Differential spatial expression of A- and B-type CDKs, and distribution of auxins and cytokinins in the open transverse root apical meristem of Cucurbita maxima

† Background and Aims Aside from those on Arabidopsis, very few studies have focused on spatial expression ofcyclin-dependent kinases (CDKs) in root apical meristems (RAMs), and, indeed, none has been undertaken foropen meristems. The extent of interfacing between cell cycle genes and plant growth regulators is also an increasinglyimportant issue in plant cell cycle studies. Here spatial expression/localization of an A-type and B-typeCDK, auxin and cytokinins are reported in relation to the hitherto unexplored anatomy of RAMs of Cucurbitamaxima.† Methods Median longitudinal sections were cut from 1-cm-long primary root tips of C. maxima. Full-lengthA-type CDKs and a B-type CDK were cloned from C. maxima using degenerate primers, probes of whichwere localized on sections of RAMs using in situ hybridization. Isopentenyladenine (iPA), trans-zeatin (t-Z)and indole-3yl-acetic acid (IAA) were identified on sections by immunolocalization.† Key Results The C. cucurbita RAM conformed to an open transverse (OT) meristem typified by an absence of aclear boundary between the eumeristem and root cap columella, but with a distinctive longitudinally thickenedepidermis. Cucma;CDKA;1 expression was detected strongly in the longitudinally thickened epidermis, a tissuewith mitotic competence that contributes cells radially to the root cap of OT meristems. Cucma;CDKB2 wasexpressed mainly in proliferative regions of the RAM and in lateral root primordia. iPA and t-Z were mainly distributedin differentiated cells whilst IAA was distributed more uniformly in all tissues of the RAM.†Conclusions Cucma;CDKA;1 was expressed most strongly in cells that have proliferative competence whereasCucma;CDKB2 was confined mainly to mitotic cells. iPA and t-Z marked differentiated cells in the RAM, consistentwith the known effect of cytokinins in promoting differentiation in root systems. iPA/t-Z were distributedin a converse pattern to Cucma;CDKB2 expression whereas IAA was detected in most cells in the RAM regardlessof their proliferative potential