Muñiz et al, , 2010Brugière et al, 2008;Gómez et al, 2009), including BETL-1, 3, 4, 9, and 10; BAP-1A, 1B, 2, 3A, and 3B; TCRR-1 and 2; and INCW2 (CELL WALL INVERTASE2), EBE-2 (EMBRYO SAC/BASAL ENDOSPERM-LAYER/EMBRYO-SURROUNDING REGION-2), IPT-2 (ISOPENTENYL TRANS-FERASE-2), and CC-8 (CORN CYSTATIN-8) (Supplemental Data Set 4). In addition, this module contained 11 of the 13 MEG genes (with MEG-4 and MEG-14 being the two exceptions) that have been identified in the B73 genome (Supplemental Data Set 4), including MEG-1, which is a BETL-specific gene that has been shown to be important for the development and differentiation of the BETL (Costa et al, 2012).…”

“…In addition, this module contained 11 of the 13 MEG genes (with MEG-4 and MEG-14 being the two exceptions) that have been identified in the B73 genome (Supplemental Data Set 4), including MEG-1, which is a BETL-specific gene that has been shown to be important for the development and differentiation of the BETL (Costa et al, 2012). The BETL-, BAP-, and MEG-type genes encode small, secreted, cysteine-rich proteins that have been suggested to protect the embryo from maternally transmitted pathogens (Tailor et al, 1997) and to serve as signaling molecules that coordinate the supply of nutrients to the embryo during kernel development (Marshall et al, 2011), while the TCRR genes encode type-A response regulators (Muñiz et al, , 2010. Significantly, all of these genes showed high MM (>0.90) to module M18, and many of them were among the top-scoring hubs (Supplemental Figure 14E and Supplemental Data Set 4).…”

“…The BETL transports nutrients from the maternal tissue into the endosperm and is important for the proper development of the endosperm and the endosperm's capacity as a storage organ Costa et al, 2012). Based on the available data (Baranowskij et al, 1994;Gómez et al, 2002Gómez et al, , 2009Gutiérrez-Marcos et al, 2004;Barrero et al, 2006;Muñiz et al, 2006Muñiz et al, , 2010 including our identification of the coexpression module M18 and its association with GATA-rich sequence motifs (discussed above), we hypothesized that a GRN for BETL differentiation is minimally composed of MRP-1 and a large set of target genes that are activated upon binding of MRP-1 to Motifs 3, 6, and 8. A close examination of all the submotifs of the 10 motifs enriched for M18 showed that each of the motifs is represented by two to 21 submotifs that appeared one to 90 times within the promoters of the M18 genes (Supplemental Table 10).…”