Plant biology

Signaling in Absentia

Science Signaling  02 Jun 2009:
Vol. 2, Issue 73, pp. ec181
DOI: 10.1126/scisignal.273ec181

We normally think of the presence of a hormone or growth factor as an initiating event for cellular signaling, but the absence of hormonal signaling could be a similarly effective cue. Sorefan et al. describe an example in the plant Arabidopsis thaliana in which depletion of the hormone auxin and consequent signaling from a small group of cells (an auxin “minimum,” as the authors call it) have an essential role in seed production. The authors monitored a fluorescent reporter to visualize auxin signaling and saw that auxin signaling was absent in a small group of cells that differentiate into a narrow stripe of cells and then produce enzymes to degrade the cell wall, thus allowing the fruit to split open to release seed. Proper fruit development was disrupted if auxin production was enhanced in these tissues by expression of a bacterial gene. The product of the IND gene, a transcription factor, was necessary for formation of the auxin minimum, and IND was found to regulate expression of auxin efflux carriers that are critical for polarized transport of auxin. Polarized localization of the carrier PIN3 was regulated by IND. Such localization of PIN is in turn regulated by the kinases PID and WAG2, and expression of mRNAs encoding these proteins was regulated by IND. Chromatin immunoprecipitation assays indicated that the PID and WAG2 genes are direct targets of IND. Thus, the authors emphasize that it is local depletion of auxin, rather than its presence, that is a key event for determining cell fate in this developmental process, a mechanism made feasible by the ability of plant cells to efficiently move auxin in a polarized manner. It seems possible that regulated “minima” or other signaling molecules may function in a similar way.

K. Sorefan, T. Girin, S. J. Liljegren, K. Ljung, P. Robles, C. S. Galván-Ampudia, R. Offringa, J. Friml, M. F. Yanofsky, L. Østergaard, A regulated auxin minimum is required for seed dispersal in Arabidopsis. Nature 459, 583–586 (2009). [PubMed]