Institute of Molecular, Cell, and Systems Biology, College of Medical, Veterinary, and Life Sciences, University of Glasgow, Glasgow G12 8QQ, United Kingdom.
Department of Botany, St. Edmund's College, Shillong 793003, Meghalaya, India.
Plant Physiol. 2019 Jun;180(2):1119-1131. doi: 10.1104/pp.19.00206. Epub 2019 Mar 27.
Phototropin (phot) receptor kinases play important roles in promoting plant growth by controlling light-capturing processes, such as phototropism. Phototropism is mediated through the action of NON-PHOTOTROPIC HYPOCOTYL3 (NPH3), which is dephosphorylated following phot activation. However, the functional significance of this early signaling event remains unclear. Here, we show that the onset of phototropism in dark-grown (etiolated) seedlings of Arabidopsis () and tomato () is enhanced by greening (deetiolation). Red and blue light were equally effective in promoting phototropism in Arabidopsis, consistent with our observations that deetiolation by phytochrome or cryptochrome was sufficient to enhance phototropism. Increased responsiveness did not result from an enhanced sensitivity to the phytohormone auxin, nor does it involve the phot-interacting protein, ROOT PHOTOTROPISM2. Instead, deetiolated seedlings showed attenuated levels of NPH3 dephosphorylation and diminished relocalization of NPH3 from the plasma membrane during phototropism. Likewise, etiolated seedlings that lack the PHYTOCHROME-INTERACTING FACTORS (PIFs) PIF1, PIF3, PIF4, and PIF5 displayed reduced NPH3 dephosphorylation and enhanced phototropism, consistent with their constitutive photomorphogenic phenotype in darkness. Phototropic enhancement could also be achieved in etiolated seedlings by lowering the light intensity to diminish NPH3 dephosphorylation. Thus, phototropism is enhanced following deetiolation through the modulation of a phosphorylation rheostat, which in turn sustains the activity of NPH3. We propose that this dynamic mode of regulation enables young seedlings to maximize their establishment under changing light conditions, depending on their photoautotrophic capacity.
光受体激酶（phot）在控制光捕获过程（如向光性）方面发挥着重要作用，促进植物生长。向光性是通过非光形态建成下胚轴 3（NPH3）的作用介导的，NPH3 在光激活后去磷酸化。然而，这一早期信号事件的功能意义尚不清楚。在这里，我们表明，拟南芥（Arabidopsis）和番茄（tomato）黑暗生长（黄化）幼苗的向光性起始被绿化（脱黄化）增强。红光和蓝光在促进拟南芥向光性方面同样有效，这与我们的观察结果一致，即光敏色素或隐花色素的脱黄化足以增强向光性。增加的反应性不是由于对植物激素生长素的敏感性增强，也不涉及光相互作用蛋白 ROOT PHOTOTROPISM2。相反，脱黄化幼苗显示出 NPH3 去磷酸化水平降低，并且在向光性过程中 NPH3 从质膜重新定位减少。同样，缺乏 PHYTOCHROME-INTERACTING FACTORS (PIFs) PIF1、PIF3、PIF4 和 PIF5 的黄化幼苗显示出 NPH3 去磷酸化减少和增强的向光性，这与它们在黑暗中固有的光形态建成表型一致。通过降低光强度以减少 NPH3 去磷酸化，也可以在黄化幼苗中实现向光性增强。因此，脱黄化后通过调节磷酸化变阻器来增强向光性，这反过来又维持了 NPH3 的活性。我们提出，这种动态调节模式使幼苗能够根据其光自养能力，在不断变化的光照条件下最大限度地建立自身。
