Plant biology

Preventing Seed Germination

Science Signaling  28 Oct 2008:
Vol. 1, Issue 43, pp. ec370
DOI: 10.1126/scisignal.143ec370

The abundance of two phytohormones, gibberellic acid (GA) and abscisic acid (ABA), controls seed germination. When environmental conditions are right, GA concentrations increase and ABA concentrations decrease and germination proceeds (defined as the protrusion of the embryonic axis from the seed coat). Two transcriptional repressors play key roles in seed germination: GA triggers the degradation of the transcriptional repressor RGL2 through a proteasome-mediated process, and ABA triggers the accumulation of a second transcriptional repressor ABI5, as well as the phosphorylation of ABI5, which is necessary for its activity. Piskurewicz et al. provide evidence that there is crosstalk between GA signaling and ABA signaling and that RGL2 appears to serve a key role in this process in Arabidopsis thaliana. If GA synthesis was prevented, seeds failed to germinate, RGL2 abundance remained elevated, and ABI5 abundance was elevated and the protein appeared to be phosphorylated based on retardation of mobility in gels. If seeds overexpressed ABI5, then inhibition of germination was more sensitive to GA inhibition. In seeds from abi5 mutants, inhibition of GA synthesis failed to prevent germination. In seeds from rgl2 mutants, inhibition of GA synthesis fails to trigger the accumulation of ABI5 and the seeds germinate, suggesting that ABI5 is a key repressor of germination and that RGL2 communicates that GA abundance is low and stimulates the ABA pathway. XERICO, which is a RING-H2 zinc finger factor, is known to stimulate the production of ABA biosynthesis genes and, in rgl2 mutants, XERICO mRNA was not induced by inhibition of GA synthesis. Thus, GA acting through RGL2 appears to stimulate the production of XERICO, which stimulates ABA production and, consequently, ABI5 production and activation, thus repressing seed germination.

U. Piskurewicz, Y. Jikumaru, N. Kinoshita, E. Nambara, Y. Kamiya, L. Lopez-Molina, The gibberellic acid signaling repressor RGL2 inhibits Arabidopsis seed germination by stimulating abscisic acid synthesis and ABI5 activity. Plant Cell preview, published 21 October 2008 as 10.1105/tpc.108.061515. [PubMed]