HD-ZIP I Transcription Factor (PtHB13) Negatively Regulates Citrus Flowering through Binding to FLOWERING LOCUS C Promoter

Abstract

For floral induction in adult citrus, low temperature is one of the most important environmental factors. FLOWERING LOCUS C (FLC) plays a very important role in low-temperature-induced Arabidopsis flowering by repressed FLC expression under exposure to prolonged low-temperature conditions. However, little is known about the FLC regulation mechanism in perennial woody plants such as citrus. In this study, the functions of citrus FLC homolog (PtFLC) were investigated by ectopic expression in Arabidopsis. Transcription factor of homeodomain leucine zipper I (HD-ZIP I) as an upstream regulator of PtFLC was identified by yeast one-hybrid screen to regulate its transcription. The HD-ZIP I transcription factor was highly homologous to Arabidopsis ATHB13 and thus was named PtHB13. Ectopically expressed PtHB13 inhibited flowering in transgenic Arabidopsis. Furthermore, the expression of PtFLC and PtHB13 showed a seasonal change during the floral induction period and was also affected by low temperature. Thus, we propose that PtHB13 binds to PtFLC promoter to regulate its activity during the citrus floral induction process.

HD-ZIP Gene Family: Potential Roles in Improving Plant Growth and Regulating Stress-Responsive Mechanisms in Plants

“…This regulation could be the outcome of the direct interaction of START domain-containing proteins with lipid/sterol proteins or by third mediator protein [25]. A study supported this notion by reporting that an HD-ZIP class IV gene regulates the phospholipid signaling in arabidopsis roots [26]. Another report concluded that the START domain is essential for the proper functioning of HD-ZIP genes in the cotton plant [27].…”

“…For example, the HDZI-4 promoter drives DREB/CBF expression under severe drought conditions, which mitigates the negative effects of drought stress and restricts the declination in yield and other growth attributes in wheat and barley [25]. Recently, Ma et al [26] addressed the crucial role of ATHB13 in floral induction. Flower induction at an appropriate time is crucial for seed setting, survival, and germination [27,28].…”

“…The citrus PtHB13 is homologous to Arabidopsis ATHB13. The ectopic expression of PtHB13 in Arabidopsis inhibited the floral induction process and could regulate the floweringrelated genes [26]. Majority of the reports available on HD-ZIP I TFs suggested that they are mostly induced under abiotic stresses and thus crucial for maintaining plant growth under unfavorable environments.…”

A MADS‐box gene with similarity to FLC is induced by cold and correlated with epigenetic changes to control budbreak in kiwifruit

“…In citrus, seasonal fluctuation in expression of FLC homologs was reported (Zhang et al ., 2009; Y.‐J. Ma et al ., 2020), suggestive of negative regulation of citrus flowering. Furthermore, a mandarin FLC CcMADS19 (Hou et al ., 2014; Agustí et al ., 2020) acted to repress the citrus FT and drive the biennial bearing pattern.…”

A MADS-Box Gene CiMADS43 Is Involved in Citrus Flowering and Leaf Development through Interaction with CiAGL9

“…For example, FLOWERING LOCUS C ( FLC ) encodes a specific MADS-box protein and as a typical flowering suppressor [ 10 , 11 , 12 ]. Citrus FLC homolog is conserved with model plants, but it regulates the phase changes of citrus through alternative splicing [ 13 , 14 ]. AGAMOUSLIKE16 ( AGL16 ) is a target of miR824, which inhibits plant flowering [ 15 , 16 ].…”