FRUCTOKINASE-LIKE PROTEIN 1 interacts with TRXz to regulate chloroplast development in rice

Chloroplast genes are transcribed by the plastidencoded RNA polymerase（PEP） or nucleus-encoded RNA polymerase. FRUCTOKINASE-LIKE PROTEINS（FLNs） are phosphofructokinase-B（Pfk B）-type carbohydrate kinases that act as part of the PEP complex; however, the molecular mechanisms underlying FLN activity in rice remain elusive.Previously, we identified and characterized a heat-stress sensitive albino（hsa_1） mutant in rice. Map-based cloning revealed that HSA_1 encodes a putative OsFLN_2. Here, we further demonstrated that knockdown or knockout of the OsFLN_1, a close homolog of HSA_1/OsFLN_2, considerably inhibits chloroplast biogenesis and the fln_1 knockout mutants, created by clustered regularly interspaced short palindromic repeats（CRISPR） and CRISPR-associate protein_9, exhibit severe albino phenotype and seedling lethality. Moreover, OsFLN_1 localizes to the chloroplast.Yeast two-hybrid, pull-down and bimolecular fluorescencecomplementation experiments revealed that OsFLN_1 and HSA_1/OsFLN_2 interact with THIOREDOXINZ（OsTRXz） to regulate chloroplast development. In agreement with this,knockout of OsTRXz resulted in a similar albino and seedling lethality phenotype to that of the fln_1 mutants. Quantitative reverse transcription polymerase chain reaction and immunoblot analysis revealed that the transcription and translation of PEP-dependent genes were strongly inhibited in fln_1 and trxz mutants, indicating that loss of OsFLN_1, HSA_1/OsFLN_2, or OsTRXz function perturbs the stability of the transcriptionally active chromosome complex and PEP activity. These results show that OsFLN_1 and HSA_1/OsFLN_2 contribute to chloroplast biogenesis and plant growth.