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

Abstract

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.

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

“…This latter finding is in agreement with our results showing that flowering time regulation by R:FR does not require COP1/SPA. Moreover, SPA proteins and phyB act in distinct tissues in the control of flowering time: while phyB acts in the mesophyll, SPA1 acts in the phloem (Endo et al, 2005;Ranjan et al, 2011). Previous results have shown that white light + FR increases CO transcript levels and also enhances CO protein stability .…”

“…This was necessary to allow DR5::GUS detection. Seedlings were then either kept in high R:FR or shifted to low R:FR for 7 h. They were incubated in GUS staining buffer (Ranjan et al, 2011), but without vacuum infiltration, at 37°C for 16 h in darkness and subsequently destained in 70% ethanol.…”

TEMPRANILLO Reveals the Mesophyll as Crucial for Epidermal Trichome Formation

“…The observed GA 3 -Fl broader distribution throughout the leaf might be directly related to normal TEM repression of GA biosynthesis in the leaf mesophyll structure and/or due to an effect on GA transport. To further support this, we first cloned an artificial microRNA (amiRNA), previously used for partial silencing of both TEM genes (Osnato et al, 2012), under the mesophyll-specific CHLOROPHYLL A/B BINDING PROTEIN 3 promoter (PCAB3; Endo et al, 2007;Ranjan et al, 2011). Transgenic plants expressing amiRTEM under the mesophyll-specific PCAB3 promoter, showed similar GA 3 -Fl accumulation patterns to tem1-1 tem2-2.…”

Spatially distinct regulatory roles for gibberellins in the promotion of flowering of Arabidopsis under long photoperiods

“…To test the effect of reducing GA levels in specific tissues, GA2ox7 cDNA was fused to promoters with specific expression patterns that have been used previously to misexpress regulatory proteins Ranjan et al, 2011). The KNAT1 promoter, which is active in the shoot apical meristem, and the SUC2 promoter, which is specific to the companion cells of the phloem, were used.…”