Plant development is governed by signaling molecules called phytohormones. Typically, in certain developmental processes more than 1 hormone is implicated and, thus, coordination of their overlapping activities is crucial for correct plant development. However, molecular mechanisms underlying the hormonal crosstalk are only poorly understood. Multiple hormones including cytokinin and auxin have been implicated in the regulation of root development. Here we dissect the roles of cytokinin in modulating growth of the primary root. We show that cytokinin effect on root elongation occurs through ethylene signaling whereas cytokinin effect on the root meristem size involves ethylene-independent modulation of transport-dependent asymmetric auxin distribution. Exogenous or endogenous modification of cytokinin levels and cytokinin signaling lead to specific changes in transcription of several auxin efflux carrier genes from the PIN family having a direct impact on auxin efflux from cultured cells and on auxin distribution in the root apex. We propose a novel model for cytokinin action in regulating root growth: Cytokinin influences cell-to-cell auxin transport by modification of expression of several auxin transport components and thus modulates auxin distribution important for regulation of activity and size of the root meristem.auxin ͉ auxin transport ͉ cytokinin ͉ hormonal crosstalk ͉ root meristem P lant hormones play a crucial role in regulating plant development and the flexible shaping of the plant architecture in response to variable environmental conditions. The final developmental and physiological output of the hormonal signaling in plants is the typical result of combined actions of several hormonal pathways. However, our knowledge of the mechanisms involved in the hormonal crosstalk is still poor.In the regulation of root development, several hormonal pathways are involved, with auxin and cytokinin being the principal players. The whole process of root organogenesis, starting with the initiation of the root pole in embryos (1), positioning and formation of stem cell niche (2, 3), maintenance of mitotic activity in proximal meristem (4-6), and rapid elongation and differentiation of cells leaving the root meristem (7) has been demonstrated to be controlled by auxin. In this context, the differential auxin distribution between cells is crucial (3,8,9). The auxin gradients or local auxin maxima can be generated by auxin metabolic reactions, mainly by local auxin biosynthesis (6, 10, 11) and intercellular auxin transport dependent on the coordinated action of influx carriers of the AUX/LAX family (12), PIN efflux carriers (13,14), and members of the multidrugresistant/P-glycoprotein (MDR/PGP) subfamily B of ATPbinding cassette (ABCB) proteins (15, 16). Accordingly, interference with the polar auxin transport disrupts the auxin distribution and results in dramatic patterning defects in the root meristem (2, 3, 9).Besides auxin, cytokinin (CK) is also involved in root organogenesis. Increase in CK levels by exogen...

SUMMARYUpon hormonal signaling, ovules develop as lateral organs from the placenta. Ovule numbers ultimately determine the number of seeds that develop, and thereby contribute to the final seed yield in crop plants. We demonstrate here that CUP-SHAPED COTYLEDON 1 (CUC1), CUC2 and AINTEGUMENTA (ANT) have additive effects on ovule primordia formation. We show that expression of the CUC1 and CUC2 genes is required to redundantly regulate expression of PINFORMED1 (PIN1), which in turn is required for ovule primordia formation. Furthermore, our results suggest that the auxin response factor MONOPTEROS (MP/ARF5) may directly bind ANT, CUC1 and CUC2 and promote their transcription. Based on our findings, we propose an integrative model to describe the molecular mechanisms of the early stages of ovule development.

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