COP1 promotes seed germination by destabilizing RGA-LIKE2 (RGL2) in Arabidopsis

Under favorable moisture, temperature and light conditions, gibberellin (GA) biosynthesis is induced and triggers seed germination. A major mechanism by which GA promotes seed germination is by promoting the degradation of the DELLA protein RGL2, a major repressor of germination in Arabidopsis seeds. Analysis of seed germination phenotypes of constitutively photomorphogenic 1 (cop1) mutants and complemented COP1-OX/cop1-4 lines in response to GA and paclobutrazol (PAC) suggested a positive role for COP1 in seed germination and a relation with GA signaling. cop1-4 mutant seeds showed PAC hypersensitivity, but transformation with a COP1 overexpression construct rendered them PAC insensitive, with a phenotype similar to that of rgl2 mutant (rgl2-SK54) seeds. Furthermore, cop1-4 rgl2-SK54 double mutants showed a PAC-insensitive germination phenotype like that of rgl2-SK54, identifying COP1 as an upstream negative regulator of RGL2. COP1 interacts directly with RGL2 and in vivo this interaction is strongly enhanced by SPA1. COP1 directly ubiquitinates RGL2 to promote its degradation. Moreover, GA stabilizes COP1 with consequent RGL2 destabilization. By uncovering this COP1-RGL2 regulatory module, we reveal a novel mechanism whereby COP1 positively regulates seed germination and controls the expression of germination-promoting genes.

Rice Non-Specific Phospholipase C6 Is Involved in Mesocotyl Elongation

ABSTRACT Mesocotyl elongation of rice is crucial for seedlings pushing out of deep soil. Underlying mechanisms on the phospholipid signaling in mesocotyl growth of rice are elusive. Here we report that the rice non-specific phospholipase C6 (OsNPC6) is involved in the mesocotyl elongation. Our results indicated that all five rice OsNPCs (OsNPC1, OsNPC2, OsNPC3, OsNPC4 and OsNPC6) hydrolyzed the substrate phosphatidylcholine (PC) to phosphocholine (PCho), and all of them showed plasma membrane localization. Overexpression (OE) of OsNPC6 produced plants with shorter mesocotyls, while comparing to those of Nipponbare (Nip) and npc6 mutants. Although the mesocotyl growth of npc6 mutants was not much affected without GA3, it was obviously elongated by the treatment of GA. Upon GA3 treatment, SLENDER RICE1 (SLR1), the DELLA protein of gibberellin (GA) signaling, was drastically increased in OE plants, in contrary, the level of SLR1 was found decreased in npc6 mutants. The GA-enhanced mesocotyl elongation and the GA-impaired SLR1 level in npc6 mutants were attenuated by the supplementation of PCho. Further analysis indicated that the GA-induced expression of phospho-base N-methyltransferase 1 (PMT1) in npc6 mutants was significantly weakened by the addition of PCho. In summary, our results suggest that OsNPC6 is involved in mesocotyl development via modulation of phosphocholine in rice.