植物蓝光受体研究进展

植物蓝光调节的反应主要有向光性、抑制幼茎伸长、叶绿体迁移、刺激气孔张开和调节基因表达等。蓝光反应的有效波长是蓝光和近紫外光(320—400am),故蓝光受体也叫蓝光／近紫外光受体。植物蓝光受体研究近年来取得较大进展。以拟南芥为例,已得到确认的受体至少有隐花色素(CRY1、2)和向光蛋白(phototropin)两大类。转基因拟南芥对蓝光、紫外光和绿光敏感,并发现CRY1是一个可溶性蛋白。CRY2编码一个核蛋白,蓝光在转录水平对该蛋白进行调节,它的作用是增加拟南芥对蓝光的敏感性。CRY1和CRY2共同介导了拟南芥植物的向光性。隐花色素的蛋白与辅基之间以非共价键连接,可以吸收蓝光和近紫外光。CRY1和CRY2蛋白之间,尤其是N端相似性很高。向光蛋白目前只发现PHOT1和PHO12两种,向光蛋白作为丝／苏氨酸激酶蓝光受体含有两个光氧化结构域(LOV)并参与了植物向光性叶绿体运动、气孔开放等。     

An experimental test of the adaptive evolution of phototropins: blue-light photoreceptors controlling phototropism in Arabidopsis thaliana

Phototropins are blue-light photoreceptor molecules mediating the capacity for phototropism or bending toward or away from directional light. Like the red-light sensing phytochromes that control shade avoidance, phototropins modulate developmental plasticity in plant architecture. Yet, unlike phytochromes, the adaptive significance of phototropins has been largely a topic of conjecture. In Arabidopsis thaliana, phototropism of seedling and plant stems is under the control of two paralogous genes, PHOT1 and PHOT2, that encode different phototropins with partially redundant light response qualities. The PHOT1 gene product interacts with the NPH3 gene product to cause phototropic bending over a broad range of light intensity, from very weak light in the soil to stronger light in the aerial environment. The PHOT2 gene product modulates shoot bending in response to light of higher intensity only. We compared the fitness of wild-type, phot1, phot2, and nph3 genotypes over a range of light conditions in the field. Seeds were sown in the field on the soil surface and left bare or covered with either gravel or bark mulch chips. Plantings were made under full sun and dense canopy cover. Rates of seedling emergence, survival to flowering, and total seed set were measured. All mutant genotypes had significantly reduced lifetime fitness compared to wild-type. Consistent with their different fluence rate sensitivities, phot1 and phot2 signaling pathways affected fitness at discrete life-cycle stages. Fitness costs of phot1 and nph3 were expressed mainly during seedling emergence from the soil whereas that of phot2 was expressed solely after emergence. Surprisingly, the only significant genotype-by-environment interaction for fitness occurred during emergence: genotypes blind to dim blue light (phot1 and nph3) had poor emergence in the open, but not in the shade. Possibly, the loss of negative phototropism in seedling roots of mutant genotypes reduced establishment success in open (dry soil) conditions. Results show that phototropin-modulated pathways are adaptive and that their evolution has involved functional specialization. However, mechanism(s) of selection on these pathways remain a mystery.     

Plasma membrane H+‐ATPase regulation is required for auxin gradient formation preceding phototropic growth

Phototropism is a growth response allowing plants to align their photosynthetic organs toward incoming light and thereby to optimize photosynthetic activity. Formation of a lateral gradient of the phytohormone auxin is a key step to trigger asymmetric growth of the shoot leading to phototropic reorientation. To identify important regulators of auxin gradient formation, we developed an auxin flux model that enabled us to test in silico the impact of different morphological and biophysical parameters on gradient formation, including the contribution of the extracellular space (cell wall) or apoplast. Our model indicates that cell size, cell distributions, and apoplast thickness are all important factors affecting gradient formation. Among all tested variables, regulation of apoplastic pH was the most important to enable the formation of a lateral auxin gradient. To test this prediction, we interfered with the activity of plasma membrane H⁺‐ATPases that are required to control apoplastic pH. Our results show that H⁺‐ATPases are indeed important for the establishment of a lateral auxin gradient and phototropism. Moreover, we show that during phototropism, H⁺‐ATPase activity is regulated by the phototropin photoreceptors, providing a mechanism by which light influences apoplastic pH.     

Multiple steps of leaf thickening during sun‐leaf formation in Arabidopsis

Plant morphological and physiological traits exhibit plasticity in response to light intensity. Leaf thickness is enhanced under high light (HL) conditions compared with low light (LL) conditions through increases in both cell number and size in the dorsoventral direction; however, the regulation of such phenotypic plasticity in leaf thickness (namely, sun‐ or shade‐leaf formation) during the developmental process remains largely unclear. By modifying observation techniques for tiny leaf primordia in Arabidopsis thaliana, we analysed sun‐ and shade‐leaf development in a time‐course manner and found that the process of leaf thickening can be divided into early and late phases. In the early phase, anisotropic cell elongation and periclinal cell division on the adaxial side of mesophyll tissue occurred under the HL conditions used, which resulted in the dorsoventral growth of sun leaves. Anisotropic cell elongation in the palisade tissue is triggered by blue‐light irradiation. We discovered that anisotropic cell elongation processes before or after periclinal cell division were differentially regulated independent of or dependent upon signalling through blue‐light receptors. In contrast, during the late phase, isotropic cell expansion associated with the endocycle, which determined the final leaf thickness, occurred irrespective of the light conditions. Sucrose production was high under HL conditions, and we found that sucrose promoted isotropic cell expansion and the endocycle even under LL conditions. Our analyses based on this method of time‐course observation addressed the developmental framework of sun‐ and shade‐leaf formation.     

Hypocotyl growth orientation in blue light is determined by phytochrome A inhibition of gravitropism and phototropin promotion of phototropism

How developing seedlings integrate gravitropic and phototropic stimuli to determine their direction of growth is poorly understood. In this study we tested whether blue light influences hypocotyl gravitropism in Arabidopsis. Phototropin1 (phot1) triggers phototropism under low fluence rates of blue light but, at least in the dark, has no effect on gravitropism. By analyzing the growth orientation of phototropism-deficient seedlings in response to gravitropic and phototropic stimulations we show that blue light not only triggers phototropism but also represses hypocotyl gravitropism. At low fluence rates of blue light phot1 mutants were agravitropic. In contrast, phyAphot1 double mutants grew exclusively according to gravity demonstrating that phytochrome A (phyA) is necessary to inhibit gravitropism. Analyses of phot1cry1cry2 triple mutants indicate that cryptochromes play a minor role in this response. Thus the optimal growth orientation of hypocotyls is determined by the action of phyA-suppressing gravitropism and the phototropin-triggering phototropism. It has long been known that phytochromes promote phototropism but the mechanism involved is still unknown. Our data show that by inhibiting gravitropism phyA acts as a positive regulator of phototropism.     

植物的蓝光受体及其信号转导

近年来,对拟南芥及其它植物的分子遗传研究,在隐花色素和向光素的分子、基因和蓝光信号转导方面取得了显著进展。本文就这两种蓝光受体的基本结构及蓝光信号转导进行介绍。