Phytochrome-interacting factors (PIFs) play important roles in photomorphogenesis, the shade avoidance response, and other aspects of plant growth and development. PIF family proteins have been well-studied in Arabidopsis thaliana, but little is known about their physiological functions and molecular mechanisms in maize (Zea mays). In this study, we investigated the physiological functions of ZmPIF4 and ZmPIF5, two highly conserved members of the PIF gene family. RT-qPCR and western blot analyses revealed that ZmPIF4 and ZmPIF5 expression and ZmPIF4 and ZmPIF5 levels peak at night and remain low during the day. Overexpression of ZmPIF4 and ZmPIF5 in Arabidopsis partially rescued the reduced hypocotyl elongation and defective response to gravity in pif1 pif3 pif4 pif5 quadruple mutants (pifq). In addition, under high red: far-red light conditions, Arabidopsis lines overexpressing ZmPIF4 exhibited a constitutive shade avoidance response, including early flowering, slender leaves and inflorescences, plant lodging and precocious leaf senescence. Furthermore, ZmPIF4 physically interacted with the Arabidopsis DELLA protein REPRESSOR OF GA1-3 (RGA), indicating a potential interaction between ZmPIF4 and gibberellin signaling pathway on plant growth. Taken together, our results revealed that ZmPIF4 and ZmPIF5 are functionally conserved proteins that may play conserved roles in the response to phytochrome signaling in plants.Highlights:In this study, the functions of ZmPIF4 and ZmPIF5 were characterized by expression in Arabidopsis, revealing conserved roles of PIF family proteins in photomorphogenesis and the shade avoidance response in land plants.

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22Light is one of the most important environmental factors affecting plant growth and 23 development. Plants use shade avoidance and shade tolerance strategies to adjust 24 their growth and development thus increase their success in the competition for 25 incoming light. To investigate the mechanism of shade responses in maize (Zea mays), 26 we examined the anatomical and transcriptional dynamics of the early shade response 27 in seedlings of the B73 inbred line. Transcriptome analysis identified 912 differentially 28 expressed genes, including genes involved in light signaling, auxin responses, and cell 29 elongation pathways. Grouping transcription factor family genes and performing 30 enrichment analysis identified multiple types of transcription factors that are 31 differentially regulated by shade and predicted putative core genes responsible for 32 regulating shade avoidance syndrome. For functional tests, we ectopically over-33 expressed ZmHB53, a type II HD-ZIP transcription factor gene significantly induced by 34 shade, in Arabidopsis thaliana. Transgenic Arabidopsis plants overexpressing 35 ZmHB53 exhibited narrower leaves, earlier flowering, and enhanced expression of 36 shade-responsive genes, suggesting that ZmHB53 participates in the regulation of 37 shade responses in maize. This study increases our understanding of the regulatory 38 network of the shade response in maize and provides a useful resource for maize 39 genetics and breeding. 40

Polar auxin transport mediated by PIN-FORMED (PIN) proteins is critical for plant growth and development. As an environmental cue, shade stimulates hypocotyls, petiole, and stem elongation by inducing auxin synthesis and asymmetric distributions, which is modulated by PIN3,4,7 in Arabidopsis. Here, we characterize the MtPIN1 and MtPIN3, which are the orthologs of PIN3,4,7, in model legume species Medicago truncatula. Under the low Red:Far-Red (R:FR) ratio light, the expression of MtPIN1 and MtPIN3 is induced, and shadeavoidance response is disrupted in mtpin1 mtpin3 double mutant, indicating that MtPIN1 and MtPIN3 have a conserved function in shade response. Surprisingly, under the normal growth condition, mtpin1 mtpin3 displayed the constitutive shade avoidance responses, such as the elongated petiole, smaller leaf, and increased auxin and chlorophyll content. Therefore, MtPIN1 and MtPIN3 play dual roles in regulation of shadeavoidance response under different environments. Furthermore, these data suggest that PIN3,4,7 and its orthologs have evolved conserved and specific functions among species.