**Tong Su, Mingyue Yang, Pingping Wang, Yanxiu Zhao and Changle Ma \***

Shandong Provincial Key Laboratory of Plant Stress, College of Life Sciences, Shandong Normal University, Jinan 250014, China; sut0229@sdnu.edu.cn (T.S.); yangmingyue1@stu.sdnu.edu.cn (M.Y.); pingping.wang@sdnu.edu.cn (P.W.); zhaoyx@sdnu.edu.cn (Y.Z.)

**\*** Correspondence: machangle@sdnu.edu.cn; Tel.: +86-0531-86180792

Received: 21 August 2020; Accepted: 25 September 2020; Published: 1 October 2020

**Abstract:** All eukaryotes rely on the ubiquitin-proteasome system (UPS) and autophagy to control the abundance of key regulatory proteins and maintain a healthy intracellular environment. In the UPS, damaged or superfluous proteins are ubiquitinated and degraded in the proteasome, mediated by three types of ubiquitin enzymes: E1s (ubiquitin activating enzymes), E2s (ubiquitin conjugating enzymes), and E3s (ubiquitin protein ligases). Conversely, in autophagy, a vesicular autophagosome is formed that transfers damaged proteins and organelles to the vacuole, mediated by a series of ATGs (autophagy related genes). Despite the use of two completely different componential systems, the UPS and autophagy are closely interconnected and mutually regulated. During autophagy, ATG8 proteins, which are autophagosome markers, decorate the autophagosome membrane similarly to ubiquitination of damaged proteins. Ubiquitin is also involved in many selective autophagy processes and is thus a common factor of the UPS and autophagy. Additionally, the components of the UPS, such as the 26S proteasome, can be degraded via autophagy, and conversely, ATGs can be degraded by the UPS, indicating cross regulation between the two pathways. The UPS and autophagy cooperate and jointly regulate homeostasis of cellular components during plant development and stress response.

**Keywords:** autophagy; degradation; the ubiquitin-proteasome system; ubiquitin; plants
