Parks B M, Hoecker U, Spalding E P
Department of Botany, University of Wisconsin, 430 Lincoln Drive, Madison, Wisconsin 53706, USA.
Plant Physiol. 2001 Jul;126(3):1291-8. doi: 10.1104/pp.126.3.1291.
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.
先前的证据表明，SPA1是一种信号转导成分，在幼苗光形态建成中发挥功能似乎需要光敏色素A。我们使用数字图像分析，检测了红光在spa1突变体中诱导的生长抑制的时间进程，以验证SPA1在phyA特异性信号通路中早期发挥作用的这一解释。通过比较野生型和突变体的反应，我们发现，在持续红光照射约2小时后，SPA1使下胚轴生长速率增加，而phyA介导的生长抑制在几分钟内就可检测到。因此，SPA1依赖性的生长促进作用在phyA开始抑制生长之后才开始。SPA1的作用在红光照射约2天的时间内持续存在，这一时期远远超过了phyA光受体及其对生长的影响都已衰减到无法检测水平的时间。此外，当红光下生长的幼苗转移到黑暗中时，在完全没有光刺激的情况下，SPA1仍能促进生长数小时。我们提出，SPA1在一种光诱导机制中发挥作用，该机制促进生长，从而抵消phyA和phyB介导的生长抑制。我们的发现，即spa1幼苗即使在phyA缺失的背景下，对远红光的日终脉冲也不表现出生长促进作用，支持了这一解释。综合这些结果，我们得出这样的观点：光照下下胚轴伸长的速率至少由两个独立的、相反的过程决定；光敏色素对生长的抑制作用以及光激活的SPA1对生长的促进作用。
