NERD1 is required for primexine formation and plasma membrane undulation during microsporogenesis in Arabidopsis thaliana

The primexine formation and plasma membrane undulation are the crucial steps of pollen wall formation in many angiosperms.However,the molecular mechanism underlining these processes is largely unknown.In Arabidopsis,NEW ENHANCER OF ROOT DWARFISM1(NERD1),a transmembrane protein,was reported to play pleiotropic roles in plant development including male fertility control;while,how NERD1 disruption impacts male reproduction is yet unclear.Here,we revealed that the male sterility of nerd1 mutants is attributed to defects in early steps of pollen wall formation.We found that nerd1-2 is void of primexine formation and microspore plasma membrane undulation,defective in callose deposition.Consequently,sporopollenin precursors are unable to deposit and assemble on the microspore surface,but instead accumulated in the anther locule and tapetal cells,and ultimately leading to microspore abortion.NERD1 is localized in the Golgi and is expressed in both vegetative and reproductive organs,with the highest expression in reproductive tissues,including the tapetum,male meiocytes,tetrads and mature pollen grains.Our results suggest that NERD1 is required for the primexine deposition and microspore plasma membrane undulation,thus essential for sporopollenin assembly and pollen exine formation.

Pollen wall pattern in Arabidopsis

The pollen wall is a solid and variously sculptured structure. This pattern is determined inside a tetrad. During meiosis, the callose wall is formed outside of the meiocyte/microspore to form a tetrad. Then, primexine is deposited between the callose wall and the microspore plasma membrane which will become undulated. The sporopollenin deposits on top of the undulated membrane and develops into the pollen wall pattern, while the callose wall is gradually degraded. In recent years, much progress has been made in the study of pollen wall pattern formation, at both molecular and genetic levels. In this review,we summarize these achievements mainly in Arabidopsis.

Ultrastructure analysis reveals sporopollenin deposition and nexine formation at early stage of pollen wall development in Arabidopsis

In angiosperm, pollen wall formation is a critical step for male gametophyte development. Pollen wall constitutes of the outer layer exine and the inner layer intine. Exine is further divided into sexine and nexine. In Arabidopsis, the general process of pollen wall formation has been reported. However, the nexine formation has not been revealed. Here, we observed the process of pollen wall formation in Arabidopsis thaliana using transmission electron microscope. After callose wall is formed, the primexine is present between plasma membrane and the callose layer in the tetrad. With plasma membrane undulation, sporopollenin precursors accumulated on the peak of undulated membrane which is further developed into probacula. The primexine determines plasma membrane undulation and sporopollenin accumulation based on previous analysis of an undulation-deficient mutant. Some materials obviously different from sporopollenin are filled between the primexine and plasma membrane. These materials cover all the surface of plasma membrane and gradually develop into nexine. After microspore is released from tetrad, the nexine layer is formed and the probacula is further developed into sexine with continued accumulation of sporopollenin. Based on these observations, we proposed a developmental model of early pollen wall formation.