The Rice Basic Helix-Loop-Helix Transcription Factor TDR INTERACTING PROTEIN2 Is a Central Switch in Early Anther Development  

Abstract

In male reproductive development in plants, meristemoid precursor cells possessing transient, stem cell-like features undergo cell divisions and differentiation to produce the anther, the male reproductive organ. The anther contains centrally positioned microsporocytes surrounded by four distinct layers of wall: the epidermis, endothecium, middle layer, and tapetum. Here, we report that the rice (Oryza sativa) basic helix-loop-helix (bHLH) protein TDR INTERACTING PROTEIN2 (TIP2) functions as a crucial switch in the meristemoid transition and differentiation during early anther development. The tip2 mutants display undifferentiated inner three anther wall layers and abort tapetal programmed cell death, causing complete male sterility. TIP2 has two paralogs in rice, TDR and EAT1, which are key regulators of tapetal programmed cell death. We revealed that TIP2 acts upstream of TDR and EAT1 and directly regulates the expression of TDR and EAT1. In addition, TIP2 can interact with TDR, indicating a role of TIP2 in later anther development. Our findings suggest that the bHLH proteins TIP2, TDR, and EAT1 play a central role in regulating differentiation, morphogenesis, and degradation of anther somatic cell layers, highlighting the role of paralogous bHLH proteins in regulating distinct steps of plant cell-type determination.

MS23, a master basic helix-loop-helix factor, regulates the specification and development of tapetum in maize

“…MS23 interacts with bHLH51 but not bHLH122 (Fig. S14) paralleling its counterpart in rice (Fu et al, 2014;Ko et al, 2014). Additionally, MS32 only forms strong dimers with bHLH122 in maize whereas bHLH51 forms both homodimers and strong dimers with bHLH122.…”

“…Mutants of tip2, the rice ortholog of Ms23, have sporadic, periclinal divisions in the pre-tapetal layer but never form a complete bilayer as in ms23-ref (Fu et al, 2014;Ko et al, 2014). Furthermore, in situ hybridization reveals that transcript localizations are distinct in these grasses.…”

“…The expression of Ms23 in the ML is transitory and might reflect only pre-existing, SPC-derived transcripts. In rice, TIP2 transcripts are in the ML and TP as well as weakly present in the EN when all four wall layers are present (Fu et al, 2014). Interestingly, in maize, the initial site of the lower signal in the ML is restricted along the lobe circumference; it constitutes a stripe along the outer arc along the length of each lobe.…”

“…In Arabidopsis, the ortholog of Ms32 (DYT1) is also upstream of bHLH10, bHLH89, bHLH91 and AMS. Interestingly, Ms32, UDT1 and DYT1 are all upregulated in the mutants corresponding to ms23-ref (rice tip2 and Arabidopsis bhlh10/bhlh89/bhlh91) (Fu et al, 2014;Ko et al, 2014;Zhu et al, 2015).…”

“…Mutant analysis demonstrates that several tapetal bHLHs are crucial for (1) pre-meiotic to meiotic stages: Arabidopsis DYSFUNCTIONAL TAPETUM1 (DYT1) Feng et al, 2012), its rice homolog UNDEVELOPED TAPETUM1 (UDT1) (Jung et al, 2005), and the maize homolog, Male Sterility32 (Ms32) (Moon et al, 2013); rice ETERNAL TAPETUM1 (EAT1 -OS04G0599300, also named DELAYED TAPETUM DEGENERATION or DTD) (Niu et al, 2013;Ji et al, 2013); rice TDR INTERACTING PARTNER2 (TIP2 -OS01T0293100; also named bHLH142) (Fu et al, 2014;Ko et al, 2014); Arabidopsis bHLH10, bHLH89 and bHLH91 (Zhu et al, 2015); as well as for (2) meiotic to post-meiotic stages: Arabidopsis ABORTED MICROSPORES (AMS) (Sorensen et al, 2003;Xu et al, 2010Xu et al, , 2014 and its rice homolog TAPETUM DEGENERATION RETARDATION (TDR) (Li et al, 2006;Zhang et al, 2008).…”