Drought stress from relative humidity control induces nutrient starvation stress

Autophagy contributes to the maintenance of life by degrading and removing proteins and organelles damaged by various environmental stresses. Autophagy-deficient (atg) mutants are known to be sensitive to environmental stresses, such as oxidative stress. In Physcomitrium, atg mutants exhibit greater sensitivity to nutrient starvation (carbon, nitrogen, and phosphate) compared to wild-type (WT). Furthermore, when Physcomitrium is incubated in the dark, chlorophyll is degraded, leading to decolorization, indicating that atg mutants are sensitive not only to nutrient starvation but also to darkness. In this study, we focused on drought stress, which plants frequently experience in their natural environment, and examined the response of atg mutants in Physcomitrium under drought stress conditions of 13% relative humidity (RH) using LiCl and 43% RH using K₂CO₃. Immediately after exposure to drought stress, both WT and atg mutants retained their green color under 13% RH; however, atg mutants faded under 43% RH. This difference may be attributed to reduced intracellular metabolic activity and suppressed chlorophyll degradation under 13% RH, whereas chlorophyll degradation was enhanced under 43% RH. These findings suggest that nutrient starvation stress should be considered even under mild drought conditions, such as 43% RH.自噬（Autophagy）通过降解并清除受各类环境胁迫损伤的蛋白质与细胞器，助力生命稳态的维持。已知自噬缺陷（autophagy-deficient, atg）突变体对氧化应激等环境胁迫敏感。相较于野生型（wild-type, WT），小立碗藓（Physcomitrium）中的atg突变体对营养饥饿胁迫（碳、氮、磷酸盐匮乏）表现出更强的敏感性。此外，当小立碗藓置于黑暗环境中培养时，叶绿素会发生降解并导致植株脱色，这表明atg突变体不仅对营养饥饿胁迫敏感，同时也对黑暗胁迫敏感。本研究聚焦于植物在自然生境中频繁遭遇的干旱胁迫，以小立碗藓为实验材料，分别采用13%相对湿度（relative humidity, RH）结合氯化锂（LiCl）、43%相对湿度结合碳酸钾（K₂CO₃）的干旱胁迫处理体系，探究atg突变体的胁迫响应特征。干旱胁迫处理即刻，13%相对湿度条件下的野生型与atg突变体均保持绿色；但在43%相对湿度条件下，atg突变体出现了褪色现象。该差异或源于13%相对湿度条件下细胞内代谢活性降低、叶绿素降解受到抑制，而43%相对湿度条件下叶绿素降解过程被显著增强。本研究结果提示，即便在43%相对湿度这类轻度干旱条件下，也应将营养饥饿胁迫纳入干旱胁迫的考量范畴。