Kernel size is an important factor determining grain yield. Although a number of genes affecting kernel development in maize (Zea mays) have been identified by analyzing kernel mutants, most of the corresponding mutants cannot be used in maize breeding programs due to low germination or incomplete seed development. Here, we characterized small kernel7, a recessive small-kernel mutant with a mutation in the gene encoding the second-largest subunit of RNA polymerase III (RNAPIII; NRPC2). A frame shift in ZmNRPC2 leads to a premature stop codon, resulting in significantly reduced levels of transfer RNAs and 5S ribosomal RNA, which are transcribed by RNAPIII. Loss-of-function nrpc2 mutants created by CRISPR/CAS9 showed significantly reduced kernel size due to altered endosperm cell size and number. ZmNRPC2 affects RNAPIII activity and the expression of genes involved in cell proliferation and endoreduplication to control kernel development via physically interacting with RNAPIII subunits RPC53 and AC40, transcription factor class C1 and Floury3. Notably, unlike the semidominant negative mutant floury3, which has defects in starchy endosperm, small kernel7 only affects kernel size but not the composition of kernel storage proteins. Our findings provide novel insights into the molecular network underlying maize kernel size, which could facilitate the genetic improvement of maize in the future.

籽粒大小是决定粮食产量的重要因素。尽管通过分析籽粒突变体已经鉴定出许多影响玉米籽粒发育的基因，但由于发芽率低或种子发育不完全，大多数相应的突变体不能用于玉米育种计划。在这里，我们表征了小内核7，这是一种隐性小内核突变体，其编码RNA聚合酶III第二大亚基（RNAPIII；NRPC2）的基因发生突变。ZmNRPC2中的移码导致过早终止密码子，从而导致由 RNAPIII 转录的转移 RNA 和 5S 核糖体 RNA 的水平显着降低。由 CRISPR/CAS9 创建的功能缺失型nrpc2突变体由于胚乳细胞大小和数量的改变而显示出籽粒大小显着减小。ZmNRPC2 通过与 RNAPIII 亚基 RPC53 和 AC40、转录因子 C1 和 Floury3 发生物理相互作用，影响 RNAPIII 活性以及参与细胞增殖和核内复制的基因表达，从而控制内核发育。值得注意的是，与淀粉胚乳缺陷的半显性失活突变体floury3不同，小kernel7仅影响籽粒大小，但不影响籽粒储藏蛋白的组成。我们的研究结果为玉米粒大小的分子网络提供了新的见解，这可能有助于未来玉米的遗传改良。