EXS, a Putative LRR Receptor Kinase, Regulates Male Germline Cell Number and Tapetal Identity and Promotes Seed Development in Arabidopsis

BACKGROUND Plant germlines arise late in development from archesporial initials in the L2 layer of the anther and ovule primordia. These cells generate a radially symmetrical array of tissues that, in the Arabidopsis anther, comprises a core of sporogenous cells (meiocytes) and the enveloping tapetum, middle cell, and endothecium layers. The putative transcription factor NZZ/SPL is required for the specification of archesporial cells, but nothing is known of how their number is regulated, or what controls cell fate in the lineages they generate. Here, we report detailed characterization of extra sporogenous cells (exs), a male sterile mutant that generates extra meiocytes but lacks tapetal and middle cell layers. RESULTS We identified the EXS locus by map-based cloning and found it to encode a putative LRR receptor kinase. In the anther, an increased number of L2 layer cells assume an archesporial fate and divide to generate a larger number of sporogenous cells. In seeds, the exs mutation results in smaller embryonic cells, delayed embryo development, and smaller mature embryos. Consistent with the observed phenotype, EXS is expressed in the inflorescence meristem, floral apices, anthers, and in developing seeds. CONCLUSIONS EXS regulates the number of cells that divide in the L2 layer of the anther, and thus the number of functional male archesporial initials. In the young seed, EXS affects cell size in the embryo and the rate at which it develops. The apparently contrasting roles of EXS in the anther and embryo suggest that signaling through the EXS receptor kinase is a feature of a number of regulatory pathways in Arabidopsis.