75 Integrating Membrane Transport with Male Gametophyte ... - TAIR

193 Cytokinins regulate both the identity and proliferation of vascular cell lineages
Ana Campilho 1 , Melanie Decourteix 1 , Ari Pekka Mahonen 1 , Anthony Bishopp 1 , Kaisa Nieminen 1 , Masayuki Higuchi 2 ,
Tatsuo Kakimoto 2 , Yka Helariutta 1
1
Institute of Biotechnology, University of Helsinki, Finland, 2 Biology Department , Osaka University, Japan
>We are studying the genetic control of vascular morphogenesis during root development in Arabidopsis. The cell
lineages which form xylem and phloem and the intervening pluripotent procambial tissue originate from stem cells
near the root tip. We have recently shown that cytokinins promote the pluripotent cell identity and inhibit the default
protoxylem identity. Either a decrease in cytokinin levels in the procambium, or a disruption of cytokinin-signalling
through the two-component phosphorelay (as exemplified by the phenotype of the cre1 ahk2 ahk3 receptor triple knockout
mutant), causes all vascular cells to differentiate into protoxylem cells. AHP6, an inhibitory pseudo phosphotransfer
protein, counteracts cytokinin signaling allowing protoxylem formation (Mähönen et al, 2006 Science). On the other
hand, we have shown that CRE1/WOL cytokinin receptor is a bifunctional kinase/phosphatase and that replacing CRE1
with AHK2 (with no or low phosphatase activity) results in stimulation of proliferation of vascular cell files (Mähönen,
Higuchi et al, in press Current Biology). This indicates that in addition to specifying vascular cell identity, cytokinins
have a second role in controlling the rate of proliferation of vascular cell files. Our current progress in understanding
which genes control and are regulated by cytokinin action in these two processes will be presented.
194 Gene Expression Programs during Shoot, Root and Callus Development in Arabidopsis
Tissue Culture
Ping Che 1 , Sonia Lall 1 , Dan Nettleton 2 , Stephen Howell 1
1
Plant Sciences Institute, Iowa State University, Ames IA 50011, USA, 2 Statistics Department, Iowa State
University, Ames IA 50011
Shoots can be regenerated from Arabidopsis root explants in tissue culture through a two-step process requiring
preincubation on callus induction medium (CIM). Regenerating tissues can be directed along different developmental
pathways leading to the formation of shoots, new roots or callus by transferring to shoot- or root-induction medium (SIM
or RIM) or continued incubation on CIM. Using gene-profiling methods, we identified genes that were specifically up- or
downregulated on one developmental pathway, but not on others. One gene (At5g13330) upregulated during early shoot
development encoded an AP2/ERF transcription factor, RAP2.6L. RAP2.6L plays a pivotal role in shoot regeneration
because T-DNA knock-down mutations in the gene reduced the efficiency of shoot formation in tissue culture. These same
T-DNA mutations reduced the expression of 35% of the genes normally upregulated during SIM incubation, including
CUP-SHAPED COTYLEDON 2 (CUC2, At5g53950) a gene involved in shoot meristem specification and required for
efficient shoot regeneration. During CIM preincubation, root explants acquire the ability to form green callus or shoots
when transferred to SIM. Genes that depend on CIM preincubation for subsequent expression on SIM were identified
from expression profiles of explants in which CIM preincubation was omitted. One gene that required CIM preincubation
was an A-type response regulator gene, RESPONSE REGULATOR 15 (ARR15, At1g74890). ARR15 appeared to be to
a marker for competency to form green callus, because both required only one day preincubation on CIM and were not
affected by reversible cell cycle inhibitors. Competence to form shoots requires two or more days of CIM preincubation
and can be blocked by reversible inhibitors of cell division.
This work was supported by grants from the National Science Foundation and from the National Research Initiative of the USDA Cooperative
State Research, Education and Extension Service.