Arabidopsis noncoding RNA modulates seedling greening during deetiolation

Seedling greening is essential for the survival of plants emerging from the soil. The abundance of chlorophyll precursors, including protochlorophyllide (Pchlide), is precisely controlled during the dark-to-light transition, as over-accumulation of Pchlide can lead to cellular photooxidative damage. Previous studies have identified and characterized multiple regulators controlling this important process. HID1 (hidden treasure 1) is the first noncoding RNA (ncRNA) found in photomorphogenesis. Under continuous red light, HID1 has been shown to inhibit hypocotyl elongation by repressing the transcription of PIF3 (phytochrome interacting factor 3). Here, we report that HID1 acts as a negative regulator of cotyledon greening. Knockdown of HID1 resulted in an increased greening rate of etiolated seedlings relative to wild type when exposed to white light. Genetically, HID1 acts downstream of PIF3 during the dark-to-light transition. The expression of HID1 is not regulated by PIF3 in the dark. Molecularly, the Pchlide content was reduced in dark-grown hid1 mutants than WT. Meanwhile, transcript levels of the protochlorophyllide oxidoreductases known to catalyze Pchlide to chlorophyllide conversion were significantly increased in hid1 seedlings. Thus, our study reveals an additional role of HID1 in the dark-to-light transition in Arabidopsis. Moreover, these results suggest HID1 could regulate distinct targets in different light-mediated developmental processes, and thus is essential to the control of these mechanisms.