Hypocotyl negative gravitropism is one of major physiological responses regulated by phytochrome which determines fitness of seedlings by enhancing phototropic response. Phytochrome inhibits hypocotyl negative gravitropism by randomizing the orientation of hypocotyl which is first identified by up-straight growth of phytochrome mutants. PifQ mutant (pif1;pif3;pif4;pif5) showed agravitropism in dark condition and lack endodermal starch granule, while only endodermal PIF1 expression in pifQ was sufficient to rescue the agravitropism of pifQ by rescuing endodermal starch granule, most well-known gravity sensor of hypocotyl. Moreover, endodermal PIF3dN expression in wild type induced agravitropism in dark condition, suggests endodermal PIFs induce hypocotyl negative gravitropism by stabilization of endodermal starch granule. To identify phytochrome functions in cell-autonomous manner, I analyzed several tissue-specific PHYB expressing lines, and figured out epidermal phyB inhibits hypocotyl negative gravitropism while endodermal phyB could not in continuous red light grown seedlings. Epidermal PHYB expressing line stained weaker than endodermal PHYB expressing line for endodermal starch granule in continuous red light grown seedlings. Moreover, epidermal phyB induced degradation of endodermal PIFs in similar kinetics with endogenous phyB. In conclusion, epidermal phyB generates mobile signal to endodermal PIFs, subsequently inducing PIF degradation to inhibit hypocotyl negative gravitropism.
As an independent study, I studied hypocotyl orientation of bzr1-1D, which was bent to root-ward in continuous red light grown seedlings. This required phytochrome, and bending proportion was increased by increasing dose of exogenous brassinosteroid in wild type. Genetic analysis with various gravitropism-required genes indicated hypocotyl bending of bzr1-1D also requires gravitropism-processing-genes. Surprisingly, red light induced endodermal starch granules of bzr1-1D to mobilize toward cotyledon-side of endodermis asymmetrically in both endodermis before hypocotyl bends. Moreover, PIN3 asymmetrically re-localized and resulted in asymmetric auxin distribution. In conclusion, I propose phytochrome induce hypocotyl bending in accordance with relative brassinosteroid signaling activity to inhibit hypocotyl negative gravitropism.
본 연구에서는 피토크롬의 배축음성굴지성 방해에 관해 연구해보았다. 우선 조직특이적인 피토크롬 형질전환체를 이용해 배축음성굴지성 표현형을 조사한 결과 내피특이적 피토크롬은 배축음성굴지성을 방해하지 못하는데 반해 외피특이적 피토크롬은 배축음성굴지성을 방해하고 있었다. 또한, 내피특이적 피토크롬은 내피녹말과립을 분해하지 못한데 반해 외피특이적 피토크롬은 내피녹말과립을 분해했다. 더불어, 외피특이적 피토크롬은 내피특이적 PIF를 야생형 정도의 속도로 분해할 수 있는 것으로 조사됐다. 이는 외피의 피토크롬이 원격신호를 통해 배축음성굴지성을 방해함을 나타낸다. 별개의 연구에서 브라시노스테로이드의 신호전달이 줄어들지 않는 bzr1-1D 돌연변이가 피토크롬-의존적으로 배축 방향이 뿌리를 향하는 것을 발견했다. 이 표현형은 배축 내의 내피녹말과립이 떡잎 쪽으로 이동하고 PIN3가 비대칭적으로 배치되면서 옥신이 비대칭적으로 배치돼서 일어난 현상으로 분석됐다. 또한 브라시노스테로이드의 신호전달이 강해질수록 뿌리를 향하는 배축이 많아지고 약해질수록 음성굴지성을 보이는 배축이 많아지는 것을 관찰했다. 따라서, 피토크롬은 브라시노스테로이드의 신호전달 강도에 따라 배축의 굽기를 조절해 배축음성굴지성을 방해한다.
