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在次生生長過程中，維管形成層不斷產生植物器官、木材(木質部)和韌皮部(韌皮部)的增厚。在擬南芥下胚軸和根中，我們可以根據細胞形態和產量來區分二次生長的兩個階段。第一階段是木質部和韌皮部同樣產生的階段，在木質部擴展階段之前，木質部形成增強，木質部纖維分化。
已知赤黴素(GA)通過降解Della蛋白來觸發這一發育轉變，形成層主控調控因子BREVIPEDICELLUS/Knat1(BP/Knat1)以及類似受體的激酶ERECTA和ERL1調節GA下遊的這一過程。然而，我們對GA介導的次級生長背後的調控網絡的理解仍然有限。在這裡，我們證明Della介導的木質部擴展主要是通過RGA和GAI實現的，RGA和GAI促進形成層衰老。我們進一步證明，生長素反應因子(ARF6)和ARF8在木質部膨大階段能特異性地抑制韌皮部增殖和誘導形成層衰老，它們與Dellas發生物理上的相互作用。
此外，ARF6和ARF8背景下BP的失活揭示了ARF6和ARF8在形成層建立和維持中的重要作用。總體而言，我們的研究結果闡明了GA和生長素在植物次生生長過程中的關鍵激素相互作用。
During secondary growth, the thickening of plant organs, wood (xylem) and bast (phloem) are continuously produced by the vascular cambium. In Arabidopsis hypocotyl and root, we can distinguish two phases of secondary growth based on cell morphology and production rate. The first phase, in which xylem and phloem are equally produced, precedes the xylem expansion phase in which xylem formation is enhanced and xylem fibers differentiate. It is known that Gibberellins (GA) trigger this developmental transition via the degradation of DELLA proteins and that the cambium master regulator BREVIPEDICELLUS/KNAT1 (BP/KNAT1) and the receptor like kinases ERECTA and ERL1 regulate this process downstream of GA. However, our understandings on the regulatory network underlying GA-mediated secondary growth, are still limited. Here, we demonstrate that DELLA-mediated xylem expansion is mainly achieved through RGA and GAI and that RGA and GAI promote cambium senescence. We further show that AUXIN RESPONSE FACTOR (ARF6) and ARF8, which physically interact with DELLAs, specifically repress phloem proliferation and induce cambium senescence during the xylem expansion phase. Moreover, the inactivation of BP in arf6 arf8 background revealed an essential role for ARF6 and ARF8 in cambium establishment and maintenance. Overall, our results shed light on a pivotal hormone cross-talk between GA and auxin in the context of plant secondary growth.
https://www.biorxiv.org/content/10.1101/2020.11.13.381558v1?rss=1
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