Antagonistic Actions of HLH/bHLH Proteins Are Involved in Grain Length and Weight in Rice

Grain size is a major yield component in rice, and partly controlled by the sizes of the lemma and palea. Molecular mechanisms controlling the sizes of these organs largely remain unknown. In this study, we show that an antagonistic pair of basic helix-loop-helix (bHLH) proteins is involved in determining rice grain length by controlling cell length in the lemma/palea. Overexpression of an atypical bHLH, named POSITIVE REGULATOR OF GRAIN LENGTH 1 (PGL1), in lemma/palea increased grain length and weight in transgenic rice. PGL1 is an atypical non-DNA-binding bHLH and assumed to function as an inhibitor of a typical DNA-binding bHLH through heterodimerization. We identified the interaction partner of PGL1 and named it ANTAGONIST OF PGL1 (APG). PGL1 and APG interacted in vivo and localized in the nucleus. As expected, silencing of APG produced the same phenotype as overexpression of PGL1, suggesting antagonistic roles for the two genes. Transcription of two known grain-length-related genes, GS3 and SRS3, was largely unaffected in the PGL1-overexpressing and APG-silenced plants. Observation of the inner epidermal cells of lemma revealed that are caused by increased cell length. PGL1-APG represents a new grain length and weight-controlling pathway in which APG is a negative regulator whose function is inhibited by PGL1.

籽粒大小是水稻重要的产量构成因子，其发育部分受外稃（lemma）与内稃（palea）的尺寸调控。目前，调控这类器官尺寸的分子机制在很大程度上仍未被阐明。本研究发现，一对拮抗的碱性螺旋-环-螺旋（basic helix-loop-helix, bHLH）蛋白可通过调控水稻外稃/内稃的细胞长度，参与籽粒长度的决定过程。将一个命名为籽粒长度正调控因子1（POSITIVE REGULATOR OF GRAIN LENGTH 1，PGL1）的非典型bHLH蛋白在外稃与内稃中过表达，可使转基因水稻的籽粒长度与粒重显著提升。PGL1属于不结合DNA的非典型bHLH蛋白，推测其通过与典型的DNA结合型bHLH蛋白形成异源二聚体，发挥抑制作用。我们鉴定得到了PGL1的互作蛋白，并将其命名为PGL1拮抗因子（ANTAGONIST OF PGL1，APG）。PGL1与APG可在体内发生互作，且二者均定位于细胞核。正如预期，APG基因沉默所产生的表型与PGL1过表达的表型完全一致，这表明二者在功能上具有拮抗作用。在PGL1过表达与APG沉默的转基因植株中，两个已知的籽粒长度相关基因GS3与SRS3的转录水平基本未受影响。对外稃内表皮细胞的观察显示，籽粒长度的增加源于细胞长度的增大。PGL1-APG介导了一条全新的水稻籽粒长度与粒重调控通路，在该通路中APG作为负调控因子，其功能可被PGL1所抑制。