TIP2-UDT1-OsUPEX1/2 module regulates tapetum development and function in rice

The tapetum in the anther wall is essential for plant fertility, secreting many components essential for pollen development. Development of the tapetum is controlled by multiple transcription factors and signaling pathways. UDT1, TIP2, TDR, and EAT1 constitute a sequential regulatory cascade crucial for tapetal differentiation in rice, but UDT1- and TIP2-dependent regulatory networks, particularly in early anther development, remain largely unknown. Functional analysis of knockout mutants and spatial-temporal expression analysis demonstrated overlapping expression of TIP2 and UDT1 in the middle layer and tapetum and that the tip2 mutation was epistatic to udt1. Moreover, TIP2 and UDT1 were shown to heterodimerize to activate the expression of downstream genes essential for early anther development. We identified two genes activated by TIP2-UDT1, OsUPEX1 and OsUPEX2, predicted to encode galactosyltransferases, that were preferentially expressed in the tapetum. Analysis of their single mutants demonstrated their functional redundancy, while the double knockout mutant revealed their critical roles in tapetum development and function, likely in enabling tapetal secretion. Overall, this study provides insights into the regulation of rice anther development by TIP2 and UDT1 and identifies downstream targets OsUPEX1 and OsUPEX2 essential for tapetum function and rice male fertility.