PNAS|EXO70D亚型介导A型ARR蛋白的选择性自噬降解以调节细胞分裂素敏感性

植物激素细胞分裂素影响植物生长发育的多个方面，其中一些还涉及自噬的细胞过程，包括叶片衰老、营养物质再动员和发育转变。

拟南芥A型反应调节因子(type-AARR)是细胞分裂素信号的负调控因子，在转录水平上诱导对细胞分裂素的应答。在这里，我们描述了细胞分裂素信号和自噬之间的机制联系，证明了植物通过A型ARR蛋白的自噬调节来调节细胞分裂素的敏感性。

A型ARR蛋白通过自噬以自噬相关(ATG)5依赖的方式降解，这种降解是通过A型ARRS受体结构域中保守的天冬氨酸磷酸化来促进的。

EXO70D家族成员可能以磷酸化依赖的方式与A型ARR蛋白相互作用，并通过EXO70D AIM与核心自噬蛋白ATG8的相互作用将它们招募到自噬小体中。一致地，功能丧失的exo70D1、2、3突变体表现出A型ARR对自噬小泡的靶向性受损，A型ARR蛋白水平升高，并且对细胞分裂素不敏感。A型ARRS和EXO70D1、2、3的中断都降低了缺碳条件下的存活率，这表明自噬和细胞分裂素对胁迫的反应之间存在相互作用。

这些结果表明，EXO70D蛋白作为选择性自噬受体，靶向A型ARR货物进行自噬降解，证实了选择性自噬对细胞分裂素信号的调节。

The phytohormone cytokinin influences many aspects of plant growth and development, several of which also involve the cellular process of autophagy, including leaf senescence, nutrient remobilization, and developmental transitions. The Arabidopsis type-A response regulators (type-A ARR) are negative regulators of cytokinin signaling that are transcriptionally induced in response to cytokinin. Here, we describe a mechanistic link between cytokinin signaling and autophagy, demonstrating that plants modulate cytokinin sensitivity through autophagic regulation of type-A ARR proteins. Type-A ARR proteins were degraded by autophagy in an AUTOPHAGY-RELATED (ATG)5-dependent manner, and this degradation is promoted by phosphorylation on a conserved aspartate in the receiver domain of the type-A ARRs. EXO70D family members interacted with type-A ARR proteins, likely in a phosphorylation-dependent manner, and recruited them to autophagosomes via interaction of the EXO70D AIM with the core autophagy protein, ATG8. Consistently, loss-of-function exo70D1,2,3 mutants exhibited compromised targeting of type-A ARRs to autophagic vesicles, have elevated levels of type-A ARR proteins, and are hyposensitive to cytokinin. Disruption of both type-A ARRs and EXO70D1,2,3 compromised survival in carbon-deficient conditions, suggesting interaction between autophagy and cytokinin responsiveness in response to stress. These results indicate that the EXO70D proteins act as selective autophagy receptors to target type-A ARR cargos for autophagic degradation, demonstrating modulation of cytokinin signaling by selective autophagy.

EXO70D isoforms mediate selective autophagic degradation of type-A ARR proteins to regulate cytokinin sensitivity

Atiako Kwame Acheampong, Carly Shanks, Chia-Yi Cheng, G. Eric Schaller, Yasin Dagdas, Joseph J. Kieber

Proceedings of the National Academy of Sciences Oct 2020, 117 (43) 27034-27043; DOI: 10.1073/pnas.2013161117

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