21Histone chaperones mediate assembly and disassembly of nucleosomes and 22 participate in essentially all DNA-dependent cellular processes. In Arabidopsis thaliana, 23 loss-of-functions of FAS1 or FAS2 subunits of the H3-H4 histone chaperone complex 24 CHROMATIN ASSEMBLY FACTOR 1(CAF-1) has a dramatic effect on plant 25 morphology, growth and overall fitness. Altered chromatin compaction, systematic loss 26 of repetitive elements or increased DNA damage clearly demonstrate the severity of 27 CAF-1 dysfunction. How histone chaperone molecular networks change without a 28 functional CAF-1 remains elusive. Here we present an intriguing observation that 29 disruption of the H2A-H2B histone chaperone NUCLEOSOME ASSEMBLY PROTEIN 1 30 (NAP1) supresses FAS1 loss-of function. The quadruple mutant fas1nap1;1-3 shows 31 wild-type growth and decreased sensitivity to genotoxic stress. Chromatin of fas1nap1;1-32 3 plants is less accessible to micrococcal nuclease and progressive loss of telomeres 33 and 45S rDNA is supressed. Interestingly, the strong genetic interaction between FAS1 34 and NAP1 does not occur via direct protein-protein interaction. We propose that 35 NAP1;1-3 play an essential role in nucleosome assembly in fas1, thus their disruption 36 abolishes fas1 defects. Our data altogether reveal a novel function of NAP1 proteins, 37 unmasked by CAF-1 dysfunction. It emphasizes the importance of a balanced 38 composition of chromatin and shed light on the histone chaperone molecular network. 39 40 42 ability to directly bind histones and mediate their assembly into nucleosomes (Nakagawa 43 et al. , 2001; Takeuchi et al., 2003). This process begins with the interaction of the H3-H4 44 tetramer with DNA (~147 bp), followed by addition of H2A-H2B dimers, which completes 45