F-actin regulates the polarized secretion of pollen tube attractants in Arabidopsis synergid cells.

Pollen tube attraction is a key event of sexual reproduction in flowering plants. In the ovule, two synergid cells neighboring the egg cell control pollen tube arrival via the active secretion of attractant peptides such as AtLURE1 and XIUQIU from the filiform apparatus (FA) facing toward the micropyle. Distinctive cell polarity together with longitudinal F-actin and microtubules are hallmarks of the synergid cell in various species, though the functions of these cellular structures are unclear. In this study, we used genetic and pharmacological approaches to indicate the roles of cytoskeletal components in FA formation and pollen tube guidance in Arabidopsis thaliana. Genetic inhibition of microtubule formation reduced invaginations of the plasma membrane but did not abolish micropylar AtLURE1.2 accumulation. By contrast, the expression of a dominant-negative form of ACTIN8 induced disorganization of the FA and loss of polar AtLURE1.2 distribution toward the FA. Interestingly, after pollen tube reception, F-actin became unclear for a few hours in the persistent synergid cell, which may be involved in pausing and resuming pollen tube attraction during early polytubey block. Our data suggest that F-actin plays a central role in maintaining cell polarity and in mediating male-female communication in the synergid cell.