Light-Induced Growth Promotion by SPA1 Counteracts Phytochrome-Mediated Growth Inhibition during De-Etiolation

Previous evidence has suggested that SPA1 is a signal transduction component that appears to require phytochrome A for function in seedling photomorphogenesis. Using digital image analysis, we examined the time course of growth inhibition induced by red light in spa1 mutants to test the interpretation that SPA1 functions early in a phyA-specific signaling pathway. By comparing wild-type and mutant responses, we found that SPA1 caused an increase in hypocotyl growth rate after approximately 2 h of continuous red light, whereas the onset of phyA-mediated inhibition was detected within several minutes. Thus, SPA1-dependent growth promotion began after phyA started to inhibit growth. The action of SPA1 persisted for approximately 2 d of red light, a period well beyond the time when the phyA photoreceptor and its influence on growth have both decayed to undetectable levels. Also, SPA1 promoted growth for many hours in the complete absence of a light stimulus when red-light-grown seedlings were shifted to darkness. We propose that SPA1 functions in a light-induced mechanism that promotes growth and thereby counteracts growth inhibition mediated by phyA and phyB. Our finding that spa1 seedlings do not display growth promotion in response to end-of-day pulses of far-red light, even in a phyA-null background, supports this interpretation. Combined, these results lead us to the view that the rate of hypocotyl elongation in light is determined by at least two independent, opposing processes; an inhibition of growth by the phytochromes and a promotion of growth by light-activated SPA1.

既往研究证据表明，SPA1是一种信号转导组分（signal transduction component），其在幼苗光形态建成（photomorphogenesis）中的功能似乎依赖于光敏色素A（phytochrome A）。本研究通过数字图像分析（digital image analysis）技术，检测了spa1突变体在红光（red light）诱导下的生长抑制时间进程，以验证“SPA1在光敏色素A特异性信号通路中发挥早期功能”这一假说。通过对比野生型（wild-type）与突变体的响应，我们发现，在持续红光照射约2小时后，SPA1可提升下胚轴（hypocotyl）生长速率；而光敏色素A介导的生长抑制可在数分钟内启动。由此可见，依赖SPA1的生长促进过程始于光敏色素A启动生长抑制之后。SPA1的作用可持续约2天的红光照射，这一时长远超光敏色素A光受体及其对生长的影响衰减至无法检测水平的时间。此外，当将红光预培养的幼苗转移至完全黑暗环境时，SPA1仍可在无光照刺激的情况下促进生长长达数小时。我们提出，SPA1可通过光诱导的机制发挥功能，该机制能够促进生长，进而抵消光敏色素A与光敏色素B（phytochrome B）介导的生长抑制。我们的研究发现，即使处于phyA缺失型（phyA-null）背景中，spa1幼苗也不会对日末远红光脉冲（end-of-day far-red light pulses）产生生长促进响应，这一结果支持了上述假说。综合上述结果，我们认为，光照条件下的下胚轴伸长速率由至少两种独立且相互拮抗的过程决定：一是光敏色素介导的生长抑制，二是光激活SPA1介导的生长促进。