Microspore reprogramming to embryogenesis induces changes in cell wall and starch accumulation dynamics associated with proliferation and differentiation events.

Plant cell wall polymers are regulated during development, but the specific roles of their different molecular components and the functional meaning of cell wall changes in different cell types and cell processes are still unclear. In the present work the presence and distribution of different cell wall components in Capsicum annuum L. pollen have been analyzed in situ in order to monitor how they change during two developmental programs. These programs are: pollen development, which is a differentiation process, and stress-induced pollen reprogramming to embryogenesis, which involves proliferation followed later by differentiation processes. Specific antibodies recognizing the major cell wall polymers, the major hemicellulose, xyloglucan (XG), the rhamnogalacturonan II (RGII) pectin domain and high- and low-methyl-esterified pectins were used for both dot-blot and immunolocalization assays at light and electron microscopy levels during defined developmental stages. For comparison purposes, a similar approach was also used in zygotic embryogenesis and root apical tip growth. Results showed differences in the distribution pattern of these molecular complexes, in the proportion of esterified and de-esterified pectins in the two pollen developmental pathways, and defined wall changes during microspore reprogramming. These changes were associated with proliferation and differentiation events where highly esterified pectins were characteristic of proliferation, while de-esterified pectins, XG and RGII were abundant in walls of differentiating cells. Starch deposits were also studied and the results revealed changes in starch synthesis dynamics after switching the pollen embryogenic developmental program. These changes occurred together with modifications in the distribution patterns of cell wall polymers, starch accumulation being associated with cell differentiation. As in the case of proliferating cells, esterified pectins were also abundant in the apertures of developing microspores, regions of new cell wall formation. The different distribution patterns of cell wall polymers were common for proliferating cells and differentiating cells in all the plant systems analyzed, including zygotic embryos and root tip cells, suggesting that these patterns are markers of proliferation and differentiation events as well as markers of pollen reprogramming to embryogenesis.