Defying Gravity: WEEP promotes negative gravitropism in Prunus persica by establishing asymmetric auxin gradients

Trees with weeping shoot architectures are valued for their beauty and are a resource for understanding how plants regulate posture control. The Prunus persica (peach) weeping phenotype, which has elliptical downward arching branches, is caused by a homozygous mutation in the WEEP gene. Until now, little was known about the function of WEEP protein despite its high conservation throughout Plantae. Here, we present the results of anatomical, biochemical, biomechanical, physiological, and molecular experiments that provide insight into WEEP function. Our data suggest that weeping peach trees do not have defects in branch structure. Rather, transcriptomes from the adaxial (upper) and abaxial (lower) sides of standard and weeping branch shoot tips revealed flipped expression patterns for genes associated with early auxin response, tissue patterning, cell elongation, and tension wood development. This suggests that WEEP promotes polar auxin transport toward the lower side during shoot gravitropic response, leading to cell elongation and tension wood development. In addition, weeping peach trees exhibited steeper root systems and faster lateral root gravitropic response. This suggests that WEEP moderates root gravitropism and is essential to establishing the set-point angle of lateral roots from the gravity vector. Additionally, size-exclusion chromatography indicated that WEEP proteins self-oligomerize, like other proteins with sterile alpha motif domains. Collectively, our results from weeping peach provide new insight into polar auxin transport mechanisms associated with gravitropism and lateral shoot and root orientation. To investigate WEEP function in the development of the gravitropic polar auxin gradient, we compared gene expression in upper and lower tissues of the first two internodes of horizontal lateral shoots from standard and weeping peaches.

具有下垂枝型的树木不仅兼具极高观赏价值，同时也是解析植物姿态调控机制的宝贵研究资源。桃（Prunus persica）的下垂枝表型表现为枝条呈椭圆形向下弯曲，该表型由WEEP基因的纯合突变所引发。尽管WEEP蛋白在整个植物界（Plantae）中高度保守，但截至目前，学界对其功能仍所知寥寥。本研究通过解剖学、生物化学、生物力学、生理学及分子生物学实验，揭示了WEEP蛋白的相关功能机制。我们的研究数据表明，垂枝桃树的枝条结构并无异常。与之相反，对普通型与垂枝型桃枝梢的近轴（adaxial，上）侧与远轴（abaxial，下）侧进行转录组（transcriptome）分析后发现，与生长素（auxin）早期响应、组织模式建成、细胞伸长以及张力木（tension wood）发育相关的基因呈现出表达模式反转的现象。这提示WEEP蛋白在枝条向重力性响应过程中，可促进生长素向远轴（下）侧的极性运输，进而介导细胞伸长与张力木发育。此外，垂枝桃树的根系更为直立，且侧根的向重力性响应速度更快。这表明WEEP蛋白可调控根系的向重力性，对于确定侧根相对于重力矢量的设定角度至关重要。此外，尺寸排阻色谱法（size-exclusion chromatography）分析结果显示，WEEP蛋白可发生自身寡聚化，这与其他携带无菌α基序（sterile alpha motif, SAM）结构域的蛋白特征一致。综上，本研究以垂枝桃为研究对象，所得结果为解析与向重力性以及侧枝、侧根定向生长相关的生长素极性运输机制提供了全新的见解。为探究WEEP蛋白在向重力性生长素极性梯度形成过程中的功能，本研究对比了普通型与垂枝桃水平侧枝前两个节间的上、下组织的基因表达差异。