Identification of PAN2 by quantitative proteomics as a leucine-rich repeat-receptor-like kinase acting upstream of PAN1 to polarize cell division in maize.

Mechanisms governing the polarization of plant cell division are poorly understood. Previously, we identified pangloss1 (PAN1) as a leucine-rich repeat-receptor-like kinase (LRR-RLK) that promotes the polarization of subsidiary mother cell (SMC) divisions toward the adjacent guard mother cell (GMC) during stomatal development in maize (Zea mays). Here, we identify pangloss2 (PAN2) as a second LRR-RLK promoting SMC polarization. Quantitative proteomic analysis identified a PAN2 candidate by its depletion from membranes of pan2 single and pan1;pan2 double mutants. Genetic mapping and sequencing of mutant alleles confirmed the identity of this protein as PAN2. Like PAN1, PAN2 has a catalytically inactive kinase domain and accumulates in SMCs at sites of GMC contact before nuclear polarization. The timing of polarized PAN1 and PAN2 localization is very similar, but PAN2 acts upstream because it is required for polarized accumulation of PAN1 but is independent of PAN1 for its own localization. We find no evidence that PAN2 recruits PAN1 to the GMC contact site via a direct or indirect physical interaction, but PAN2 interacts with itself. Together, these results place PAN2 at the top of a cascade of events promoting the polarization of SMC divisions, potentially functioning to perceive or amplify GMC-derived polarizing cues.

ROP GTPases act with the receptor-like protein PAN1 to polarize asymmetric cell division in maize.

Plant Rho family GTPases (ROPs) have been investigated primarily for their functions in polarized cell growth. We previously showed that the maize (Zea mays) Leu-rich repeat receptor-like protein PANGLOSS1 (PAN1) promotes the polarization of asymmetric subsidiary mother cell (SMC) divisions during stomatal development. Here, we show that maize Type I ROPs 2 and 9 function together with PAN1 in this process. Partial loss of ROP2/9 function causes a weak SMC division polarity phenotype and strongly enhances this phenotype in pan1 mutants. Like PAN1, ROPs accumulate in an asymmetric manner in SMCs. Overexpression of yellow fluorescent protein-ROP2 is associated with its delocalization in SMCs and with aberrantly oriented SMC divisions. Polarized localization of ROPs depends on PAN1, but PAN1 localization is insensitive to depletion and depolarization of ROP. Membrane-associated Type I ROPs display increased nonionic detergent solubility in pan1 mutants, suggesting a role for PAN1 in membrane partitioning of ROPs. Finally, endogenous PAN1 and ROP proteins are physically associated with each other in maize tissue extracts, as demonstrated by reciprocal coimmunoprecipitation experiments. This study demonstrates that ROPs play a key role in polarization of plant cell division and cell growth and reveals a role for a receptor-like protein in spatial localization of ROPs.

Determination of symmetric and asymmetric division planes in plant cells.

The cellular organization of plant tissues is determined by patterns of cell division and growth coupled with cellular differentiation. Cells proliferate mainly via symmetric division, whereas asymmetric divisions are associated with initiation of new developmental patterns and cell types. Division planes in both symmetrically and asymmetrically dividing cells are established through the action of a cortical preprophase band (PPB) of cytoskeletal filaments, which is disassembled upon transition to metaphase, leaving behind a cortical division site (CDS) to which the cytokinetic phragmoplast is later guided to position the cell plate. Recent progress has been made in understanding PPB formation and function as well as the nature and function of the CDS. In asymmetrically dividing cells, division plane establishment is governed by cell polarity. Recent work is beginning to shed light on polarization mechanisms in asymmetrically dividing cells, with receptor-like proteins and potential downstream effectors emerging as important players in this process.

PAN1: a receptor-like protein that promotes polarization of an asymmetric cell division in maize.

Polarization of cell division is essential for eukaryotic development, but little is known about how this is accomplished in plants. The formation of stomatal complexes in maize involves the polarization of asymmetric subsidiary mother cell (SMC) divisions toward the adjacent guard mother cell (GMC), apparently under the influence of a GMC-derived signal. We found that the maize pan1 gene promotes the premitotic polarization of SMCs and encodes a leucine-rich repeat receptor-like protein that becomes localized in SMCs at sites of GMC contact. PAN1 has an inactive kinase domain but is required for the accumulation of a membrane-associated phosphoprotein, suggesting a function for PAN1 in signal transduction. Our findings implicate PAN1 in the transmission of an extrinsic signal that polarizes asymmetric SMC divisions toward GMCs.

Two WD-repeat genes from cotton are functional homologues of the Arabidopsis thaliana TRANSPARENT TESTA GLABRA1 (TTG1) gene.

Cotton fibres are single, highly elongated cells derived from the outer epidermis of ovules, and are developmentally similar to the trichomes of Arabidopsis thaliana. To identify genes involved in the molecular control of cotton fibre initiation, we isolated four putative homologues of the Arabidopsis trichome-associated gene TRANSPARENT TESTA GLABRA1 (TTG1). All four WD-repeat genes are derived from the ancestral D diploid genome of tetraploid cotton and are expressed in many tissues throughout the plant, including ovules and growing fibres. Two of the cotton genes were able to restore trichome formation in ttg1 mutant Arabidopsis plants. Both these genes also complemented the anthocyanin defect in a white-flowered Matthiola incana ttg1 mutant. These results demonstrate parallels in differentiation between trichomes in cotton and Arabidopsis, and indicate that these cotton genes may be functional homologues of AtTTG1.