Arabidopsis COP1 and SPA genes are essential for plant elongation but not for acceleration of flowering time in response to a low red light to far-red light ratio.

Plants sense vegetative shade as a reduction in the ratio of red light to far-red light (R:FR). Arabidopsis (Arabidopsis thaliana) responds to a reduced R:FR with increased elongation of the hypocotyl and the leaf petioles as well as with an acceleration of flowering time. The repressor of light signaling, CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1), has been shown previously to be essential for the shade-avoidance response in seedlings. Here, we have investigated the roles of COP1 and the COP1-interacting SUPPRESSOR OF PHYA-105 (SPA) proteins in seedling and adult facets of the shade-avoidance response. We show that COP1 and the four SPA genes are essential for hypocotyl and leaf petiole elongation in response to low R:FR, in a fashion that involves the COP1/SPA ubiquitination target LONG HYPOCOTYL IN FR LIGHT1 but not ELONGATED HYPOCOTYL5. In contrast, the acceleration of flowering in response to a low R:FR was normal in cop1 and spa mutants, thus demonstrating that the COP1/SPA complex is only required for elongation responses to vegetative shade and not for shade-induced early flowering. We further show that spa mutant seedlings fail to exhibit an increase in the transcript levels of the auxin biosynthesis genes YUCCA2 (YUC2), YUC8, and YUC9 in response to low R:FR, suggesting that an increase in auxin biosynthesis in vegetative shade requires SPA function. Consistent with this finding, expression of the auxin-response marker gene DR5::GUS did not increase in spa mutant seedlings exposed to low R:FR. We propose that COP1/SPA activity, via LONG HYPOCOTYL IN FR LIGHT1, is required for shade-induced modulation of the auxin biosynthesis pathway and thereby enhances cell elongation in low R:FR.

The Arabidopsis repressor of light signaling SPA1 acts in the phloem to regulate seedling de-etiolation, leaf expansion and flowering time.

Plants adjust their growth and development in response to the ambient light environment. These light responses involve systemic signals that coordinate differentiation of different tissues and organs. Here, we have investigated the function of the key repressor of photomorphogenesis SPA1 in different tissues of the plant by expressing GUS-SPA1 under the control of tissue-specific promoters in a spa mutant background. We show that SPA1 expression in the phloem vasculature is sufficient to rescue the spa1 mutant phenotype in dark-grown spa mutant seedlings. Expression of SPA1 in mesophyll, epidermis or root tissues of the seedling, by contrast, has no or only slight effects. In the leaf, SPA1 expression in both the phloem and the mesophyll is required for full complementation of the defect in leaf expansion. SPA1 in phloem and mesophyll tissues affected division and expansion of cells in the epidermal layer, indicating that SPA1 induces non-cell-autonomous responses also in the leaf. Photoperiodic flowering is exclusively controlled by SPA1 expression in the phloem, which is consistent with previous results showing that the direct substrate of the COP1/SPA complex, CONSTANS, also acts in the phloem. Taken together, our results highlight the importance of phloem vascular tissue in coordinating growth and development. Because the SPA1 protein itself is incapable of moving from cell to cell, we suggest that SPA1 regulates the activity of downstream component(s) of light signaling that subsequently act in a non-cell-autonomous manner. SPA1 action in the phloem may also result in mechanical stimuli that affect cell elongation and cell division in other tissues.

Light exposure of Arabidopsis seedlings causes rapid de-stabilization as well as selective post-translational inactivation of the repressor of photomorphogenesis SPA2.

The COP1/SPA complex acts as an E3 ubiquitin ligase to repress photomorphogenesis by targeting activators of the light response for degradation. Genetic analysis has shown that the four members of the SPA gene family (SPA1-SPA4) have overlapping but distinct functions. In particular, SPA1 and SPA2 differ in that SPA1 encodes a potent repressor in light- and dark-grown seedlings, but SPA2 fully loses its function when seedlings are exposed to light, indicating that SPA2 function is hyper-inactivated by light. Here, we have used chimeric SPA1/SPA2 constructs to show that the distinct functions of SPA1 and SPA2 genes in light-grown seedlings are due to the SPA protein sequences and independent of the SPA promoter sequences. Biochemical analysis of SPA1 and SPA2 protein levels shows that light exposure leads to rapid proteasomal degradation of SPA2, and, more weakly, of SPA1, but not of COP1. This suggests that light inactivates the COP1/SPA complex partly by reducing SPA protein levels. Although SPA2 was more strongly degraded than SPA1, this was not the sole reason for the lack of SPA2 function in the light. We found that the SPA2 protein is inherently incapable of repressing photomorphogenesis in light-grown seedlings. The data therefore indicate that light inactivates the function of SPA2 through a post-translational mechanism that eliminates the activity of the remaining SPA2 protein in the cell.

Functional and expression analysis of Arabidopsis SPA genes during seedling photomorphogenesis and adult growth.

The four members of the Suppressor of phyA-105 (SPA) gene family function to inhibit photomorphogenesis in dark- and light-grown seedlings. Additionally, SPA1-SPA4 regulate elongation growth of adult plants. In these processes, SPA2, SPA3 and SPA4 have overlapping but distinct functions. Here, we have further investigated the role of SPA1 which is partially masked by functional redundancy. We show that SPA1 represses not only red, far-red and blue light responses in a PHYA-dependent fashion, but also acts to suppress light signaling in darkness. We demonstrate that deletion-derivatives of SPA1 lacking the complete N-terminus or part of the kinase-like domain retain SPA1 function in light- and dark-grown seedlings, while deletion of the constitutive photomorphogenesis 1 (COP1)-interacting coiled-coil domain eliminates SPA1 activity. This suggests that the coiled-coil domain and the WD-repeat domain of SPA1 are sufficient for SPA1 function. An analysis of spa2 spa3 spa4 triple mutants demonstrates that SPA1, like SPA2, is sufficient for normal etiolation of dark-grown seedlings. In light-grown seedlings and adult plants, in contrast, SPA1 function is divergent from SPA2 function, with SPA1 playing the predominant role. Levels of SPA1, SPA3 and SPA4 transcript are increased by red, far-red and blue light, consistent with a role of these three SPA genes in light-grown seedlings. The abundance of SPA2 mRNA, in contrast, is not altered by light. Taken together, the analysis of SPA transcript levels suggests that differences in SPA gene expression patterns contribute to divergence in SPA1-SPA4 function.