Chloroplast arrangement in finger millet under low-temperature conditions

Finger millet, a C₄ plant with mesophyll and bundle sheath cells, has been cultivated at high altitudes in the Himalayas owing to its adaptability to stressful environments. Under environmental stresses such as high light and drought, finger millet mesophyll chloroplasts move toward the bundle sheath, a phenomenon known as aggregative arrangement.

To investigate the effect of low temperatures on mesophyll chloroplast arrangement in finger millet, we conducted microscopic observations and photochemical measurements using leaves treated at different temperatures in light or darkness, with or without pharmacological inhibitors. Abscisic acid (ABA) content was also quantified.

Chloroplast aggregative arrangement was induced at 5 °C in a light- and actin-dependent manner. This response required a lower intensity of blue light than that previously observed at moderate temperatures. Low temperature significantly reduced the maximum quantum efficiency of photosystem II and increased leaf ABA content in the light. Conversely, in the absence of blue light at low temperatures or under actin-inhibited conditions, mesophyll chloroplasts exhibited a doughnut-like arrangement, characterized by a distribution away from the bundle sheath side.

In finger millet, mesophyll chloroplasts move toward the bundle sheath through a blue light and actin-based mechanism at low temperatures. The doughnut-like arrangement appears to be a contingent phenomenon that manifests when the dispersion of mesophyll chloroplasts toward the bundle sheath is impeded.

The aggregative arrangement is a response to various environmental stresses, including low temperatures, and may be advantageous for finger millet seedlings in mitigating photoinhibition during cool mornings.